Pharmacologic characterization of BMS-191095, a mitochondrial K(ATP) opener with no peripheral vasodilator or cardiac action potential shortening activity

Previous work described ATP-sensitive K(+) channel (K(ATP)) openers (e.g., BMS-180448), which retain the cardioprotective activity of agents such as cromakalim while being significantly less potent as vasodilators. In this study, we describe the pharmacologic profile of BMS-191095, which is devoid of peripheral vasodilating activity while retaining glyburide-reversible cardioprotective activity. In isolated rat hearts subjected to 25 min of global ischemia and 30 min of reperfusion, BMS-191095 increased the time to onset of ischemic contracture with an EC(25) of 1.5 microM, which is comparable to 4.7 microM and 3.0 microM for cromakalim and BMS-180448, respectively. Comparisons of cardioprotective and vasorelaxant potencies in vitro and in vivo showed BMS-191095 to be significantly more selective for cardioprotection with virtually no effect on peripheral smooth muscle, whereas cromakalim showed little selectivity. In addition to increasing the time to the onset of contracture, BMS-191095 improved postischemic recovery of function and reduced lactate dehydrogenase release in the isolated rat hearts. The cardioprotective effects of BMS-191095 were abolished by glyburide and sodium 5-hydroxydecanoate (5-HD). BMS-191095 did not shorten action potential duration in normal or hypoxic myocardium within its cardioprotective concentration range nor did it activate sarcolemmal K(ATP) current (< or =30 microM). BMS-191095 opened cardiac mitochondrial K(ATP) with a K(1/2) of 83 nM, and this was abolished by glyburide and 5-HD. These results show that the cardioprotective effects of BMS-191095 are dissociated from peripheral vasodilator and cardiac sarcolemmal K(ATP) activation. Agents like BMS-191095 may owe their cardioprotective selectivity to selective mitochondrial K(ATP) activation.

Cardioselective antiischemic ATP-sensitive potassium channel (K(ATP)) openers. 6. Effect of modifications at C6 of benzopyranyl cyanoguanidines

The effect on potency and selectivity of modifications at the C6 position of the cardioprotective K(ATP) opener BMS-180448 (2) is described. Structure-activity studies show that a variety of electron-withdrawing groups (ketone, sulfone, sulfonamide, etc.) are tolerated for cardioprotective activity as measured by EC(25) values for an increase in time to the onset of contracture in globally ischemic rat hearts. Changes made to the sulfonamido substituent indicate that compounds derived from secondary lipophilic amines are preferred for good cardioprotective potency and selectivity. The diisobutyl analogue 27 (EC(25) = 0.04 microM) is the most potent compound of this series. The cardiac selectivity of 27 results from a combination of reduced vasorelaxant potency and enhanced cardioprotective potency relative to the potent vasodilating K(ATP) openers (e.g., cromakalim). The diisobutylsulfonamide analogue 27 is over 4 orders of magnitude more cardiac selective than cromakalim (1). These results support the hypothesis that the cardioprotective and vasorelaxant properties of K(ATP) openers follow distinct structure-activity relationships. The mechanism of action of 27 appears to involve opening of the cardiac K(ATP) as its cardioprotective effects are abolished by the K(ATP) blocker glyburide.

Preconditioning in rat hearts is independent of mitochondrial F1F0 ATPase inhibition

Mitochondrial F1F0 adenosinetriphosphatase (ATPase) is responsible for the majority of ATP synthesis during normoxic conditions, but under ischemic conditions it accounts for significant ATP hydrolysis. A previous study showed that preconditioning in isolated rat hearts is mediated by inhibition of this ATPase during ischemia. We tested this hypothesis in our isolated rat heart model of preconditioning. Preconditioning was accomplished by three 5-min periods of global ischemia separated by 5 min of reperfusion. This was followed by 20 min of global ischemia and 30 min of reperfusion. Preconditioning significantly enhanced reperfusion contractile function and reduced lactate dehydrogenase release but paradoxically reduced the time to onset of contracture during global ischemia. Myocardial ATP was depleted at a faster rate during the prolonged ischemia in preconditioned than in sham-treated hearts, which is consistent with the reduced time to contracture. ATP during reperfusion was repleted more rapidly in preconditioned hearts, which is consistent with their enhanced contractile function. Preconditioning significantly reduced lactate accumulation during the prolonged ischemia. We were not able to demonstrate that mitochondrial F1F0 ATPase (measured in submitochondrial particles) was inhibited by preconditioning before or during the prolonged ischemia. The mitochondrial ATPase inhibitor oligomycin significantly conserved ATP during ischemia and increased the time to the onset of contracture, which is consistent with inhibition of the mitochondrial ATPase. Our results show that preconditioning in rat hearts can be independent of mitochondrial ATPase inhibition as well as ATP conservation.

Adenosine A1 receptor blockade does not abolish the cardioprotective effects of the adenosine triphosphate-sensitive potassium channel opener bimakalim

There has been controversy regarding whether ATP-sensitive potassium channel activation protects hearts through adenosine A1 receptor activation or the converse. We addressed this issue by determining the effect of the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) on the cardioprotective activity of the ATP-sensitive potassium channel opener bimakalim. In isolated rat hearts subjected to 25 min of global ischemia and 30 min of reperfusion, bimakalim significantly reduced lactate dehydrogenase release and improved postischemic recovery of contractile function. Bimakalim increased the time to the onset of ischemic contracture (EC25 = 1.2 microM), compared with vehicle, and 10 microM DPCPX had no effect on this protective action (EC25 = 1.1 microM). The 10 microM concentration of DPCPX was sufficient to abolish the bradycardic and cardioprotective effects of the adenosine A1 receptor agonist (R)-(-)-N6-(2-phenylisopropyl)adenosine. DPCPX alone had no effect on the severity of ischemia/reperfusion damage. Glyburide completely abolished the cardioprotective effects of bimakalim. Bimakalim (1 microg/kg, intracoronarily) given over four periods of 5 min, interspersed with 10-min drug-free periods, before a 60-min occlusion and 3-hr reperfusion significantly reduced infarction size in anesthetized dogs (25 +/- 5 and 8 +/- 2% of the left ventricular area at risk for vehicle- and bimakalim-treated groups, respectively). DPCPX had no effect on the infarction-sparing activity of bimakalim (9 +/- 3% of the left ventricular area at risk). The protective effect of bimakalim was not accompanied by marked hemodynamic changes or by changes in regional myocardial blood flow. The results of this study suggest that the cardioprotective effects of ATP-sensitive potassium channel openers are not dependent on adenosine A1 receptor activation in rat or dog models of ischemia.

Cardioselective antiischemic ATP-sensitive potassium channel (KATP) openers. 5. Identification of 4-(N-aryl)-substituted benzopyran derivatives with high selectivity

This paper describes our studies aimed at the discovery of structurally distinct analogs of the cardioprotective KATP opener BMS-180448 (2) with improved selectivity for the ischemic myocardium. The starting compound 6, derived from the indole analog 4. showed good cardioprotective potency and excellent selectivity compared to 2 and the first-generation KATP opener cromakalim (1). The structure-activity studies indicate that increasing the size of the alkyl ester leads to diminished potency as does its replacement with a variety of other groups (nitrile, methyl sulfone). Replacement of the ethyl ester of 6 with an imidazole gave the best compound 3 (BMS-191095) of this series which maintains the potency and selectivity of its predecessor 6. The results described in this publication further support that there is no correlation between vasorelaxant and cardioprotective potencies of KATP openers. Compound 3 is over 20- and 4000-fold more selective for the ischemic myocardium than 2 and cromakalim (1), respectively. The selectivity for the ischemic myocardium is achieved by reduction of vasorelaxant potency rather than enhancement in antiischemic potency. As for cromakalim (1) and 2, the cardioprotective effects of compound 3 are inhibited by cotreatment with the KATP blocker glyburide, indicating that the KATP opening is involved in its mechanism of cardioprotection. With its good oral bioavailability (47%) and plasma elimination half-life (3 h) in rats, compound 3 offers an excellent candidate to investigate the role of residual vasorelaxant potency of 2 toward its cardioprotective activity in vivo.

Glyburide-reversible cardioprotective effects of BMS-180448: functional and energetic considerations

Adenosine triphosphate (ATP)-sensitive potassium channel openers as a class exert cardioprotective effects, and we can separate vasodilator from glyburide-reversible cardioprotective activity in cromakalim analogs (e.g., BMS-180448). The purpose of this study was to determine the relation between cardiac function, energy status, and cardioprotective effects for BMS-180448 in isolated rat hearts compared with diltiazem. BMS-180448 (1-30 microM) or 0.1-1 microM diltiazem were given 10 min before 25-min global ischemia in rat hearts followed by 30 min of reperfusion. Both compounds significantly increased time to the onset of contracture during ischemia and improved postischemic recovery of contractile function in a concentration-dependent manner. At equivalent cardioprotective concentrations, BMS-180448 depressed preischemic cardiac function significantly less than did diltiazem. During ischemia, diltiazem significantly accelerated the functional decline observed in vehicle-treated hearts, whereas BMS-180448 attenuated the net rate of decline of function. Despite these different effects on preischemic and ischemic cardiac function, diltiazem and BMS-180448 conserved cardiac ATP during ischemia to a similar degree. BMS-180448 enhanced the recovery of ATP (also seen for diltiazem, but not to the same magnitude) and creatine phosphate during reperfusion compared with vehicle-treated hearts. For BMS-180448, this enhanced ATP recovery was accompanied by a significant improvement in the efficiency of oxygen use, which was profoundly reduced in reperfused vehicle-treated hearts. BMS-180448 also significantly enhanced the functional reserve after the 25-min period of global ischemia. Thus BMS-180448 protects ischemic myocardium and conserves ATP with less reduction in cardiac function compared with diltiazem.

Lack of a pharmacologic interaction between ATP-sensitive potassium channels and adenosine A1 receptors in ischemic rat hearts

OBJECTIVES

An interaction between adenosine A1 receptors and ATP-sensitive potassium channels (KATP) has been hypothesized to mediate preconditioning in several species. Unlike other species tested, KATP blockers and A1 antagonists do not abolish preconditioning in rat hearts. The purpose of this study was to determine if KATP and A1 receptors are pharmacologically linked in rat hearts as they are in other species.

METHODS

Isolated rat hearts were given 0.03-1.00 microM R-PIA (adenosine A1 receptor agonist) with or without concomitant 0.3 microM glyburide starting 10 min pre-ischemia. After 25 min global ischemia, the hearts were reperfused for 30 min. Rat hearts were also treated with 1-30 microM cromakalim in the presence of 10 microM DPCPX (adenosine A1 antagonist).

RESULTS

R-PIA produced a concentration-dependent bradycardia before ischemia which was blocked by DPCPX. R-PIA increased the time to onset of contracture in a concentration-dependent manner (EC25 = 0.13 microM) and this was unaffected by 0.3 microM glyburide (EC25 = 0.20 microM). This concentration of glyburide completely abolished the protective effects of 10 microM cromakalim. R-PIA also significantly enhanced post-ischemic recovery of function and reduced LDH release, and glyburide did not alter these responses. Cromakalim significantly increased the time to onset of contracture (EC25 = 4.5 microM) and 10 microM DPCPX had no effect on this (EC25 = 5.6 microM). Cromakalim also significantly enhanced post-ischemic recovery of function and reduced LDH release. DPCPX did not alter these cardioprotective effects while glyburide completely abolished the cardioprotective effects of cromakalim.

CONCLUSIONS

While both cromakalim and R-PIA are cardioprotective in isolated rat hearts, they are not pharmacologically linked, possibly explaining why preconditioning may be different in this species.

Hemodynamic and cardiac effects of BMS-180448, a novel K+ATP opener, in anesthetized dogs and isolated rat hearts

Hemodynamic and cardiac effects of BMS-180448 (0.3-10 mg/kg i.v.) or cromakalim (0.01-0.3 mg/kg i.v.) were evaluated in anesthetized open-chest dogs and isolated perfused rat hearts. In the canine studies, heart rate (HR), mean arterial pressure and left ventricular pressure were measured as well as electromagnetic blood flows recorded from the aortic, renal, coronary and femoral vascular beds. BMS-180448 was 187-fold less potent than cromakalim in lowering blood pressure (ED-20 values of 7.84, and 0.042 mg/kg for BMS-180448 and cromakalim, respectively). Both compounds increased HR. Effects of BMS-180448 occurred at doses higher than those of cromakalim, but at doses slightly lower than those needed to cause hypotension (ED(HR)/ED(MABP) ratio of 0.18 for BMS-180448). BMS-180448 had no effect on myocardial contractility or relaxation over the doses studied, whereas cromakalim significantly increased +dP/dt and lowered -dP/dt. Effects on +dP/dt were associated with a decrease in blood pressure. Although BMS-180448 reduced total peripheral resistance (ED-25 = 5.75 mg/kg), it had little effect on specific vascular beds, with the exception of the coronary bed. BMS-180448, unlike cromakalim which caused more general vasodilating effects, appeared to be relatively selective in dilating the coronary vascular bed. In isolated perfused rat hearts, BMS-180448, 10-fold more potent as a cardioprotectant (EC25 = 2.7 microM) than as a cardiodepressant (ED-25 = 27.8 microM), had no effect on HR, suggesting a lack of effect of BMS-180448 on myocardial conduction. In conclusion, BMS-180448, a recently developed K+ATP opener, exerted less hypotensive and more selective vascular effects than did cromakalim. These results suggest that BMS-180448, at doses previously reported to give cardioprotection, should have a safe hemodynamic profile.

Cardioselective antiischemic ATP-sensitive potassium channel openers. 4. Structure-activity studies on benzopyranylcyanoguanidines: replacement of the benzopyran portion

The results of our efforts aimed at the replacement of the benzopyran ring of the lead cardiac selective antiischemic ATP-sensitive potassium channel (KATP) opener (4) are described. Systematic modification of the benzopyran ring of 4 resulted in the discovery of a structurally simpler acyclic analog (8) with slightly lower antiischemic potency than the lead compound 4. Further structure-activity studies on the acyclic analog 8 provided the 2-phenoxy-3-pyridylurea analog 18 with improved antiischemic potency and selectivity compared to the benzopyran-based compound 4. These data demonstrate that the benzopyran ring of 4 and its congeners is not mandatory for antiischemic activity and cardiac selectivity. The results described in this paper also show that, as for the benzopyran class of compounds, the structure-activity relationships for the antiischemic and vasorelaxant activities of KATP openers are distinct. The mechanism of action of the acyclic analogs (e.g., 18) still appears to involve KATP opening as their cardioprotective effects are abolished by pretreatment with the KATP blocker glyburide.

A comparison between the effects of BMS-180448, a novel K+ channel opener, and cromakalim in rat and dog

BMS-180448 [(3S-trans)-N-(4-chlorophenyl)-N'-cyano-N"-(6-cyano-3, 4-dihydro-3-hydroxy-2,2-dimethyl-2H-1-benzopyran-4-yl) guanidine] is a structural analog of cromakalim, which was found to similarly decrease ischemic injury, but was 18- to 100-fold less potent as a vasodilator. In the present study, the vascular and cardiac effects of cromakalim and BMS-180448 were evaluated in both in vitro and in vivo preparations. Cromakalim evoked a concentration-dependent relaxation to a K(+)-induced contracture in rat aorta. BMS-180448 behaved in a similar fashion but was 18-fold less potent than cromakalim. Measurements of ischemic damage made in isolated perfused rat hearts demonstrated that cromakalim and BMS-180448 were equipotent as cardioprotective agents; time to contracture was increased with an EC25 value of 4.8 and 4.7 microM, respectively, and lactate dehydrogenase levels were significantly reduced compared to those in the presence of vehicle. In vivo electrophysiologic studies in anesthetized dogs were conducted at basic cycle lengths of 400, 333, and 286 ms, and showed that BMS-180448 caused no significant effect on electrophysiologic parameters with the exception of decreasing atrial effective refractory periods by 12 +/- 3% and 17 +/- 4% at 3 and 10 mg/kg, respectively. There was also a significant drop in mean blood pressure of 18 +/- 5% and 33 +/- 4% at these doses. In contrast, cromakalim was shown to produce shortening of atrial to His conduction time (20 +/- 7%; basic cycle length = 286 ms), atrial effective refractory period (34 +/- 3%; basic cycle length = 400 ms), ventricular effective refractory period (14 +/- 2%; basic cycle length = 400 ms), wavelength (13 +/- 3%; basic cycle length = 400 ms), PR-interval (14 +/- 3%; basic cycle length = 333 ms) and mean blood pressure (65 +/- 3%; basic cycle length = 400 ms) at a dose of 0.3 mg/kg. No supraventricular or ventricular arrhythmias were observed for either compound tested. Based on the reduced cardiac electrophysiologic and vascular effects of BMS-180448, we suggest that BMS-180448 should provide cardioprotective efficacy similar to cromakalim with reduced risk of hypotension or arrhythmias.

Glyburide-reversible cardioprotective effect of BMS-180448 is independent of action potential shortening

OBJECTIVE

We determine if action potential duration (APD) shortening and cardioprotection are separable phenomena in ATP-sensitive potassium channel (KATP) openers which protect ischemic myocardium via a glyburide-reversible mechanism.

METHODS

We determined the effect of the weakly vasodilating KATP opener, BMS-180448, and the less cardiac-selective cromakalim, on APD in normal, hypoxic or ischemic guinea pig papillary muscles or isolated hearts and compared their APD effects with their cardioprotective activity in isolated guinea pig hearts.

RESULTS

In isolated ischemic guinea pig hearts, cromakalim and BMS-180448 had similar cardioprotective potencies (EC25 of 3.2 and 3.3 microM, respectively, for increasing time to the onset of contracture). At 10 microM, BMS-180448 produced no APD shortening, yet was equally protective at this concentration compared to cromakalim, which produced profound APD shortening under either hypoxic or ischemic conditions. The cardioprotective effects of both compounds at 10 microM were abolished by 0.3 microM glyburide.

CONCLUSIONS

APD shortening is not correlated with cardioprotective activity for BMS-180448 and cromakalim while their cardioprotective effects are abolished by glyburide. These results suggest the possibility of reduced proarrhythmic activity in some KATP openers and that their cardioprotective activity is not associated with sarcolemmal KATP opening.

Protective effect of K(ATP) openers in ischemic rat hearts treated with a potassium cardioplegic solution

ATP-sensitive potassium channel (KATP) openers directly protect ischemic myocardium, which may make them useful for treating patients undergoing cardiopulmonary bypass, but whether high-potassium-containing cardioplegic solutions would inhibit their protective effects is not clear. We determined whether additional protection greater than that provided by cardioplegia could be found for KATP openers. We studied the effect of 10 microM cromakalim or BMS-180448 pretreatment (10 min before cardioplegia) on severity of ischemia in isolated rat hearts given normothermic or cold St. Thomas' cardioplegic solution (16 mM K+). After cardioplegic arrest, the hearts were subjected to 30-min (normothermic) or 150-min (hypothermic) global ischemia, each followed by 30-min reperfusion. The cardioplegic solutions significantly protected the hearts, as measured by increased time to onset of contracture, enhanced recovery of function, and reduced lactate dehydrogenase (LDH) release. Cromakalim and BMS-180448 both further significantly increased time to contracture in both normothermic and hypothermic arrested hearts; this was accompanied by enhanced recovery of reperfusion contractile function and reduced cumulative LDH release. This additional protective effect of the K ATP openers was abolished by glyburide. Because administration of the K ATP openers only with the cardioplegic solution (1 min before global ischemia) was not efficacious, >1-min pretreatment apparently is necessary. K ATP openers provide additional protection to that afforded by cold or normothermic potassium cardioplegia in rat heart, although the timing of treatment may be crucial.

Cardioselective anti-ischemic ATP-sensitive potassium channel openers. 3. Structure-activity studies on benzopyranyl cyanoguanidines: modification of the cyanoguanidine portion

Structure-activity relationships for the cyanoguanidine portion of the lead cardiac selective ATP-sensitive potassium channel (KATP) opener (3) are described. The cyanoguanidine moity appears to be optimal since increasing or decreasing the distance between the aniline nitrogen and the pendant aromatic ring attenuates anti-ischemic potency/selectivity. Similarly, unfavorable results are obtained by replacement of the aniline nitrogen with other linkers (CH2, S, O). Replacement of the phenyl ring with a methyl group diminishes cardiac selectivity. Constraining the urea moiety into a benzimidazolone or imidazolone ring retains anti-ischemic potency with significant improvement in cardiac selectivity. As shown by the ratio of vasorelaxant and anti-ischemic potencies, the cardiac selectivity in vitro varies over 3 orders of magnitude. These data are in agreement with previous results indicating that distinct structure-activity relationships exist for the anti-ischemic and vasorelaxant activities. Since the anti-ischemic effects of this series of compounds are abolished by pretreatment with structurally different KATP blockers (glyburide, sodium 5-hydroxydecanoate, meclofenamic acid), the mechanism for the anti-ischemic actions of these compounds still appears to involve the opening of KATP.

Pharmacologic profile of the dihydropyrimidine calcium channel blockers SQ 32,547 and SQ 32,926 [correction of SQ 32,946]

SQ 32,926 and SQ 32,547, two dihydropyrimidine calcium channel blockers, were characterized as potent inhibitors of depolarization-induced contractions of isolated smooth muscle preparations. In rat aorta, the IC50 values were 5.5 nM for SQ 32,547 and 8.1 nM for SQ 32,926, values which compare favorably with that of 2.9 nM for nifedipine. The dihydropyrimidines were also tested in a model of stable angina: pacing-induced ischemia in dogs. Both SQ 32,547 and SQ 32,926 reduced the ST-segment elevation observed in vehicle-treated animals. No significant changes in hemodynamic status were detected, suggesting that a reduction in cardiac work secondary to afterload reduction was probably not a major contributor to the protective effects. Neither was increased coronary blood flow important for the antiischemic outcome because no significant effects of the dihydropyrimidines were observed on ischemic regional blood flow. SQ 32,547 was also studied in globally ischemic, isolated rat hearts. In this model, SQ 32,547 was protective because it significantly reduced contracture formation and lactate dehydrogenase (LDH) release. Washing out the effect of SQ 32,547 in isolated hearts and smooth muscles was difficult, presumably due to its lipophilicity. In the smooth muscle preparations, the effects of both nifedipine and SQ 32,926 were much more easily washed out. As with other calcium channel blockers, increasing the antiischemic effects of SQ 32,547 was associated with a higher cost in terms of cardiac function. In summary, the two novel dihydropyrimidines, SQ 32,547 and SQ 32,926 showed antiischemic properties in animal models.

Cardioselective antiischemic ATP-sensitive potassium channel openers. 2. Structure-activity studies on benzopyranylcyanoguanidines: modification of the benzopyran ring

The ATP-sensitive potassium channel (KATP) openers are of considerable interest as myocardial protecting agents. However, there exists a narrow window of safety for the use of first-generation compounds as antiischemic agents due to their powerful peripheral vasodilating effects, which can result in underperfusion of the area already at risk. We have recently disclosed the discovery of benzopyranylcyanoguanidine type KATP openers (BMS-180448) which are more selective for the ischemic myocardium compared to the first-generation compounds. This publication deals with structure-activity relationships for the antiischemic activity of the lead compound 8. The presence of an electron-withdrawing group at C6, an sp3 center at C4, and a gem-dimethyl group at C2 appears to be essential for antiischemic activity. Cyanoguanidine can be replaced with a urea moiety. The results reported here support the hypothesis that distinct structure-activity relationships exist for antiischemic and vasorelaxant activities of compounds related to 8 and cromakalim. The trifluoromethyl analog 10 is 550-fold more selective in vitro for the ischemic myocardium compared to the first-generation agent cromakalim. The reasons for the selectivity of these compounds for the ischemic myocardium are not clear at the present time. They may be related to the existence of receptor subtypes in smooth muscle and the myocardium.

In vitro effects of capsaicin: antiarrhythmic and antiischemic activity

The antiarrhythmic effects of vehicle (0.1% dimethyl sulfoxide: DMSO) or capsaicin were evaluated in isolated perfused rat and guinea pig heart preparations. In the rat, capsaicin reduced ischemic ventricular tachycardia from 100% in control to 0%, and ischemic ventricular fibrillation from 60% in control to 0% at 30 microM, and diltiazem reduced the incidence of ischemic ventricular tachycardia and ventricular fibrillation to 55% and 0%, respectively. Reperfusion ventricular fibrillation was reduced from 90% to 20% and 33% for capsaicin and diltiazem, respectively, at these concentrations. In isolated perfused globally ischemic rat hearts, antiischemic efficacy was assessed as a significant extension (36% and 50%) in time to contracture with 30 microM capsaicin and 1 microM diltiazem, respectively. Capsaicin reduced left ventricular developed pressure by 35% in non-ischemic rat hearts, and increased coronary flow by 40%. The increased time to contracture for either compound was not blocked by glyburide (0.1 microM) suggesting a lack of any involvement of ATP-sensitive K+ channels. In isolated guinea pig hearts subjected to global ischemia, capsaicin and diltiazem reduced reperfusion ventricular fibrillation from 100% to 10% and 0% at 30 and 3 microM, respectively. Electrophysiologic evaluation in guinea pig papillary muscles using standard microelectrode techniques demonstrated significant (P < 0.05) action potential durations at 90% repolarization shortening at 1 Hz by 9%, 28% and 39%, and 23%, 37% and 51% at 10, 30, and 100 microM of capsaicin or diltiazem, respectively. Unlike diltiazem, no changes in action potential duration were observed with capsaicin (up to 100 microM) at faster stimulation rates (5 Hz). In conclusion, capsaicin displays both antiarrhythmic and antiischemic efficacy. These data suggest that the effects of capsaicin are mediated primarily through block of Ca2+ channels in these preparations.

Endogenous catecholamines are not necessary for ischaemic preconditioning in the isolated perfused rat heart

OBJECTIVE

The mechanism of the protective effect of ischaemic preconditioning in the myocardium is not yet known. The aim of this study was to test the hypothesis that endogenous myocardial catecholamines may be mediators of preconditioning.

METHODS

To test whether endogenous catecholamines are involved in preconditioning, experiments were performed in hearts from rats which had been catecholamine depleted with either reserpine or 6-hydroxydopamine. Experiments were also done to determine if noradrenaline can mimic preconditioning.

RESULTS

Catecholamine depletion with either reserpine or 6-hydroxydopamine had no effect on preischaemic coronary flow or cardiac function. Ischaemic preconditioning (four episodes of 5 min global ischaemia and 5 min reperfusion) resulted in a significant increase in postischaemic cardiac function and a 50% decrease in lactate dehydrogenase (LDH) release following 30 min ischaemia and 30 min reperfusion compared with non-preconditioned hearts. Reserpine pretreatment did not affect the response to ischaemia or to preconditioning, although LDH release tended to be greater than in normal hearts, especially in the non-preconditioned group. Although 6-hydroxydopamine significantly increased postischaemic cardiac function in the preconditioned group, no other index of ischaemic damage (for example, LDH release, left ventricular end diastolic pressure) was affected. Further studies showed that 10 nmol.min-1 noradrenaline did not affect the severity of ischaemia, indicating that it does not mimic preconditioning.

CONCLUSIONS

Endogenous catecholamines are not necessary for ischaemic preconditioning in isolated rat hearts and play little or no role in the functional responses to ischaemia.

Effect of potassium on the action of the KATP modulators cromakalim, pinacidil, or glibenclamide on arrhythmias in isolated perfused rat heart subjected to regional ischaemia

OBJECTIVE

The ATP sensitive potassium channel openers cromakalim (n = 10) and pinacidil (n = 10), and a blocker of this channel, glibenclamide (n = 10), were studied in isolated perfused rat hearts subjected to regional ischaemia at varying concentrations (2 to 8 mM) of external potassium ([K+]o).

METHODS

Hearts were isolated and perfused on a Langendorff apparatus. Vehicle (0.1% DMSO), cromakalim (10 microM), pinacidil (10 microM), or glibenclamide (10 microM) were given 10 min before ischaemia. The left coronary artery was then occluded for 15 min and reperfused for 5 min.

RESULTS

No agent caused more than a 10% change in heart rate. Both cromakalim and pinacidil increased (30%), and glibenclamide decreased (30%) coronary flow at 4 and 6 mM [K+]o. In the vehicle group, increases in [K+]o produced concentration dependent reductions in arrhythmia scores by decreasing ventricular fibrillation. No concentration dependent effects of [K+]o on ischaemic ventricular tachycardia was observed. Under ischaemic conditions, potassium channel openers and glibenclamide more markedly reduced ischaemic ventricular tachycardia and fibrillation relative to the effects of increased [K+]o.

CONCLUSIONS

Ischaemic ventricular fibrillation was inversely related to changes in [K+]o, whereas effects on ventricular tachycardia were all-or-none. Neither potassium channel openers nor glibenclamide elicited significant proarrhythmic activity despite variations in [K+]o. These data suggest that both potassium channel openers and glibenclamide display potential antiarrhythmic activity through their ability to abolish two distinct arrhythmogenic mechanisms during ischaemia. It is also suggested that the underlying mechanisms of ventricular tachycardia and fibrillation are coupled during ischaemia in the rat.

The cardioprotective and electrophysiological effects of cromakalim are attenuated by meclofenamate through a cyclooxygenase-independent mechanism

Recent published studies indicate that the cyclooxygenase inhibitor meclofenamate can abolish preconditioning. Unpublished preliminary data from this laboratory suggest that meclofenamate may be blocking cardiac ATP-sensitive potassium channels (KATP channels), which may also mediate preconditioning. The purpose of the present study was to determine whether meclofenamate is a cardiac KATP channel blocker and it can abolish the anti-ischemic activity of the KATP channel opener cromakalim. This concept was tested initially in an isolated rat heart model of 25 min of ischemia and 30 min of reperfusion. Meclofenamate, in a concentration (5 microM) that did not cause proischemic effects alone, abolished the protective effect of cromakalim, as measured by recovery of contractile function, lactate dehydrogenase release and contracture formation. The preischemic coronary dilating activity of cromakalim was not attenuated by meclofenamate. The cyclooxygenase inhibitors indomethacin and SQ 29,109 had no effect on the cardioprotection afforded by cromakalim. Concentration-response curves for the ability of cromakalim to increase time to contracture during ischemia in rat hearts were generated alone or in the presence of 5 or 10 microM meclofenamate. Cromakalim increased the time to contracture with an EC25 of approximately 3 microM. Meclofenamate appeared to block this effect in a manner that was not surmountable by 100 microM cromakalim. Studies in guinea pig hearts showed that meclofenamate had no effect on action potential duration or effective refractory period when given alone. Meclofenamate attenuated the action potential duration shortening effects of cromakalim in this model. Thus, meclofenamate blocked the cardioprotective effects of cromakalim and this effect was not related to cyclooxygenase inhibition. Meclofenamate appears to be a cardiac KATP channel blocker.

The cardioprotective effects of R 75231 and lidoflazine are not caused by adenosine A1 receptor activation

The goal of this study was to determine the cardioprotective profile for the nucleoside transport inhibitor 2-(aminocarboxyl)-N-(4-amino-2,6-dichlorophenyl)-4-[5,5-bis(4- fluorophenyl)pentyl]-1-piperazinylacetamide trihydrochloride-2,5 hydrate (R 75231) in isolated rat hearts and whether its protective effects are caused by adenosine A1 activation. R 75231 increased time to contracture during global ischemia in a concentration-dependent manner (EC25 = 2.6 microM) that was comparable to the structurally related compound lidoflazine (EC25 = 1.2 microM). R 75231 caused only modest improvements in reperfusion contractile function, whereas it profoundly reduced LDH release. The cardioprotective effects of R 75231 were accompanied by preischemic negative inotropy with modest bradycardic effects. Adenosine also increased time to contracture, although it was not very potent (EC25 > 300 microM), and this effect was accompanied by significant preischemic bradycardia without measurable negative inotropic activity. Both the preischemia bradycardia and increase in ischemic time to contracture with adenosine were abolished completely by the A1 blocker 8-cyclopentyl-1,3-dipropylxanthine. The adenosine-induced increase in time to contracture was reversed partially by glybenclamide. Neither the pre- nor postischemic effects of R 75231 were abolished by 8-cyclopentyl-1,3-dipropylxanthine or glybenclamide, except for the preischemic bradycardia. Similar results were observed for lidoflazine. Thus, the cardioprotective effects of R 75231 are not mediated by adenosine A1 receptor activation and, thus, probably are not caused by its activity as a nucleoside transport inhibitor. It may be acting as a calcium antagonist in this model.

Cardioprotective effects of the cyanoguanidine potassium channel opener P-1075

P-1075 is a cyanoguanidine ATP-sensitive potassium channel opener (KATP) that relaxes smooth muscle and shortens myocardial action potential duration (APD) at concentrations in the nanomolar range. Most KATP openers have antiischemic potencies in the micromolar range. We wished to determine if the relatively high cardiac potency of P-1075 could be translated into high antiischemic potency. Isolated rat hearts were pretreated with 10-300 nM P-1075 followed by 25-min global ischemia and 30-min reperfusion. Before ischemia, P-1075 had little effect on cardiac function, although it did increase coronary flow. During ischemia, P-1075 significantly increased time to contracture in a concentration-dependent manner (EC25 = 57 nM). P-1075 also improved recovery of contractile function significantly and reduced lactate dehydrogenase (LDH) release during reperfusion (at concentrations > or = 60 nM). Treatment with 75 nM P-1075 both before and after ischemia did not add to the protective effects observed after preischemic treatment. Treatment with P-1075 only during reperfusion was not cardioprotective. The protective effects of P-1075 were completely abolished by the KATP blocker glyburide (100 nM). In addition, P-1075 relaxed methoxamine-constricted aorta with a higher potency relative to antiischemic potency. Thus, P-1075 has cardioprotective effects similar to that of other reference KATP openers, except that P-1075 is approximately 100-fold more potent relative to most other tested KATP openers. These results demonstrate that P-1075 is the first KATP opener that protects ischemic myocardium at nanomolar concentrations.

Cardioprotective effects of a novel calcium antagonist related to the structure of cromakalim

A novel pyranoquinoline analog (BMS-188107) of the ATP-sensitive potassium channel (KATP) opener cromakalim was previously shown to be devoid of KATP opening activity in nonischemic myocardium and vascular smooth muscle, but appeared to be a relatively potent calcium antagonist. This clear differentiation between channels within a structural series is a novel finding. With the idea that KATP openers are often more active in ischemic relative to nonischemic myocardium, we determined the cardioprotective effects of this agent in isolated rat hearts and whether these anti-ischemic effects are abolished by KATP blockade. Isolated rat hearts were subjected to 25 min of global ischemia and 30 min of reperfusion and the severity of ischemic/reperfusion injury was determined. BMS-188107 was given before ischemia at 0.5 to 10 microM. Pretreatment (before ischemia) with BMS-188107 caused significant cardiodepressant activity and increased coronary flow only at a concentration of 10 microM, although modest negative inotropic effects were observed at the 0.5 and 1 microM concentrations. Significant improvements in postischemic contractile function and reductions in lactate dehydrogenase release were observed with 1 to 10 microM BMS-188107, indicating significant reductions in ischemic/reperfusion injury. Neither the pre- nor the postischemic effects of 1 to 10 microM BMS-188107 were significantly altered by the KATP blockers sodium 5-hydroxydecanoate (100 microM) or glyburide (1 microM). Previous studies did not determine the effect of BMS-188107 on sodium channels and thus, the effect of this agent on maximum upstroke velocity of the action potential was determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardioprotection in ischemic rat hearts with the SH-containing angiotensin-converting enzyme inhibitor zofenopril: possible involvement of the ATP-sensitive potassium channel

The SH-containing angiotensin-converting enzyme (ACE) inhibitors zofenopril and captopril have been shown to protect the ischemic myocardium independently of ACE inhibition. Zofenopril (30-100 microM) enhanced reperfusion contractile function and reduced lactate dehydrogenase release. The cardioprotective activity of zofenopril was stereoselective in isolated globally ischemic rat hearts (S, S,R stereoisomer of zofenopril was inactive). The role of ATP-sensitive potassium channel (KATP) activation was investigated using two structurally different KATP blockers, 1 microM glyburide and 100 microM sodium 5-hydroxydecanoate. The cardioprotective activity of 100 microM zofenopril was abolished by both KATP blockers. Cardioprotection with the SH-containing compound n-acetyl cysteine (300 microM) was also reversed by glyburide, further demonstrating that ACE inhibition is not a prerequisite. Isobolographic analysis demonstrated that cotreatment with zofenopril and the KATP opener cromakalim resulted in a super-additive response in the ischemic myocardium. KB analysis demonstrated glyburide was a noncompetitive antagonist in the presence of zofenopril and a competitive antagonist in the presence of cromakalim. Zofenopril has been reported to cause relaxation in aortic smooth muscle rings via an endothelium-dependent component. This relaxation was shifted to the right by both glyburide and sodium 5-hydroxydecanoate. Isobolographic analysis of zofenopril and cromakalim in smooth muscle also demonstrated a super-additive response. These results demonstrate for the first time a link between the cardioprotective effects of the SH-containing compounds zofenopril and n-acetyl cysteine and the KATP. The activity appears to be a receptor-mediated event which occurs in a manner different from classical KATP openers such as cromakalim.

The ATP-sensitive potassium channel blocker glibenclamide (glyburide) does not abolish preconditioning in isolated ischemic rat hearts

Previous studies have indicated that the ATP-sensitive potassium channels blocker glibenclamide (glyburide) can abolish preconditioning in canine models of myocardial ischemia. Recently, an isolated rat heart model of preconditioning has been developed that may be ideal for studying the mechanisms of preconditioning. In the present study, we determined the effect of glyburide on preconditioning in isolated rat hearts. Rat hearts were isolated and retrogradely perfused with oxygenated Krebs'-Henseleit solution. They were then subjected to four periods of total global ischemia of 5-min duration, separated by 5-min reperfusion. The hearts were then subjected to 30-min global ischemia followed by 30-min reperfusion and contractile function and lactate dehydrogenase release determined. Non-preconditioned hearts sustained severe damage. Glyburide (1-100 microM) pretreatment had no effect on the severity of 30-min global ischemia and 30-min reperfusion. Preconditioning caused significant improvements in reperfusion contractile function (25-fold increase in left ventricular developed pressure) and reductions in reperfusion lactate dehydrogenase release and reperfusion end diastolic pressure (contracture). Glyburide had modest preischemic cardiodepressant and vasoconstrictor effects at 1-30 microM, whereas 100 microM caused a 50% reduction in preischemic coronary flow. Despite these effects, none of these concentrations of glyburide affected the efficacy of preconditioning. These studies indicate that preconditioning in isolated rat hearts does not occur via a glyburide- (and thus presumably ATP-sensitive potassium channel) sensitive mechanism.

Role of endothelin-1 and big endothelin-1 in modulating coronary vascular tone, contractile function and severity of ischemia in rat hearts

The effect of endothelin-1 (ET-1) and big ET-1 on coronary flow and contractile function was determined in isolated nonischemic and ischemic rat hearts. Both ET-1 (IC50 = 12 pMol) and big ET-1 (IC50 = 2 nMol) reduced coronary flow in a concentration-dependent manner, although ET-1 was > 100-fold more potent. Both compounds decreased contractility, an effect which was lost when coronary flow was held constant, indicating that ET-1 and big ET-1 decrease contractility secondary to reducing coronary flow. Mechanical reduction in coronary flow to levels equivalent to those seen for ET-1 or big ET-1 caused similar reductions in contractility. Both 30 pMol ET-1 and 10 nMol big ET-1 pretreatment significantly reduced the time to contracture in globally ischemic rat hearts, suggesting a proischemic effect. Phosphoramidon (100 microM, endothelin-converting enzyme inhibitor) and BQ-123 (0.3 microM, ETA receptor antagonist) abolished the preischemic increase in coronary perfusion pressure induced by big ET-1 as well as its proischemic effect, whereas only BQ-123 abolished the cardiac effect of ET-1. Neither phosphoramidon nor BQ-123 had an effect on severity of ischemia when given alone. Phosphoramidon was also given i.v. to rats subjected to coronary occlusion and reperfusion and was found to significantly reduce infarct size 24 hr postischemia. Thus, in isolated rat hearts, big ET-1 appears to be converted to ET-1 and is a potent coronary constrictor.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of myocardial ATP-sensitive potassium channels in mediating preconditioning in the dog heart and their possible interaction with adenosine A1-receptors

BACKGROUND

A brief period of myocardial ischemia can result in an increased resistance to subsequent, more severe episodes of ischemia. Recent studies have indicated that activation of adenosine A1-receptors may mediate this preconditioning effect. It is also known that A1-activation can lead to ATP-sensitive potassium channel (KATP) opening via a G(i) protein-mediated effect. Thus, we determined whether the KATP blocker glyburide could abolish preconditioning or the protective effects of A1-receptor activation.

METHODS AND RESULTS

Anesthetized dogs were subjected to 5 minutes of left circumflex coronary artery (LCx) occlusion (or sham) followed by 10 minutes of reperfusion. The hearts were then subjected to 60 minutes of LCx occlusion and 5 hours of reperfusion. Glyburide (5 micrograms/kg/min) or vehicle was given directly into the LCx 20 minutes before preconditioning or sham preconditioning. Preconditioning resulted in a significantly reduced infarct size compared with nonpreconditioned animals. Glyburide abolished the protective effect of preconditioning. To establish a link between KATP and A1-receptor activation, the effect of the A1-agonist R-PIA with or without glyburide on infarct size was determined. R-PIA (0.4 microgram/kg/min, directly into the LCx) significantly reduced infarct size, and this protective effect was abolished by glyburide. None of the treatments described above had a significant effect on peripheral hemodynamic status or myocardial blood flow.

CONCLUSIONS

Preconditioning may be mediated by KATP activation, and this may be linked to A1-receptor stimulation.

Effect of calmodulin and protein kinase C inhibitors on globally ischemic rat hearts

Several calmodulin inhibitors have been reported to be cardioprotective, but the ability of these compounds to inhibit protein kinase C (PKC) suggests that calmodulin inhibition may not be the sole mechanism responsible. To distinguish between the effects, we determined the cardioprotective activity of several calmodulin inhibitors with differing PKC inhibitory potencies in isolated globally ischemic rat hearts. Twenty-five minutes of global ischemia caused significant myocardial dysfunction, contracture formation, and lactate dehydrogenase (LDH) release on reperfusion in vehicle-treated hearts. The calmodulin inhibitors trifluoperazine, W-7, calmidazolium, W-13, and CGS 9343B improved postischemic contractile function and/or reduced LDH release. They also reduced preischemic cardiac function, although cardioprotection did not appear to be correlated with cardiodepression. Calmodulin inhibitors increased preischemic coronary flow (CF) and decreased heart rate (HR), but controlling these parameters did not affect the cardioprotection. Pretreatment of ischemic hearts with trifluoperazine was associated with preservation of myocardial ATP. Pretreatment of ischemic rat hearts with the PKC inhibitors staurosporine, calphostin C, polymyxin B, and H-7 did not result in cardioprotection. Thus, calmodulin inhibition causes cardioprotection that appears to be independent of PKC inhibition.

Specific block of the anti-ischemic actions of cromakalim by sodium 5-hydroxydecanoate

The potassium channel activators cromakalim and pinacidil were recently shown to have anti-ischemic properties in isolated globally ischemic rat hearts. The effects of two reported blockers of ATP-sensitive potassium channels, glibenclamide (glyburide) and sodium 5-hydroxydecanoate, on the anti-ischemic efficacy of cromakalim were determined in this model. Buffer-perfused rat hearts were subjected to 25 minutes of ischemia followed by 30 minutes of reperfusion. Pretreatment of these hearts with 60 microM cromakalim significantly decreased indexes of contractile function but caused a significant improvement of postreperfusion function and a significant decrease in release of lactate dehydroxygenase and in end-diastolic pressure. Pretreatment with glibenclamide at concentrations that reversed the preischemic effects of cromakalim (0.05 and 1.0 microM) also significantly reversed its postischemic protective effects. Sodium 5-hydroxydecanoate (100 and 300 microM) had no effect on the preischemic (negative inotropic) effects of cromakalim but completely reversed its cardioprotective effects. Sodium 5-hydroxydecanoate did not reverse the cardioprotective effects of the calcium entry blocker diltiazem. In phenylephrine-contracted rat aorta, glibenclamide (0.1-10 microM) inhibited cromakalim-induced relaxation, whereas sodium 5-hydroxydecanoate (10-1,000 microM) had no effect. Similarly, the ability of cromakalim to shorten cardiac action potential duration in guinea pig papillary muscle and to increase outward whole-cell potassium currents in isolated myocytes was inhibited by glibenclamide, whereas sodium 5-hydroxydecanoate was without effect. Thus, both glibenclamide and sodium 5-hydroxydecanoate inhibited the effects of cromakalim after reperfusion; however, sodium 5-hydroxydecanoate, unlike glibenclamide, had no effect in nonischemic preparations. These results suggest that sodium 5-hydroxydecanoate is an ischemia-selective inhibitor of ATP-sensitive potassium channels.

Effect of potassium channel blockade on the anti-ischemic actions of mechanistically diverse agents

The ATP-sensitive potassium channel opener, cromakalim, protects ischemic hearts and its effect can be reversed by glyburide. It is presently unknown if glyburide can abolish the anti-ischemic effects of mechanistically different agents or if blockers of other potassium channels can abolish the protective effects of cromakalim. Thus, the effect of glyburide on previously reported cardioprotective agents was tested in globally ischemic/reperfused isolated rat hearts. Calcium antagonists, sodium channel blockers and calmodulin antagonists were found to significantly improve postischemic contractile function and reduce lactate-dehydrogenase release after 25 min of global ischemia and 30 min of reperfusion. Glyburide did not reverse their cardioprotective effects. 5-(N,N-dimethyl)amiloride, an inhibitor of Na+/H+ exchange, significantly reduced lactatedehydrogenase release without improving postischemic contractile function, and glyburide did not reverse this. The potassium channel opener, cromakalim, protected ischemic rat hearts (improved recovery of contractile function and reduced enzyme release) and this was abolished by glyburide. Charybdotoxin blocks both calcium-activated potassium channels and voltage-gated potassium channels and E-4031 the delayed rectifier potassium channels. Neither was found to effect the action of the potassium channel opener, cromakalim. These data indicate that glyburide is selective in that it only blocks the anti-ischemic effects of potassium channel openers and not other cardioprotective compounds. In addition, cromakalim is unaffected by blockers of other potassium channels, further indicating selectivity of glyburide for ATP-sensitive potassium channels.

Effects of different angiotensin-converting enzyme (ACE) inhibitors on ischemic isolated rat hearts: relationship between cardiac ACE inhibition and cardioprotection

We determined the relationship between cardiac angiotensin-converting enzyme (ACE) inhibition and anti-ischemic efficacy of several structurally different ACE inhibitors or their prodrug esters perfused through the isolated rat heart. Seven ACE inhibitors inhibited cardiac ACE to varying degrees due to differences in uptake during perfusion through nonischemic rat hearts. Zofenopril-sulfhydryl and fosinoprilic acid were the most effective of the free inhibitors. Among the prodrugs, zofenopril and S-benzoylcaptopril, hydrolyzed rapidly by cardiac esterase, were more effective than their component ACE-inhibitors, whereas fosinopril, ramipril and enalapril were poorly active. For studies in ischemic rat hearts, vehicle or drug treatment was initiated 10 min before a 25-min period of global ischemia and during a 30-min reperfusion period. Of five unesterified ACE inhibitors studied for anti-ischemic activity, only captopril and zofenopril-sulfhydryl were found to improve postischemic contractile function and reduce cell death in the isolated rat hearts. Fosinoprilic acid, ramiprilat and enalaprilat were not cardioprotective at high perfusion concentrations, despite the fact that nearly complete inhibition of cardiac ACE was achieved with all of the compounds studied. The S-benzoyl prodrugs of zofenopril-sulfhydryl and captopril were at least as potent as their component ACE inhibitors in reducing ischemic-reperfusion damage in the same model. Neither zofenopril nor captopril, however, had any effect on coronary flow before or after ischemia. Thus, it appears that the cardioprotective effects of zofenopril and captopril are independent of cardiac ACE inhibition or, at least, that ACE inhibition alone is not sufficient. Both captopril and zofenopril are sulfhydryl-containing compounds whereas the inactive compounds are not; and, thus, this group appears to be important in mediating their cardioprotective actions.

Cardioprotective effects of the potassium channel opener cromakalim: stereoselectivity and effects on myocardial adenine nucleotides

We determined if the cardioprotective effects of the potassium channel opener cromakalim are stereoselective and if it can preserve adenine nucleotides in ischemic myocardium. We subjected isolated isovolumically beating rat hearts to 25 min of global ischemia and reperfusion with and without pretreatment by cromakalim or its enantiomers. All of these compounds significantly increased preischemic coronary flow with the (3S,4R)-(-)-enantiomer being more potent (EC25 = 0.52 microM) compared to cromakalim (EC25 = 1.04 microM) and the (3R,4S)-(+)-enantiomer (EC25 greater than 100 microM). The (-)-enantiomer was also significantly more potent in reducing ischemic/reperfusion damage compared to cromakalim and its (+)-enantiomer. Reperfusion contractile function was improved significantly and lactate dehydrogenase release was reduced by these compounds. Time to contracture was also increased significantly by the (-)-enantiomer (EC25 = 2.27 microM), cromakalim (EC25 = 4.89 microM) and the (+)-enantiomer (EC25 greater than 100 microM). We determined if cromakalim, in a concentration which does not depress cardiac function (10 microM), can preserve high energy phosphates during ischemia in isolated rat hearts. Cromakalim significantly preserved ATP at 15 to 25 min of ischemia. Adenylate energy charge was also significantly improved by cromakalim at 20 to 25 min into an ischemic episode. Thus, the cardioprotective effects of cromakalim are stereoselective and may be due partly to preservation of myocardial energy reserves. It is significant that cromakalim can preserve adenine nucleotides despite its lack of negative inotropic effects.

Pharmacologic profile of cromakalim in the treatment of myocardial ischemia in isolated rat hearts and anesthetized dogs

The detailed antiischemic pharmacology of the potassium channel activator cromakalim was determined in isolated globally ischemic rat hearts and a canine model of coronary occlusion and reperfusion. Cromakalim significantly improved reperfusion function in rat hearts starting at a concentration of 1 microM; this effect peaked at 7 microM. No cardiodepressant effects were observed in nonischemic tissue with cromakalim until a concentration of 100 microM was achieved, and this effect was reversed by glyburide. The antiischemic effect of 7 microM cromakalim was also completely reversed by glyburide and the novel ATP-sensitive potassium channel blocker sodium 5-hydroxydecanoate (5-HD). Glyburide did not reverse the antiischemic effects of 1 microM diltiazem. Cromakalim not only improved reperfusion contractile function in rat hearts, but improved the functional reserve and efficiency of O2 utilization. In anesthetized dogs, intracoronary cromakalim (0.1 micrograms/kg/min given throughout ischemia and reperfusion) significantly reduced infarct size in hearts subjected to 90-min coronary occlusion and 5-h reperfusion. Along with this reduced infarct size, the frequency of ectopic beats and the proportion of animals fibrillating during reperfusion were significantly reduced by cromakalim. In isolated globally ischemic and reperfused rat hearts, cromakalim was significantly profibrillatory. Thus, cromakalim is significantly cardioprotective, and may have the propensity for profibrillatory activity, although this is not true under all conditions.

Reduction of ischemic damage in isolated rat hearts by the potassium channel opener, RP 52891

Potassium channel activators have been shown to protect ischemic myocardium. We studied the ability of the novel potassium channel activator, RP 52891, to also reduce ischemic damage in isolated globally ischemic rat hearts. RP 52891 (1-100 microM) was given before the hearts were subjected to 25 min of ischemia and 30 min of reperfusion. Before ischemia, RP 52891 reduced contractile function only at the highest concentration (100 microM). Significant reductions in ischemic damage were observed at 3 microM and higher concentrations. RP 52891 improved reperfusion contractile function and reduced lactate dehydrogenase release. Contracture was significantly reduced by RP 52891 during reperfusion. The protective effects of RP 52891 were completely reversed by glyburide and sodium 5-hydroxydecanoate, both blockers of ATP-sensitive potassium channels. Thus, RP 52891 has direct cardioprotective efficacy, which may be related to activation of ATP-sensitive potassium channels.

Ruthenium red improves postischemic contractile function in isolated rat hearts

The effect of ruthenium red (inhibitor of mitochondrial calcium entry) on reperfusion contractile function and enzyme release was determined in isolated perfused rat hearts and compared with that of diltiazem. The hearts were made ischemic for 25 min and reperfused for 30 min. They were pretreated with 1-10 microM ruthenium red, 1 microM diltiazem, or vehicle. All concentrations of ruthenium red significantly improved reperfusion contractile function without affecting lactate dehydrogenase (LDH) release or contracture. Diltiazem significantly improved reperfusion function and reduced LDH release and contracture formation. Ruthenium red still improved function even when given only during reperfusion. Diltiazem increased reperfusion oxygen consumption and efficiency of oxygen utilization whereas ruthenium red improved efficiency without an increase in oxygen consumption. Diltiazem significantly increased reperfusion functional reserve in these hearts, although ruthenium red did not. Ruthenium red reduced coronary flow and contractile function in nonischemic myocardial tissue and the reduced function appeared to be secondary to the reduced coronary flow as well as to a direct negative inotropic effect. Thus, ruthenium red improved reperfusion contractile function and oxygen efficiency; this may be related to its ability to block mitochondrial calcium uptake.

Myosin light-chain phosphorylation and vascular resistance in canine anterior tibial arteries in situ

Two regulatory systems are believed to control the contractile state of vascular smooth muscle in vitro. In one, crossbridges are phosphorylated by myosin light-chain (MLC) kinase; these phosphorylated crossbridges are believed to predominate during force development. In the other system, the crossbridges are unphosphorylated (latchbridges); the latchbridges are thought to prevail during force maintenance. The role of these systems in the control of vascular resistance in vivo is unknown. This study compared MLC phosphorylation with vascular tone in canine anterior tibial arteries in vitro and in situ. The arteries were frozen at various times before and during stimulation with norepinephrine. Under both conditions, the levels of MLC phosphorylation were low at rest and increased transiently during norepinephrine stimulation. This increase was associated with stress development in vitro and with an increase in vascular resistance in situ. Thus, the biochemical changes measured in arteries in situ parallel those measured in vitro suggesting that the in vitro models are appropriate and useful for studying the physiological function of blood vessels. To our knowledge, this is the first demonstration of changing levels of MLC phosphorylation in an artery in situ.

Anti-ischemic activity of the novel benzazepine calcium antagonist SQ 31,486

We tested the benzazepine, SQ 31,486 for its ability to selectively block the voltage-dependent calcium channel and to protect the ischemic myocardium. SQ 31,486 was found to be a selective calcium antagonist in vascular tissue with an IC50 value of 1.5 microM in KCl-contracted rabbit aorta. SQ 31,486 decreased contractile function and increased coronary flow in nonischemic isolated rat hearts in a concentration-dependent manner. SQ 31,486 also significantly reduced postischemic lactate dehydrogenase (LDH) release and end-diastolic pressure (EDP) compared to vehicle. Reperfusion double product [heart rate (HR) x left ventricular developed pressure (LVDP)] was also significantly improved by SQ 31,486. Diltiazem was a less potent anti-ischemic agent and was significantly more cardiodepressant relative to its anti-ischemic efficacy than was SQ 31,486. Thus, SQ 31,486 should have a larger therapeutic index. In a model of pacing-induced myocardial ischemia in anesthetized, open chest dogs, SQ 31,486 reduced pacing-induced ST-segment elevation approximately 50% at 10, 40, and 70 min after drug administration. This protective effect occurred despite a lack of effect of SQ 31,486 on ischemic regional blood flow and peripheral hemodynamic status.

Effect of thromboxane receptor blockade on oxygen supply/consumption variables during reperfusion in the anesthetized dog

The thromboxane A2/PGH2 receptor antagonist SQ 30,741 has been previously shown to reduce infarct size and to improve subendocardial reflow. The purpose of this study was to determine the effect of SQ 30,741 on reperfusion O2 supply/consumption variables. Anesthetized open-chest dogs treated with vehicle or 1 mg/kg + 1 mg/kg/hr SQ 30,741, i.v. (starting 10 min after the onset of ischemia) were subjected to 90 min left circumflex coronary occlusion and 3 hr reperfusion. Regional myocardial blood flow (radioactive microspheres) and arterial and venous O2 saturations (microspectrophotometry) were determined. Animals treated with SQ 30,741 had significantly higher subendocardial reflow at 3 hr (48 +/- 6 ml/min/100 g) compared with vehicle (27 +/- 10 ml/min/100 g). At 3 hr postreperfusion, O2 extraction was significantly higher in the reperfused region compared with the nonischemic region, although extraction was not at maximal values. O2 extraction was similar in vehicle- and SQ 30,741-treated animals despite the near doubling of reflow into the subendocardial region with SQ 30,741. O2 consumption was significantly reduced in the reperfused subendocardial region (1.65 +/- 0.9 ml O2/min/100 g) in vehicle controls compared with the nonischemic subendocardial region (10.2 +/- 2.6 ml O2/min/100 g). SQ 30,741 significantly improved subendocardial reperfused regional O2 consumption (4.03 +/- 0.41 ml O2/min/100 g) compared with vehicle and this increase was proportional to the flow increment (no change in the O2 supply/consumption ratio). SQ 30,741 is thus increasing subendocardial reflow secondary to an increase in O2 consumption and the increased O2 consumption may be due to preservation of myocardial tissue.

Nicorandil improves postischemic contractile function independently of direct myocardial effects

We determined whether any of the antiischemic effects of nicorandil were due to direct cardioprotective effects such as potassium channel activation or to its peripheral hemodynamic effects. Nicorandil was administered either intravenously (i.v.) or directly into the ischemic coronary artery (i.c.) and compared with i.c. cromakalim (a potassium channel activator previously shown to improve reperfusion function directly in rat hearts) or vehicle for their ability to improve postischemic contractile function as measured by ultrasonic crystals in anesthetized dogs or in isolated perfused rat hearts. In a model of 25-min global ischemia and reperfusion in isolated perfused rat hearts, nicorandil (10-100 microM) did not improve reperfusion function or decrease LDH release, although 300 microM nicorandil did protect the hearts. Cromakalim (7 microM) significantly improved reperfusion function and reduced lactate dehydrogenase (LDH) release. In the dog studies, the left anterior descending coronary artery (LAD) was occluded for 15 min and was reperfused for 3 h. Nicorandil improved reperfusion function only when administered i.v., although i.c. cromakalim was efficacious in improving function. Neither nicorandil nor cromakalim improved collateral flow, although cromakalim significantly improved preischemic and reperfusion blood flows, particularly in the subepicardial region. Although i.c. treatment with cromakalim and nicorandil did not result in significant changes in peripheral hemodynamic status, i.v. nicorandil reduced both preload and afterload. Thus, at the dose used, nicorandil does not appear to have direct myocardial protective effects and the beneficial effects of nicorandil do not appear to be related to potassium channel activation in the myocardium. Potassium channel activation by cromakalim does result in direct cardioprotective effects whereas nicorandil appears to be dependent on peripheral actions for its efficacy.

The protective effects of cromakalim and pinacidil on reperfusion function and infarct size in isolated perfused rat hearts and anesthetized dogs

The direct myocardial protective effects of intracoronary infusions of cromakalim and pinacidil were determined in an anesthetized canine model of coronary occlusion and reperfusion. The left circumflex coronary artery was occluded for 90 minutes and reperfused for 5 hours, at which time the infarct size was determined. Cromakalim (0.1 micrograms/kg/min) or pinacidil (0.09 micrograms/kg/min) were infused into the left circumflex coronary artery starting 10 minutes preischemia. Cromakalim significantly reduced infarct size as a percent of the left ventricular area at risk (25 +/- 5%) compared with vehicle controls (55 +/- 7%). Pinacidil did not reduce infarct size at an equimolar dose, but at the higher dose also significantly reduced infarct size. Collateral blood flow was not significantly altered by either drug, though reperfusion flow was significantly higher in cromakalim-treated animals, particularly in the subepicardial region. When the same dose of cromakalim was given starting 2 minutes before the initiation of reperfusion, no significant beneficial effect of cromakalim was observed. In another study, isolated buffer-perfused rat hearts were subjected to 25 minutes of global ischemia and 30 minutes of reperfusion. These hearts were treated with 7 microM cromakalim, either starting 10 minutes before ischemia or only during reperfusion, and its effect on reperfusion function and LDH release were determined. Cromakalim pretreatment (both when given throughout the experiment and when not present in the reperfusion buffer) resulted in significant improvements in the reperfusion function. Reperfusion contracture and LDH were also significantly reduced with this treatment. When given only during reperfusion, cromakalim did not reduce the severity of ischemia when compared with vehicle controls. Thus, both cromakalim and pinacidil reduce ischemic/reperfusion injury, though the timing of treatment may be important.

Comparative anti-ischemic effects of dihydropyridine calcium antagonists in isolated perfused rat hearts: relationship of cardiodepression and cardioprotection

A comparison was made of the anti-ischemic effects of dihydropyridine calcium antagonists in isolated globally ischemic rat hearts. Pretreatment with amlodipine, nifedipine, nitredipine, or nisoldipine reduced reperfusion enzyme (lactate dehydrogenase) release and contracture after 25 min of global ischemia and 30 min of reperfusion. Increasing concentrations of all compounds resulted in proportionally smaller reductions in the severity of ischemia, with larger decreases in nonischemic tissue contractility occurring. Reperfusion function was significantly improved at 30 min with nifedipine only; however, at 60 min reperfusion function was significantly improved for all except nisoldipine. Washout data from nonischemic hearts (rate of disappearance of cardiodepressant effects) showed that the dihydropyridines washed out in the following order (fastest to slowest): nifedipine greater than nitrendipine greater than nisoldipine greater than amlodipine. Thus, these dihydropyridines are anti-ischemic, though at higher concentrations cardiodepressant effects increase disproportionately. Differences in washout also effect the ability of these compounds to improve reperfusion function.

Anti-ischemic and vasorelaxant effects of the new benzazepine calcium channel blocker SQ 31,765

The anti-ischemic effects of SQ 31,765 or its relatively inactive enantiomer SQ 32,189 (to test for effects not related to calcium channel blockade) were determined in a model of stable angina. Anesthetized dogs were given saline (n = 6), SQ 31,765 (n = 6; 0.2 mg/kg) or SQ 32,189 (n = 6; 0.2 mg/kg) i.v. 10 min before ischemia. The effect on pacing-induced ST-segment elevation (pacing + left anterior descending coronary artery stenosis) and myocardial blood flow were determined. SQ 31,765 reduced ST-elevation (P less than .05) compared to saline at 10, 40 and 70 min after infusion (5.9 +/- 1.4 and 12.0 +/- 1.4 mV, respectively, at 70 min). SQ 32,189, which is a 10-fold less potent calcium channel blocker in coronary arteries, did not affect ST-elevation (11.8 +/- 2.1 mV). Left anterior descending coronary artery stenosis during atrial pacing resulted in a significant reduction in subendocardial flow in all groups before drug infusion (41 +/- 7, 44 +/- 7 and 35 +/- 9 ml/min/100 g for saline, SQ 31,765 and SQ 32,189, respectively) and treatment with SQ 31,765 did not affect this flow. SQ 32,189 further reduced subendocardial blood flow such that it was significantly lower compared to saline. To test for any partial agonist activity, Bay k 8644 and SQ 32,189 were tested as constrictors of porcine coronary strips in vitro. Bay k 8644 induced appreciable force whereas SQ 32,189 did not. However, the vasorelaxant potency of SQ 32,189 was decreased under hypoxic conditions. Thus, SQ 31,765 can reduce the severity of ischemia in a manner which is independent of changes in myocardial blood flow or hemodynamic alterations.

Anti-ischemic effects of the potassium channel activators pinacidil and cromakalim and the reversal of these effects with the potassium channel blocker glyburide

The direct cardioprotective efficacy of the potassium channel activators pinacidil and cromakalim was determined in isolated globally ischemic rat hearts. Isolated buffer-perfused rat hearts were subjected to 25 min of ischemia followed by 30 min of reperfusion. These hearts were pretreated with 1 to 100 microM pinacidil, 1 to 7 microM cromakalim or vehicle. Pinacidil resulted in significant improvements in reperfusion function and cardiac compliance, though it did not significantly reduce lactate dehydrogenase release at any concentration. The protective effects of pinacidil were greatest at a 10 microns concentration and were slightly diminished at higher concentrations (30 and 100 microns). Although not affecting the severity of ischemia alone, 10 microM glyburide (potassium channel blocker) completely reversed the protective effects of pinacidil on reperfusion function and compliance. Cromakalim (7 microM) resulted in a greater than 50% improvement in reperfusion function and compliance and unlike pinacidil significantly reduced lactate dehydrogenase release by approximately 50%. At 1 microM, glyburide alone did not significantly affect the severity of ischemia but reversed the protective effects of cromakalim. Not only did glyburide reverse the protective effects of cromakalim, it resulted in a worsening of ischemia compared to vehicle, an effect not seen with glyburide alone. Thus, both pinacidil and cromakalim appear to have direct cardioprotective efficacy, though some differences between them may be possible. The mechanism of their protective effects appears to be via potassium channel opening as the potassium channel blocker glyburide reverses the protective effect of these compounds. Intracellular electrophysiological studies showed that ischemia-induced depolarization was reversed with cromakalim, which increased the resting potential nearly back to preischemic levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation of cardiodepression from cardioprotection with calcium antagonists: diltiazem protects ischemic rat myocardium with a lower functional cost as compared with verapamil or nifedipine

This study was performed to determine if cardiodepression can be dissociated from cardioprotection with calcium antagonists and which one (diltiazem, nifedipine, or verapamil) can maximally protect ischemic myocardium at a given level of cardiodepression. Isolated rat hearts were subjected to 0.1, 0.5, or 1.0 microM diltiazem, verapamil, or nifedipine 10 min before global ischemia. Ischemia was maintained for 25 min, at which time reperfusion was instituted for 30 min. Pre- and postischemia function, flow, and lactate dehydrogenase (LDH) release were measured. All three drugs reduced preischemic function and improved postischemic function and reflow in a dose-dependent fashion. LDH release and contracture were also mitigated with all drugs. When the efficacy of these drugs was expressed as the ratio of LDH release versus preischemic, postdrug function (ability of drug to reduce LDH release at a given level of cardiodepression), diltiazem had a significantly lower ratio as compared with verapamil or nifedipine. When similar experiments were performed with various concentrations of calcium in the perfusion buffer (2.50, 1.25, 0.75, 0.50, 0.41 mM CaCl2) administered 10 min before ischemia and reperfusion with normal (1.25 mM) buffer, preischemic function was reduced in a concentration-dependent fashion. Despite severe reductions in function at the concentration of 0.50 mM CaCl2, LDH release was not reduced. The concentration of 0.41 mM CaCl2, which depressed function to the same degree as 0.50 mM CaCl2, reduced LDH release. This reduction in LDH release, however, was not as great as that which occurred with the high dose of the calcium antagonists. Reperfusion with 0.41 mM calcium buffer, however, nearly abolished LDH release. Thus, although all three calcium antagonists reduced the severity of ischemia, diltiazem reduces it with the lowest cost in cardiac function. Reduction in extracellular calcium reduces cardiac function, but reductions in severity of ischemia, as measured by LDH release, do not parallel these changes.

The role of thromboxane A2 in reperfusion injury

Thromboxane A2 (TXA2) receptor antagonists can limit infarct size in models of coronary occlusion and reperfusion, but it was unknown if these compounds can mitigate reperfusion injury. Anesthetized open chest dogs were subjected to left circumflex coronary (LCX) occlusion for 90 min. Two minutes before reperfusion, the dogs were given iv saline (0.9% NaCl) or the TXA2 antagonist SQ 29,548 (0.2 mg/kg + 0.2 mg/kg/hr). Reperfusion was instituted for 5 hr at which time infarct size was determined. Regional myocardial blood flow was determined before, during, and after occlusion. SQ 29,548 treatment resulted in a significant reduction in infarct size (57 +/- 7 and 34 +/- 8% of the left ventricular area at risk infarcted in the saline and SQ 29,548 groups, respectively). No differences in collateral flow during occlusion were observed between groups, but SQ 29,548 treatment resulted in a significantly higher subendocardial reperfusion flow (54 +/- 10 and 93 +/- 14 ml/min/100g for the saline and SQ 29,548 groups, respectively). Thus, TXA2 seems to play a role in exacerbating reperfusion injury and TXA2 receptor blockade may have potential as a mode of therapy for ischemia-reperfusion damage.

Effect of diltiazem on infarct size, reperfusion flow and flow reserve: the effect of timing of treatment

This study was performed to determine if improved subendocardial reflow seen with diltiazem pretreatment after left circumflex coronary (LCX) occlusion is due to a direct vasodilatory effect of diltiazem on the reperfused bed or due to an increased flow maximum or reserve. In the first part of this study anesthetized dogs were subjected to saline or diltiazem (infused starting before, 10 min after LCX occlusion or 2 min before reperfusion; 0.18 mg/kg + 0.45 mg/kg/hr i.v. for all groups) treatment with a 90-min LCX occlusion and 5-hr reperfusion and myocardial blood flow and infarct size were determined at the end of the experiment. In the second part, maximal flow using intracoronary adenosine was determined at 1 and 3 hr postreperfusion in the ischemic bed when pretreated with saline or diltiazem. Myocardial infarct size was reduced significantly only in animals pretreated with diltiazem compared to saline-treated animals. At 1-hr postreperfusion, subendocardial flow (microspheres) was significantly higher only with diltiazem pretreatment compared to the saline group (100 +/- 17 vs. 54 +/- 8 ml/min/100 g, respectively) and subendocardial reperfusion flows were negatively correlated to infarct size (r = 0.97, P less than .05). Thus, diltiazem only improves reflow and infarct size when infused before occlusion and this improved reflow does not occur via a direct vasodilator action of diltiazem. When maximal vasodilating doses of adenosine were given, flow in the ischemic region was nearly identical for saline and diltiazem pretreated groups despite higher preadenosine flows in the diltiazem group (higher resting flow occurred at the expense of the existing flow reserve).(ABSTRACT TRUNCATED AT 250 WORDS)