Nifedipine reduces adenine nucleotide breakdown in ischemic rat heart

On the mechanism of action of calcium antagonists

A short review is given of possible mechanisms of action of the organic "calcium antagonists". Calcium antagonists comprise a chemically heterogenous group of drugs, and the term may be used to denote agents that inhibit Ca2+-dependent processes or regulatory mechanisms without acting at other sites. Such drugs may be subdivided into those that decrease the availability of Ca2+ to the myoplasm, and those that decrease the cellular effects of Ca2+ without lowering the intracellular Ca2+ concentration. Accordingly, calcium channel blockers, such as verapamil, nifedipine, and diltiazem, form a subgroup of calcium availability inhibitors, as they block influx of extracellular calcium through ion selective channels in the membrane both in cardiac and smooth muscle. However, it cannot be excluded that some of these drugs, particularly in smooth muscle, may have additional sites of action, which must be taken into consideration when they are used as investigational tools.

Relation between energy metabolism, glycolysis, noradrenaline release and duration of ischemia

We studied the effect of 12-36 min of global ischemia followed by 36 min of reperfusion in Langendorff perfused rabbit hearts (n = 26). Metabolism was determined in terms of peak and total release of purines (adenosine, inosine, hypoxanthine), lactate and noradrenaline during reperfusion; and myocardial content of nucleotides (ATP, ADP, AMP), glycogen and noradrenaline at the end of reperfusion. An inverse relationship (r = -0.79) existed between duration of ischemia and developed pressure post-ischemia. Early during reperfusion, after 12 min of ischemia, the purine concentration (peak release) increased 100x (p < 0.01), that of lactate and noradrenaline 10x (p < 0.05). Total purine release rose with progression of the ischemic period (30x after 36 min of ischemia; p < 0.01), concomitant with a reduction in nucleotide content. Lactate release was independent from the duration of ischemia, although glycogen had declined by 30% (p < 0.01) after 36 min of ischemia. The acid insoluble glycogen fraction, which presumably contains proglycogen, increased substantially during short-term ischemia. Peak noradrenaline increased 100x, and 200x, (p < 0.05) after 24 and 36 min of ischemia, respectively. Total noradrenaline release due to various periods of ischemia mirrored its peak release. Function recovery was inversely related to total purine and noradrenaline efflux (both r = -0.81); it correlated with tissue nucleotide content (r = 0.84). In conclusion, larger amounts of noradrenaline are released only after a substantial drop in myocardial ATP. During severe ischemia ATP consumption more than limited ATP production by anaerobic glycolysis, is a key factor affecting recovery on subsequent reperfusion. In contrast to lactate efflux, purine and noradrenaline release are useful markers of ischemic and reperfusion damage.

Effect of antiarrhythmics on the release of adenosine in rat hearts with coronary occlusion and reperfusion

In isolated perfused rat hearts the left coronary artery was occluded for 5 min, with subsequent reperfusion for 20 min. During the reperfusion severe tachyarrhythmias were observed, with ventricular fibrillation occurring in all hearts. Simultaneously, large amounts of adenosine and its degradation products inosine, hypoxanthine, xanthine and uric acid were released into the coronary perfusate. The antiarrhythmics quinidine, lidocaine and gallopamil significantly decreased the release. The effect of quinidine and lidocaine was linked with the antifibrillatory action of these drugs. Also the interruption of fibrillation immediately after its appearance by potassium chloride decreased the release of adenosine and its metabolites in a highly significant way. The effect of gallopamil on the release was independent of an antifibrillatory action. The findings indicate that different kinds of antiarrhythmic drugs can affect the release of nucleosides and oxypurines in hearts with ischaemia and reperfusion.

Myocardial stunning--are calcium antagonists useful?

Considerable data support the point of view that calcium antagonists, whether given before the onset of ischemia or exactly at the time of reperfusion, ameliorate stunning. Benefit after the onset of reperfusion is much more controversial. It is proposed that the mechanisms whereby calcium antagonists act vary between these situations. When given before or at the onset of ischemia, then an antiischemic effect is likely. According to the hypothesis that the severity of ischemic damage determines the severity of reperfusion damage, the calcium antagonists indirectly lessen reperfusion damage. When given exactly at the time of reperfusion, the proposal is that the calcium antagonists are specifically limiting the entry of calcium ions via the calcium channel and thereby diminishing pathogenic cytosolic calcium oscillations. The reported benefit of calcium antagonists when given postreperfusion to the heart in situ, in the presence of established stunning, is of unknown mechanism and controversial significance. The hypothesis of a two-stage model of stunning with calcium as a pathogen is in accord with most of the available evidence.

How do calcium antagonists differ in clinical practice?

The majority of calcium antagonists used clinically belong to three distinct chemical classes: the phenylalkylamines, the dihydropyridines, and the benzothiazepines. In recent years their mode of action has been unravelled, their limitations recognized, and their efficacy and use in the management of patients with a broad spectrum of cardiovascular and other disorders defined. It is clear, however, that these drugs are not all alike, providing an explanation for their differing effects. The final therapeutic effect in humans depends on the mechanisms of action at the molecular level, the tissue selectivity, and the hemodynamic changes of each agent. All these aspects are examined in detail in this article. Concepts that are highlighted are as follows: (a) Molecular biology has allowed recognition of the polypeptide components of the alpha 1 subunit of the L-type Ca2+ channel and the finding of peptide segments covalently labelled by all three classes of drugs. (b) The location of these segments within the peptides is different: Binding sites for dihydropyridines are located externally, whereas those for verapamil and diltiazem are located internally, in the cytosolic part of the membrane. (c) Dihydropyridine binding is voltage dependent. This explains the selectivity of this class of drugs for vascular smooth muscle, which is more depolarized than cardiac muscle. (d) Phenylalkylamines and benzothiazepines reach their receptors at the internal surface of the sarcolemma through the channel lumen. Their binding is facilitated by the repetitive depolarization of atrioventricular and cardiac tissue, a phenomenon described as use dependence. This explains why these drugs are not highly selective, but equipotent for the myocardium, the atrioventricular conducting tissue, and the vasculature. (e) Dihydropyridines act through selective vasodilatation and may increase heart rate and contractility via a reflex mechanism. On the contrary, phenylalkylamines and diltiazem act through a combination of effects, including reduction of afterload, heart rate, and contractility. When taken together, all these differences distinguish the preferential clinical utilization of one of these compounds for a given cardiovascular pathology.

Cardioprotective effect of intracoronary nifedipine during percutaneous transluminal coronary angioplasty. A French double-blind cross-over multicentre study

The aim of this double-blind, placebo-controlled, cross-over study was to assess the cardioprotective effect of intracoronary nifedipine during percutaneous transluminal coronary angioplasty balloon occlusion. A balloon inflation without drug injection was initially made to ascertain that a shift of the ST segment (> or = 2 mm, 0.08 s after the J point) appeared (inclusion criterion). Two other balloon inflations were preceded by intracoronary injection of either 0.2 mg nifedipine or placebo, distal to the stenosis through the balloon catheter. The evaluation criteria were (1) time to ST segment shift, and (2) maximal amplitude of ST segment shift caused by balloon occlusion. Comparison of the data used an analysis of variance. Sixty-seven patients (mean age 54 +/- 8 years; 54 male, 13 female) were studied; 50 patients had 1-, 16 patients 2- and 1 patient 3-vessel disease. The dilated vessel was the left anterior descending coronary artery (n = 51), the right coronary artery (n = 12) and the left circumflex coronary artery (n = 4). Balloon inflation time was 100 +/- 31 s in the nifedipine group and 93 +/- 29 s in the placebo group. Five patients were excluded (procedure stopped after the first inflation in 1 and ST segment shift < 2 mm during the first inflation in 4). The time to 2-mm ST segment shift was longer in the nifedipine group than in the placebo group (62 +/- 40 s versus 51 +/- 40 s, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Protection of human, rat, and guinea-pig atrial muscle by mioflazine, lidoflazine, and verapamil against the destructive effects of high concentrations of Ca2+

In right atrial trabeculae from humans and in left atria from rat and guinea-pig hearts, the protective effects of mioflazine, lidoflazine, and verapamil against the accumulation of cellular calcium were investigated. Two consecutive, cumulative increases in the extracellular calcium concentration, [Ca2+]o (1-25 mmol/l), were induced, in between which the muscles were exposed for at least 30 minutes to solvent or drug. When using solvent in the 30-minute interval, the force of contraction was much lower during the second Ca2+ challenge, while the aftercontractions and the increase in passive tension at high [Ca2+]o tended to be larger. These signs of functional impairment were prevented by exposure to mioflazine or lidoflazine (3 mumol/l each) but not to verapamil (3 mumol/l). Muscles were fixed with glutaraldehyde at the end of the second Ca2+ challenge for morphological and cytochemical examination. After solvent treatment, more than half of the cells were severely damaged, showing cellular edema, contraction-band necrosis, mitochondrial swelling, and nuclear pyknosis; the sarcolemma was devoid of calcium deposits, damaged mitochondria contained either large deposits of calcium or flocculent densities, and in some cells, the cytoplasm was filled with calcium deposits. Following exposure to mioflazine and lidoflazine, but not to verapamil, the number of intact cells after the second Ca2+ challenge was not different from time-matched controls (80-90%). Furthermore, the shifts in cellular calcium distribution were prevented with mioflazine and lidoflazine, whereas verapamil was less effective. There were no species differences with respect to either morphological or contractile changes. In conclusion, exposure of atria to high [Ca2+]o induced similar ultrastructural and cytochemical changes as seen after ischemia-reperfusion induced damage. Indeed, under the mentioned conditions the sarcolemma lost its capacity to exclude Ca2+ after a challenge with high [Ca2+]o and allowed excessive Ca2+ entry. The pathway for this extra Ca2+ remains to be elucidated. L-type calcium channels are probably not involved, since verapamil cannot prevent the Ca2+ overload.

Effects of MCI-176, a new quinazolinone calcium antagonist, on myocardial energy and carbohydrate metabolism in ischemic dog hearts

The effect of 2-(2,5-dimethoxyphenylmethyl)-3-(2-dimethylaminoethyl)- 6-isopropoxy-4(3H)-quinazolinone hydrochloride (MCI-176), a calcium antagonist, on ischemic myocardial metabolism was studied in dog hearts subjected to an occlusion of the left anterior descending coronary artery (LAD) for 3 or 30 min. MCI-176 (0.03 or 0.1 mg/kg), when injected i.v. 5 min before occlusion, increased coronary blood flow and decreased systemic aortic pressure. When the LAD was ligated, the levels of creatine phosphate, ATP, total adenine nucleotides and energy change potential decreased in the ischemic myocardium. Three minutes after ischemia, MCI-176 (0.1 mg/kg) significantly (P less than 0.05) diminished these impairments of energy metabolism. Even 30 min after ischemia, pretreatment with MCI-176 tended to lessen the depletion of ATP and total adenine nucleotides, although these effects were not statistically significant. Myocardial ischemia produced a breakdown of glycogen, an accumulation of lactate, and an inhibition of glycolytic flux through phosphofructokinase reaction. MCI-176 (0.1 mg/kg) significantly (P less than 0.05) reduced these alterations of carbohydrate metabolism after 3 min of ischemia. These results suggest that pretreatment with MCI-176 reduces the impairments of myocardial energy and carbohydrate metabolism in ischemic dog hearts, suggesting that the drug is capable of improving the imbalance between oxygen supply and oxygen demand in the ischemic myocardium.

Comparison of dihydropyridine and phenylalkylamine calcium antagonists in patients with coronary heart disease

To evaluate possible differences between dihydropyridine and phenylalkylamine calcium antagonists in the setting of chronic stable angina, 2 placebo-controlled, double-blind, crossover trials were conducted comparing the effects of gallopamil and nifedipine on exercise tolerance and ischaemic ST depression, using standard as well as slow release formulations. In the first study, 30 patients received standard formulations of gallopamil (50mg 3 times daily) and nifedipine (20mg 3 times daily). This trial was stopped after 9 patients had been enrolled, because of severe exacerbation of angina in 3 nifedipine recipients. 21 patients then entered a second protocol in which the nifedipine dose was reduced to 10mg 3 times daily. Compared with the preceding placebo periods, time to angina onset and total exercise time were statistically significantly (p less than 0.01) prolonged by gallopamil (by 30 and 18%, respectively), and nonsignificantly prolonged by nifedipine (by 20 and 13%, respectively), after 4 weeks' treatment. Increases in heart rate and rate-pressure product at maximal comparable workloads were less with gallopamil than with nifedipine (p less than 0.01). In contrast to nifedipine, gallopamil was associated with very few side effects. The second trial comprised 24 patients who received slow release formulations of gallopamil (100mg twice daily) and nifedipine (20mg twice daily) over 2 weeks. Again, both drugs exhibited significant anti-ischaemic efficacy, as evidenced by reductions in ST depression at maximal comparable workloads and increases in exercise time compared with placebo, but the differences between the treatments were not statistically significant. Side effects were more frequent with nifedipine, but less severe than with the standard formulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Post-ischemic release of nucleosides and oxypurines in isolated rat hearts. Possible involvement of ventricular fibrillation

In isolated perfused rat hearts global ischemia for 2, 5, and 15 min was produced. Depending on the duration of the ischemia, postischemic reperfusion led to the release of adenosine and its catabolites, and to more or less severe ventricular tachyarrhythmias. When ventricular fibrillation occurred, a highly significant increase in the purine release was observed compared with non-fibrillating hearts. Prevention of fibrillation by antiarrhythmic drugs decreased the purine release in a highly significant way. After only 2 min of ischemia, reperfusion did not lead to ventricular fibrillation. Electrical induction of fibrillation during the reperfusion in these hearts provoked the release of very high amounts of the purine compounds. A similar effect of electrically-induced fibrillation was also obtained in hearts without a previous ischemic period. The findings suggest that ventricular fibrillation is able to induce the release of purine derivatives from the heart.

Nifedipine and experimental cardioprotection

Experimental studies using animal models designed to mimic the effect of ischemia and postischemic reperfusion have provided data indicating that the calcium antagonists might be cardioprotective. The laboratory studies have indicated consistently, however, that the timing of such drug administration is of critical importance. In the case of nifedipine (a dihydropyridine-based calcium antagonist), the laboratory studies have shown that when used prophylactically it has a protective effect during short (as in the "stunned heart") and long episodes (greater than 30 minutes) of ischemia. This protection has been quantitated in a variety of ways, including preservation of left ventricular function. A complete understanding of whether and how calcium antagonists can be used to protect the myocardium requires further detailed knowledge of not only of the voltage-activated calcium channel and its binding sites, but also of the sequence of events triggered by ischemia and reperfusion. Recent data from our laboratories indicate that cardiac membranes contain specific binding sites for the novel, endothelial-derived vasoconstrictor peptide, ET, and that the density of these sites increases during ischemia. ET promotes Ca2+ influx through the voltage-sensitive channels by a mechanism that does not involve a direct interaction with the dihydropyridine (DHP)-sensitive binding sites. Nevertheless, the ET-induced Ca2+ influx is attenuated by the dihydropyridine-based calcium channel blockers.

Effects of nifedipine on myocardial perfusion during exercise in chronic stable angina pectoris

Nifedipine may be effective in the treatment of stable angina by both decreasing myocardial oxygen demand and increasing myocardial oxygen supply. To determine the mechanism of action of nifedipine and its dose-response relation, 14 patients with stable angina were treated with nifedipine 10, 20 and 30 mg 4 times daily as single-agent therapy in a double-blind, randomized, placebo-controlled crossover trial. Treatment was continued for 1 week on each dose regimen and efficacy was determined using an exercise test at the end of each phase. Compared to placebo, a significant decrease of systolic blood pressure at peak exercise occurred with the nifedipine 20- and 30-mg regimens (p less than 0.05), accompanied by an increase in heart rate on the 10- and 20-mg regimens (p less than 0.005). There was no significant effect on the rate-pressure product compared to placebo at any exercise time on any of the nifedipine regimens. The times to onset of ST-segment depression and to angina were delayed significantly by all 3 dose regimens compared to placebo (p less than 0.02). There was a significant decrease in the magnitude of ST-segment depression at all exercise times by all dosage schedules of nifedipine compared with placebo (p less than 0.05), although there were no significant differences among the 3 dosage schedules. Data indicate that since nifedipine was effective in improving manifestations of myocardial ischemia during exercise without altering the double product at submaximal or maximal exercise, its beneficial mechanism of action may have been due to enhancing blood flow to ischemic zones or to favorably altering determinants of myocardial oxygen demand, which were not measured.

Effects of diltiazem on phosphate metabolism in ischemic and reperfused myocardium using phosphorus31 nuclear magnetic resonance spectroscopy in vivo

Diltiazem may provide a protective effect to ischemic and reperfused myocardium through preservation of high-energy phosphate metabolism. To test this hypothesis, rabbits had a 1.3 cm solenoidal coil placed over the myocardium to be rendered ischemic. Data were acquired with a 22 cm bore nuclear magnetic resonance spectrometer at 2.0 T. Animals were treated with diltiazem (200 micrograms/kg intravenous bolus of drug followed by a 15 micrograms/kg/min continuous intravenous infusion, n = 10) or by an equal volume of saline (n = 6). The left circumflex artery was occluded and reperfused using a reversible snare while electrocardiogram-gated spectra were accumulated. Levels of phosphocreatine were decreased during occlusion in both groups; however, this decrease was attenuated in the diltiazem treated animals compared to control (in relative percent area: 7.8 +/- 1.0 to 2.5 +/- 0.5, p less than 0.01). Levels of phosphocreatine promptly returned to baseline following reperfusion and there was no difference between the two groups. The inorganic phosphate metabolites of high-energy phosphate consumption increased with occlusion, though more so in the control group compared with the diltiazem-treated rabbits (in relative percent area: 72.5 +/- 0.9 to 55.4 +/- 1.3, p less than 0.01). With reperfusion, levels of inorganic phosphates returned toward baseline in both groups; however, the diltiazem group had a more complete recovery relative to control (in relative percent area: 38.8 +/- 2.1 to 47.6 +/- 2.7, p less than 0.05). Levels of adenosine triphosphate decreased in both groups relative to baseline; however, the amount of decrease was similar in the two groups. With reperfusion there was a definite though incomplete recovery of levels of adenosine triphosphate in the diltiazem-treated group (in relative percent area: 10.7 +/- 1.0 at occlusion, 12.3 +/- 0.4 during reperfusion, p less than 0.05), but in the control group levels of adenosine triphosphate remained depressed (in relative percent area: 9.8 +/- 0.6 at occlusion, 9.8 +/- 0.8 during reperfusion, p = NS). During ischemia there was a trend toward attenuation of intracellular acidosis in the diltiazem group; however, this trend did not reach statistical significance. These data indicate that diltiazem provides a protective effect on myocardial high-energy phosphate metabolism during regional ischemia and reperfusion in the intact animal.

Amlodipine pretreatment and the ischemic heart

Amlodipine is a long-acting dihydropyridine-based Ca2+ channel blocker, developed for use on a once-daily basis. Experiments using hearts from amlodipine-pretreated rats were undertaken to further test the hypothesis that Ca2+ channel blockers can be used as prophylactic therapy to reduce the severity of the mechanical and biochemical consequences of ischemia and reperfusion. Amlodipine was given intravenously, 0.25 mg/kg, 5 hours before excising the hearts. Ischemia (global) was induced at 37 degrees C for 10, 30 or 60 minutes, and was followed by reperfusion. Protection was quantitated in terms of functional recovery, adenosine triphosphate and creatine phosphate retention, tissue acidosis and Ca2+ gain. The results show that amlodipine pretreatment supplied protection, provided that the ischemic episode did not exceed 30 minutes. The protection resulted in improved recovery of peak developed tension on reperfusion, reduced Ca2+ gain, retention of tissue adenosine triphosphate and creatine phosphate, and reduced acidosis.

New anti-ischaemic drugs: cytoprotective action with no primary haemodynamic effects

Currently therapy for ischaemic heart disease is based on drugs such as beta-adrenoceptor antagonists, Ca2+ antagonists and nitrates, which have pronounced haemodynamic effects. However, these drugs can have adverse reactions including systemic haemodynamic effects, leading to low blood pressure and peripheral oedema in some patients. Recent observations that certain types of Ca2+ antagonist prevent the Ca2+ overload that occurs after ischaemia have led to the design of new anti-ischaemic drugs that are cytoprotective, but have no (or few) haemodynamic effects. In this article, Pieter van Zwieten and colleagues assess the therapeutic potential of these drugs.

Myocardial infarction in rats: effects of metabolic and pharmacologic interventions

Myocardial infarction was induced in rats by ligation of the descending branch of the left coronary artery. The time course of changes in heart function was recorded within the first nine days. There was a progressive decline in LVSP, in LV dP/dtmax and in the pressure-rate-product. LVEDP was elevated. Cardiac output and stroke volume index were depressed after two days. The ATP content in the nonischemic region was lower than control, but recovered spontaneously toward the normal value within the first four days. Three metabolic and pharmacologic interventions known to affect cardiac adenine nucleotide metabolism were applied. Continuous i.v. administration of ribose which stimulates further adenine nucleotide biosynthesis attenuated the fall and promoted the restoration of ATP in the nonischemic myocardium within four days after coronary artery ligation. The elevation of LVEDP was attenuated with ribose after two and four days. The calcium antagonist gallopamil administered as i.v. infusion for two days led to a further reduction of all parameters of left heart function, but did not influence the increase in adenine nucleotide and protein synthesis that occurred in the nonischemic heart. Coenzyme Q10 had only slight effects on LVSP, LVEDP, and LV dP/dtmax, but attenuated significantly the fall in cardiac output and stroke volume index after two days following coronary artery ligation. Thus, all interventions affected differently the infarct-induced changes in heart and circulatory function. An improvement was observed with ribose and with coenzyme Q10.

In vitro study of vascular endothelial injury by activated platelets and its prevention

We performed an in vitro study to assess injury to vascular endothelial cells by platelets. Cultured endothelial cells isolated from fetal bovine aorta were used. Addition of human platelets, activated by collagen or lysed by sonication, to the culture dish resulted in dose- and time-dependent damage to the cells as estimated by [3H]adenine release. Analysis of [3H]adenine nucleotides by thin-layer chromatography on PEI-cellulose revealed decreased intracellular ATP content in the cells treated with platelet lysate. The medium contained AMP and adenosine, the latter increasing following the treatment of the cells. Of the substances released by the activated platelets, thromboxane A2 (TxA2) and serotonin caused cell damage. Platelet-derived growth factor (PDGF), however, did not damage the endothelial cells up to a concentration of 200 ng/ml. Pretreatment of the cells with methysergide (10(-6) M) or ONO 3708 (10(-5) M), a TxA2 antagonist, only partially prevented the damage, while ZK 36374 (10(-6) M), a prostacyclin analog, and 3-isobutyl-1-methylxanthine (IBMX; 10(-3) M), a phosphodiesterase inhibitor, potently inhibited injury. We conclude that the substances released from activated platelets may injure endothelial cells in an additive or synergistic manner and that agents which produce effects that elevate cyclic AMP levels may protect the cells from damage induced by the platelets.

Captopril improves recovery of adenosine triphosphate during reperfusion of the ischemic isolated rat heart; a 31-phosphorus-nuclear magnetic resonance study

The effect of captopril on energy-rich phosphates and pH during normothermic ischemic arrest, hypothermic cardioplegic arrest and subsequent reperfusion was investigated in the isolated rat heart using 31P-nuclear magnetic resonance. The hearts remained in the probe during all perfusion procedures and captopril (80 ml.l-1) treatment was started directly after cannulation. After normothermic ischemic arrest (15 min), the ATP content of captopril-treated hearts was not significantly different from that of untreated hearts (53 +/- 9% and 52 +/- 8%, respectively). Accumulation of inorganic phosphate at the end of ischemia was significantly less in treated hearts, suggesting a higher end-ischemic nucleotide content in treated hearts. Hypothermic cardioplegic arrest (St. Thomas' Hospital solution, 4 degrees C) lasted for 3 h at 10 degrees C. Adenosine triphosphate in untreated hearts was significantly lower at the end of ischemia; 36 +/- 6% compared to 53 +/- 9% for untreated hearts. Adenosine triphosphate in untreated hearts recovered to 76 +/- 9% after normothermic ischemia and to 72 +/- 7% after hypothermic ischemia at the end of 30 min reperfusion. Captopril significantly improved adenosine triphosphate recovery in both treated groups; 89 +/- 4% after normothermic and 83 +/- 4% hypothermic ischemia. We conclude that captopril has a beneficial effect on recovery of adenosine triphosphate both after normothermic and after hypothermic ischemia.

Effects of nifedipine and felodipine on adenosine and inosine release from the hypoxemic rat cerebral cortex

The cerebral cortical cup technique has been used to study the effects of nifedipine and felodipine on adenosine and inosine release from the rat brain. After basal and hypoxia (8% 02)-evoked control levels of purine release had been established, these 1,4-dihydropyridine calcium antagonists were administered intraperitoneally (1 mg/kg). Both agents depressed basal levels of purine efflux and suppressed the hypoxia-evoked release of adenosine and inosine. An inhibition of the transporter that mediates purine efflux from brain cells is likely to account for the suppression of release from the cerebral cortex. A reduced release of adenosine into the interstitial space also explains the ability of both agents to block the increase in CBF evoked by hypoxic challenges.

Calcium channel antagonists. Part II: Use and comparative properties of the three prototypical calcium antagonists in ischemic heart disease, including recommendations based on an analysis of 41 trials

An analysis of 41 trials of angina of all varieties confirms that calcium antagonists are an important advance and are now established therapy for these syndromes. In effort angina, verapamil in a dose of 360-480 mg daily is better than propranolol in standard doses. Although nifedipine is highly effective against vasospastic angina, its use in threatened myocardial infarction or severe unstable angina is not supported by recent studies, unless combined with a beta-blocker. Diltiazem has recently been tested with apparent benefit in non-Q-wave myocardial infarction. Otherwise, these calcium antagonist agents all seem to have approximate equipotency in clinical ischemic syndromes including effort and vasospastic angina. Subjective side effects seem most troublesome in the case of nifedipine. All three calcium antagonists, especially nifedipine, have been successfully combined with beta-blocker therapy, yet occasional additive negative inotropic or chronotropic or dromotropic interactions may occur when verapamil or diltiazem is added to beta-blockade, and occasionally the direct negative inotropic potential of nifedipine may become evident. The choice between the calcium antagonists is determined not only by the clinical picture but also by the anticipated side effects in a given patient and by the overall cardiovascular status. In patients with supraventricular tachycardias or sinus tachycardia, verapamil or diltiazem is preferred, whereas in patients with a resting bradycardia or borderline heart failure nifedipine is likely to be chosen.

In vitro plasma nifedipine concentration during heart-lung machine function

The use of calcium antagonists such as nifedipine for myocardial protection during cardiac surgery has been advocated by several authors. During extracorporeal circulation many factors, such as light, interaction with circuit materials and haematocrit, may contribute to decrease plasma clearance of calcium antagonists

In an in vitro model of a heart-lung machine, plasma nifedipine and prime concentrations were detected with a series of samples at different temperatures (25 °C and 37 °C), haematocrits (0%, 20%, 30% and 40%) and light conditions (light and dark).

The results show a rapid drop of nifedipine concentration with a halflife of about 3-9 minutes and this situation is influenced with statistical significance by the presence of light and increased haematocrit.

The knowledge of this condition is useful when nifedipine is used before/ during cardiopulmonary bypass and during cardioplegia and reperfusion infusion with the use of extracorporeal devices.

Influence of inhibitors of ATP catabolism on myocardial recovery after ischemia

The loss of the catabolic products of adenosine triphosphate in the form of purine nucleosides and oxypurines during ischemia and subsequent reperfusion may limit adenine nucleotide regeneration. This study compared the effects of infusion of inhibitors of the major reactions involved in the degradation of adenosine triphosphate to inosine on the postischemic recovery of high energy phosphate and myocardial function. Isolated rat hearts were made totally ischemic after a 5-min infusion of p1,p5-diadenosine pentaphosphate, alpha, beta-methylene adenosine diphosphate, nitrobenzyl-6-thioinosine, or erythro-9-(2-hydroxy-3-nonyl) adenine, which are inhibitors of adenylate kinase, 5'-nucleotidase, adenosine translocase, and adenosine deaminase, respectively. Following 30 min of ischemia, only hearts infused with alpha, beta-methylene adenosine diphosphate recovered significantly better ventricular function than did the control (P less than 0.05), but all hearts had increased adenosine triphosphate and creatine phosphate regeneration (P less than 0.05). The formation and washout of greater than 30% of the total adenine pool metabolites were not prevented by any drug. Nevertheless all manipulations of adenine metabolism resulted in recruitment of high energy phosphate during preischemic infusion which may have potential benefits in elective ischemic arrest.

Regional cardioprotection by subselective intracoronary nifedipine is not due to enhanced collateral flow during coronary angioplasty

Twelve patients with proximal stenosis of the left anterior descending artery, normal myocardial wall motion but without angiographically demonstrable collateral circulation, were studied during transluminal occlusion. Prior to the first transluminal occlusion before crossing the lesion with the balloon, patients were randomly given 0.2 mg nifedipine or its solvent in the left mainstem. The same dose was repeated via the balloon catheter, positioned across the lesion, immediately prior to the second transluminal occlusion. In all patients great cardiac venous flow and ST-elevation were monitored during and after each transluminal occlusion. The lactate extraction ratio A-GCV/A (A = arterial, GCV = great cardiac vein) was determined prior to the angioplasty procedure, 10-15 seconds after each transluminal occlusion and 10 minutes after the third transluminal occlusion. Great cardiac venous flow rose significantly to an average of 160% of basal flow when nifedipine was administered into the mainstem before the angioplasty procedure while its solvent had no effect. During each transluminal occlusion, great cardiac venous flow diminished on average by 30% in those who received nifedipine and by 28% in those who received only its solvent. This difference was statistically not significant. After angioplasty great cardiac venous flow was slightly, but not significantly, increased in both groups with respect to basal flow (104% resp. 120% of control). Patients who received nifedipine in the post-stenotic area just before the second transluminal occlusion, had significantly lower lactate production, measured immediately after the transluminal occlusion compared with the patients who received only its solvent (P less than 0.01). The ST-elevation during the second transluminal occlusion was significantly lower in the nifedipine group (0.1 mm in nifedipine group versus 1.4 mm in solvent group; P less than 0.05, unpaired t-test). Nifedipine given intracoronary in the post-stenotic area just before coronary angioplasty reduces lactate release and electrocardiographic signs of myocardial ischemic injury. This regional cardioprotective effect seems not due to an enhanced collateral flow, but to a regional cardioplegic effect, which precedes the ischemic event.

Effects of diltiazem upon globally ischemic rat hearts

Addition of diltiazem (0.0, 0.95, 2.5 or 7.5 microM) to isolated working rat hearts before and during ischemia, produced a concentration-dependent increase in recovery of contractile function. Recovery of post-ischemic pressure-rate product showed a strong relationship with depression of pre-ischemic pressure-rate product, primarily from decreased heart rate before ischemia and increased pressure development following reperfusion. Increased recovery in treated hearts was associated with higher ATP and adenine nucleotide levels (ADN), but no relationship was observed between energy levels and degree of recovery of function or concentration of diltiazem. Hearts made ischemic for 20 min without reperfusion had increased ATP and decreased lactic acid accumulation when treated with 7.5 microM diltiazem. The results indicate contractile-dependent mechanisms of action of diltiazem in global ischemic hearts which can only be partly explained by preservation of ATP and ADN, but also are associated with reduced lactic acid accumulation.

Calcium antagonists and the ischemic myocardium

Recent laboratory studies have shown that the calcium antagonists (slow channel blockers) can protect the myocardium against the consequences of experimentally induced ischemia and reperfusion. With one recent exception, however, clinical trials relating to the effectiveness of these drugs in the management of patients with myocardial infarction have been disappointing. This paper explores this apparent discrepancy.

Calcium channel blockers and cardiac surgery

Calcium channel blockers have an important role in the pharmacotherapy of cardiovascular disorders. These agents act by inhibiting the slow inward current into excitable cells, exert direct negative inotropic, chronotropic, and dromotropic activity, and are potent vasodilators. These direct effects are modified by reflex autonomic stimulation and by pathologic states. Serious adverse effects of the calcium channel blockers are most frequently observed in patients with ventricular dysfunction, conduction system disease, or concomitant beta blockade. Calcium channel blockers are indicated in the treatment of angina pectoris, supraventricular arrhythmias, and hypertension. The use of these agents in patients with hypertrophic cardiomyopathy, congestive heart failure, and pulmonary hypertension is investigational. The calcium channel blockers are gaining increased importance in the management of patients undergoing cardiac surgery. Verapamil is indicated for the treatment of post-cardiac-surgical atrial flutter and fibrillation; however, the calcium antagonists are not effective as prophylaxis against postoperative supraventricular arrhythmias. Laboratory studies have shown that drug interactions exist between calcium channel blockers and inhalational anesthetics and nondepolarizing neuromuscular blocking agents; clinical studies have demonstrated that these interactions are rarely significant. Perioperative coronary spasm can be effectively treated with the calcium channel blockers. The timing of calcium antagonist withdrawal prior to surgery is controversial, but continuation of therapy until surgery is usually safe. The clinical significance of platelet function inhibition by the calcium antagonists is unknown. Protection of ischemic myocardium by calcium channel blockers has been demonstrated. Important interactions between the calcium antagonists, hypothermia, and the ionic constituents of cardioplegia require further study before the role of these agents as adjuncts to clinical cardioplegia is defined. Expanded indications and the introduction of new calcium channel blockers will result in increased use of these agents in the future.

Mechanism of myocardial protective action of dilazep during ischaemia and reperfusion

The aim of this study was to investigate if dilazep is able to reduce with a direct protective action on the myocardium the deleterious effects caused by ischaemia and reperfusion. For this purpose we used an isolated rabbit heart preparation. The hearts were either perfused aerobically or made totally ischaemic for 60 min (by abolishing coronary flow) or made ischaemic for 60 min and then reperfused for 30 min. Ischaemic and reperfusion damage was measured in terms of alteration in mechanical function, lactate and CPK release, mitochondrial function and tissue content of Adenosine Triphosphate (ATP), Creatine Phosphate (CP) and calcium. Dilazep (10(-5) M) was administered in the perfusate either 20 minutes before ischaemia or only during post-ischaemic reperfusion. Ischaemia induced a decline of the endogenous stores of ATP and CP, followed by an alteration of calcium homeostasis with increase of diastolic pressure, mitochondria calcium overload and impairment of the oxidative phosphorylating capacities. On reperfusion, tissue and mitochondrial calcium increase the capacity of the mitochondria to use O2 for state III respiration was further impaired and the ATP-generating capacity reduced. Diastolic pressure increased and there was only a small recovery of active tension generation associated with massive CPK release. Administration of dilazep before ischaemia induced a negative inotropic effect which, in turn, resulted in a slowing of the rate of CP and ATP depletion during ischaemia. This protected the hearts against the ischemic, and reperfusion-induced decline in the ATP-generating and O2-utilizing capacities of the mitochondria. In addition, there was a less marked increase in tissue and mitochondrial Ca++, CPK and lactate release were reduced and the recovery of developed pressure on reperfusion was significantly increased. Administration of dilazep during reperfusion failed to modify the exacerbation of ischaemic damage caused by the readmission of coronary flow. These data suggest that dilazep benefits the ischaemic myocardium via an ATP sparing action.

Fundamental mechanisms of action of calcium antagonists in myocardial ischemia

The mammalian myocardium exhibits a spectrum of damage during an ischemic episode. After relatively short periods of ischemia the damage is reversible, but with longer periods of ischemia the number of cells that are lethally injured increases. When coronary flow is restored the lethally injured cells become overloaded with Ca++ and fail to regenerate adenosine triphosphate. The calcium antagonists provide protection under these circumstances, but only if used prophylactically. When added only upon reperfusion the calcium antagonists slow, but do not inhibit, the excessive gain in Ca++ that occurs during postischemic reperfusion. Nicotine, in a concentration equivalent to that found in the plasma of smokers (0.15 microgram/ml), exacerbates the reperfusion-induced Ca++ gain. Treatment with the long-acting calcium antagonist, anipamil, on a once-daily basis attenuates the reperfusion-induced Ca++ gain in spontaneously hypertensive rats and its exacerbation by nicotine in Sprague Dawley rats. The prolonged oral administration of at least 1 calcium antagonist, verapamil (50 mg/kg body weight/day), causes a significant (p less than 0.001) depletion of left ventricular norepinephrine.

The haemodynamic effects of nifedipine, verapamil and diltiazem in patients with coronary artery disease. A review

Of the 3 most widely used calcium antagonists--nifedipine, verapamil and diltiazem--nifedipine is the most potent arterial vasodilator. Increases in cardiac output and coronary blood flow following nifedipine administration result in part from the afterload reduction. Reflex adrenergic stimulation produces an increase in heart rate and masks a direct inhibitory effect on myocardial contractility. The negative inotropic action of nifedipine is observed during intracoronary administration or may be made apparent by concurrent beta-blocker therapy. While verapamil is also a potent vasodilator, negative inotropic and dromotropic properties are more apparent in therapeutically used dosages. Reflex sympathetic activation is also triggered by verapamil, with an offsetting of the negative inotropic effects such that little change in cardiac output results. A decrease in myocardial oxygen consumption, with or without a decrease in coronary sinus blood flow, has regularly been observed following verapamil administration. Reduced oxygen demand appears to be a major mechanism of its antianginal effect. The heart rate X systolic pressure product is decreased both by the fall in arterial pressure and, particularly after oral administration, by a decrease in heart rate. Diltiazem produces similar haemodynamic and electrophysiological effects to those of verapamil but has less potency in inducing arterial dilatation and more of a tendency to slow the heart rate. Diltiazem does not appear to cause significant increases in coronary blood flow or bring about improvement in ejectional and isovolumic indices of myocardial contraction - evidence of its intrinsic negative inotropic effect.

Timely administration of nisoldipine essential for prevention of myocardial ATP catabolism

Calcium entry blockers can effectively preserve high-energy phosphates in ischemic heart. However, little is known about the optimal timing of drug therapy. The moment of nisoldipine administration in relation to its protective efficacy during ischemia and reperfusion was studied in rat hearts. Nisoldipine (50 nM), given some time before a reduction of about 90% in coronary flow diminished ATP-catabolite efflux during both ischemia and reperfusion by up to 85%. In contrast, drug administration at the onset of ischemia, or during ischemia or during reperfusion was completely without protective effect. Similarly, early nisoldipine application gave rise to ischemic ATP, adenylate charge and creatine phosphate values higher than those in untreated or late-treated hearts. Nisoldipine decreased the tension developed before ischemia by up to 66%, without affecting (post)ischemic function. Nisoldipine spares energy effectively only if administered to the heart prior to ischemia. This presumably has to do with its negative inotropy before flow reduction.

Energy conservation by nisoldipine in ischaemic heart

We studied the effect of the calcium entry blocker nisoldipine on ATP catabolism in the rat heart, perfused according to Langendorff. Even 1 nM nisoldipine induced vasodilatation; concentrations of 30 nM and higher caused significant negative inotropy. The drug had a very strong affinity for silicon rubber tubing. Myocardial ischaemia was induced by lowering the perfusion pressure, which reduced flow without nisoldipine by 85%. The efflux of purine nucleosides and oxypurines rose 14 fold. Nisoldipine reduced this efflux of ATP catabolites dose-dependently. The highest concentration, 300 nM, suppressed ischaemic purine production completely. The action of the drug was antagonized by an increase in Ca2+-concentration in the perfusion fluid. We also showed the protective effect of nisoldipine on adenine nucleotides in freeze-clamped hearts. A concentration of 20 nM partially prevented the reduction of ATP and adenylate energy charge due to ischaemia. We conclude that relatively low doses of nisoldipine effectively prevent ATP breakdown in ischaemic rat heart.

Inhibition by verapamil of ischemic Ca2+ uptake in the rabbit hippocampus

In the study described here a perfusion-dialysis system was used for recording ionic changes in the hippocampus during ischemia. Local perfusion with a Ca2+ entry blocker (verapamil) inhibited the fall in extracellular Ca2+ concentration which occurs in ischemia. The data consequently suggest that intracellular Ca2+ accumulation during ischemia can be blocked. A Ca2+ antagonist such as verapamil may thus improve post-ischemic neuronal recovery via a direct effect on parenchymal cells in addition to the previously suggested vascular effects.

The effect of two different diltiazem treatments on infarct size in ischemic, reperfused porcine hearts

The effect of diltiazem on the development of infarcts was investigated in porcine hearts. The left anterior descending coronary artery was occluded in each of 32 anesthetized pigs for 75 min and was reperfused for 4 hr. Diltiazem (15 micrograms/kg X min) was infused in 11 pigs for 30 min before occlusion (therapy A) and in another eight pigs before reperfusion (therapy B). Eleven pigs served as controls. Tissue concentrations of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NAD) were determined in transmural needle biopsy samples taken from the ischemic apex after 70 min of ischemia. The infarct size, expressed as the ratio of the infarcted tissue over the area at risk of necrosis multiplied by 100, amounted to 79 +/- 20% in the control group. Although there was no significant difference between hemodynamics in the control and the treated groups, pretreatment with diltiazem significantly reduced infarct size (53 +/- 26%; p = .025). Reduction of infarct size by therapy B did not reach the required level of significance (66 +/- 33%). The ischemic loss of ATP and NAD was significantly lower in the pretreated group, which further indicates that the beneficial effect of diltiazem was exerted primarily during ischemia and not during reperfusion.

Captopril reduces purine loss and reperfusion arrhythmias in the rat heart after coronary artery occlusion

Captopril was perfused through isolated rat hearts; its effects after local ischemia and reperfusion were assessed. Upon reperfusion all untreated (10 out of 10) but only 4 (out of 10) captopril-treated (80 micrograms/ml) hearts fibrillated (P less than 0.02). Purine overflow increased upon reperfusion but was reduced by captopril (597 +/- 62 and 333 +/- 41 nmol/min gdwt respectively; P less than 0.05). The pressure-rate index and the apex displacement were severely impaired after 30 min of reperfusion (32 +/- 16 and 10 +/- 5% respectively of initial values) but captopril reduced the injury of mechanical function (60 +/- 8; P less than 0.05 and 61 +/- 11; P less than 0.05 respectively). These results show that captopril reduces ventricular fibrillation and the loss of high energy phosphate nucleotides and thereby partly maintains mechanical function impaired by ischemia and reperfusion.

Myocardial xanthine oxidase/dehydrogenase

High-energy phosphates in heart muscle deprived of oxygen are rapidly broken down to purine nucleosides and oxypurines. We studied the role of xanthine oxidase/dehydrogenase (EC 1.2.3.2/EC 1.2.1.37) in this process with novel high-pressure liquid chromatographic techniques. Under various conditions, including ischemia and anoxia, the isolated perfused rat heart released adenosine, inosine and hypoxanthine, and also substantial amounts of xanthine and urate. Allopurinol, an inhibitor of xanthine oxidase, greatly enhanced the release of hypoxanthine. From the purine release we calculated that the rat heart contained about 18 mU xanthine oxidase per g wet weight. Subsequently, we measured a xanthine oxidase activity of 9 mU/g wet wt. in rat-heart homogenate. When endogenous low molecular weight inhibitors were removed by gel-filtration, the activity increased to 31 mU/g wet wt. Rat myocardial xanthine oxidase seems to be present mainly in the dehydrogenase form, which upon storage at -20 degrees C is converted to the oxidase form.

Synergistic effect of nifedipine and propranolol on adenosine (catabolite) release from ischemic rat heart

Both nifedipine a calcium antagonist, and propranolol a beta-adrenergic blocker, are used as protective agents of the ischemic myocardium. In the clinical setting, the combination of the two drugs is used successfully although several case reports indicate potential dangers of the combination. For this reason we decided to study the combined effect of nifedipine and DL-propranolol in the isolated rat heart made ischemic for a short period of time. Apex displacement was taken as a measure of contractility. Release of the AMP catabolites adenosine, inosine, (hypo)xanthine and uric acid was used as a marker of ATP breakdown. Contractility during ischemia was not affected by the drugs. DL-Propranolol (30 or 150 micrograms/l) had no effect on ischemic myocardial purine release, while nifedipine (15 micrograms/l) reduced purine release during ischemia by 33% (P less than 0.02). The combination of 15 micrograms/l nifedipine and 150 micrograms/l DL-propranolol decreased purine release by 53% (P less than 0.005 vs. nifedipine). We conclude from these results that propranolol has a synergistic effect, adding to the beneficial action of nifedipine on ischemic myocardium.