Endothelium-dependent calcium-induced relaxation in the presence of Ca2+-antagonists in canine depolarized coronary arteries

Prevention of cerebral vasospasm by a novel endothelin receptor antagonist, TA-0201

This study was designed to examine the preventive effect of a novel endothelin (ET)-receptor antagonist TA-0201 on the cerebral vasospasm in a canine double-hemorrhage model. TA-0201 (10(-9)-10(-7) M) inhibited ET-1-induced vasoconstriction in the isolated canine basilar artery without endothelium in a concentration-dependent manner. Its pA2 value was 9.2 (ET(A) antagonism). In a canine double-hemorrhage model, intravenous treatment with TA-0201 (3 mg/kg, twice a day for 7 days) ameliorated the basilar artery narrowing significantly on day 7 compared with that in nontreated dogs. The reductions of the basilar artery diameter were 26.1+/-3.9% and 40.5+/-4.1% with and without TA-0201 treatment, respectively (p<0.05). Histologic study on day 7 indicated that treatment with TA-0201 inhibited vessel-wall damage such as disintegration of endothelium architecture and degeneration of medial smooth-muscle cells. We conclude that intravenous treatment with TA-0201 prevents the development of cerebral vasospasm and accompanying pathologic changes of the vessel wall, probably through blockade of ET(A) receptors.

Direct activation of endothelial NO pathway by Ba2+ in canine coronary artery

1. We have reported that Ba2+ causes endothelium-dependent relaxation of canine coronary arteries through NO synthesis in Ca2+-free and depolarizing solution. To determine the cellular mechanisms by which the endothelium-dependent relaxation occurs, we used fura-2 fluorometry (F350 and F390; excitation wavelengths, 350 and 390 nm, respectively) and estimated the intracellular Ba2+ concentration in endothelial and vascular smooth muscle cells. 2. Ba2+ (10(-3) M) increased the fura-2 ratio (F350/F390) recorded from a combined preparation of smooth muscle and endothelium (0.445+/-0.073, n = 4) and contracted the arteries in the presence of 80 mM K+ (0.22+/-0.06 g, n = 4). 3. Diltiazem (3 x l0(-6) M) blocks Ba2+ entry into vascular smooth muscle cells via L-type Ca2+ channels. In this condition, Ba2+ increased the fura-2 ratio in endothelial cells (0.141+/-0.014, n = 5) and relaxed the underlying smooth muscle (0.08+/-0.01 g, n = 5) by a mechanism which was sensitive to 10(-4) M NG-methyl-L-arginine (L-NMMA). 4. Ba2+-induced relaxation was not attenuated with repeated application and was elicited even after endothelium-dependent relaxations in response to 10(-6) M bradykinin were abolished due to tachyphylaxis. Neither 10(-2) M caffeine nor 10(-6) M thapsigargin had effect upon Ba2+-induced relaxation. 5. To further rule out changes in intracellular Ca2+ as a mechanism of Ba2+-induced relaxation, fura-2 fluorescence was measured at the isosbestic wavelengths for Ca2+ (360 nm) and Ba2+ (370 nm) in endothelium-intact arteries. Ba2+ altered F360, but not F370, suggesting little or no contribution of intracellular Ca2+ to the phenomenon of Ba2+-induced relaxation. 6. These results suggest that the Ba2+-induced relaxation is due to its direct activation of endothelial NO synthesis without mobilization of intracellular Ca2+.

Modulatory role of endothelial calcium level in vascular tension of canine depolarized coronary arteries

The vascular tension in the coronary artery is modulated by factors released by endothelial cells. We investigated the relationship between the Ca2+ level in endothelium and endothelium-mediated changes in smooth muscle tone in high K(+)-depolarized canine coronary arteries by measuring intracellular Ca2+ concentration fluorimetrically with the Ca2+ indicator fura 2. Addition of Ca2+ (1 mM) caused an increase in endothelial Ca2+ and relaxed the 30 mM K(+)-depolarized arteries following inhibition of Ca2+ influx in the smooth muscle with diltiazem. This relaxation was inhibited by NG-monomethyl-L-arginine. As extracellular K+ concentration was decreased, increases of endothelial Ca2+ were augmented, whereas the relaxation was decreased. Basal muscle tone was found to be decreased in low K+ by measuring relaxation by sodium nitroprusside. These results suggest the importance of Ca2+ level in the endothelium in playing a modulatory role in coronary tension through the production of nitric oxide. The correlation of extracellular K+ to Ca2+ level in the endothelium indicates a typical characteristic of the passive Ca2+ entry pathway in the endothelium, whereas the resultant relaxation appears to be restricted by the basal muscle tone.

Calcium-channel blockers preserve coronary endothelial reactivity after ischemia-reperfusion

BACKGROUND

Calcium-channel blockers have been reported to improve myocardial recovery after ischemia-reperfusion, but their effects on coronary blood flow regulation remain to be defined. Experiments were designed to evaluate the effects of calcium antagonists on coronary artery vasoregulation exposed to ischemia-reperfusion.

METHODS

Three groups of hearts (n = 6) were pretreated with a 10-minute infusion of either diltiazem, verapamil, or nifedipine at concentrations of 10(-9) mol/L to 10(-6) mol/L and exposed to 30 minutes of no-flow ischemia and 45 minutes of reperfusion. Another group (n = 6) received no pretreatment and was used as control. Endothelium-dependent and -independent relaxations were tested by assessing coronary flow increase to 5-hydroxytryptamine (10(-6) mol/L) and sodium nitroprusside (10(-5) mol/L) infusion, respectively. Left ventricular pressure, its first derivative, and coronary basal flow were recorded before and after ischemia as well as during calcium antagonist infusion.

RESULTS

Endothelium-dependent relaxation after ischemia was significantly improved with all three drugs in a dose-dependent fashion; nifedipine was found to be the more potent. Endothelium-independent relaxation was also significantly preserved with calcium antagonists regardless of the type, whereas left ventricular hemodynamics were not. During perfusion, nifedipine was found to have the most negative inotropic effect and to be the most potent vasodilator on the coronary circulation. Diltiazem was the less effective drug on both left ventricular hemodynamics and coronary circulation.

CONCLUSIONS

This study indicates that preischemic infusion of calcium antagonists enhance endothelium-dependent and -independent coronary artery relaxation in the isolated rat heart model in a dose- and drug-dependent fashion. This can be achieved at low doses without affecting left ventricular hemodynamics and should contribute to preserve coronary artery autoregulation.

Impaired coronary sensitivity to diltiazem in experimental heart failure: involvement of the cyclooxygenase but not the nitric oxide-synthase pathway

Because controversies surround the increased negative inotropic effects of calcium antagonists in heart failure, other mechanisms may explain their lack of efficacy in this condition. We hypothesized that altered coronary sensitivity through endothelial dysfunctions may be involved. Our goal was to evaluate the effects of heart failure on coronary and cardiac sensitivity to the calcium antagonist diltiazem. Left ventricular developed pressure (LVP) and coronary flow (CF) were assessed in isovolumetrically beating, perfused, failing hearts from cardiomyopathic hamsters (UM-X7.1) and hearts from normal hamsters. Diltiazem concentration-response curves for both coronary dilation and its negative inotropic effects were charted under control conditions and in the presence of the specific nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 30 microM), and the cyclooxygenase inhibitor, indomethacin (10 microM). Diltiazem concentration-response curves for its negative inotropic action were similar in normal and failing hearts (IC50 1.2 and 2.3 microM, respectively). In contrast, the coronary dilator effects of diltiazem were impaired in failing hearts (EC50 for diltiazem-induced coronary dilation increased from 90 nM in normal hearts to 1.1 microM in failing hearts, p < 0.01). The involvement of endothelial dysfunctions in the observed coronary "desensitization" to diltiazem in heart failure was evaluated through the NO-synthase and cyclooxygenase pathways. Diltiazem concentration-response curves from failing hearts were not modified in the presence of L-NAME, whereas indomethacin normalized the coronary response to diltiazem in heart failure. These findings suggest that coronary "desensitization" to diltiazem occurs through parallel production and/or release of a vasoconstricting factor or factors originating from the cyclooxygenase pathway. Heart failure was not associated with increased cardiac sensitivity to diltiazem but rather with altered coronary sensitivity. These findings suggest that coronary desensitization may play a role in the lack of efficacy of diltiazem in heart failure and provide a better understanding of factors modulating the effects of calcium antagonists in heart failure.

Contractions and relaxations accompanied by endothelial nitric oxide production induced in the porcine coronary artery by Ca2+, Ba2+ and Sr2+

In order to investigate the effects of Ca2+, Ba2+ and Sr2+ on vascular endothelial nitric oxide (NO) production, contractile and relaxant responses of porcine depolarized coronary arteries to these divalent cations were compared. In the presence of diltiazem, Ba2+ induced NO-dependent relaxation, Sr2+ did slightly and Ca2+ did not; however all three cations increased cGMP levels in endothelium-intact arteries to similar extents. In the absence of diltiazem, these cations evoked contractions: the EC50 of Ca2+ for endothelium-denuded arteries was lower than those of Ba2+ and Sr2+. The IC50 of diltiazem for arteries precontracted with Ca2+ was higher than for arteries precontracted with Ba2+ and Sr2+. These results suggest that Ba2+ and Sr2+, as well as Ca2+, activate coronary arterial NO production, and also that the different responses of coronary arteries to these divalent cations can be explained, in part, by the different sensitivities of the smooth muscle to these cations and by the different potencies of diltiazem to inhibit the contractions the cations induced.

Barium and strontium can substitute for calcium in stimulating nitric oxide production in the endothelium of canine coronary arteries

We investigated whether Ba2+ and Sr2+ can substitute for Ca2+ in stimulating the nitric oxide (NO) production and cause relaxation in vascular smooth muscle. Ba2+ and Sr2+, like Ca2+, relaxed K(+)-depolarized canine coronary arteries in the presence of diltiazem. The Ba(2+)- and Sr(2+)-induced relaxation was endothelium-dependent and was largely inhibited by NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine (L-NNA), but not by indomethacin. These cations increased cyclic GMP levels in the coronary artery to a similar extent, and the increment was completely abolished by L-NMMA. The relaxation induced by each cation was attenuated in the presence of a combination of propranolol, phentolamine and atropine, and L-NNA markedly inhibited any remaining relaxation. This indicates that these cations produce endothelium-dependent relaxation through NO production as well as the relaxation mediated by neurotransmitters. The present study suggests that Ba2+ and Sr2+ can substitute for Ca2+ in the activation of the NO synthase pathway in the endothelium of canine coronary arteries.

The effect of a new calcium antagonist, TA3090 (clentiazem), on experimental transient focal cerebral ischemia in cats

BACKGROUND AND PURPOSE

TA3090 (Clentiazem) has been shown to have cerebrovascular protective properties in three experimental studies. An in vivo investigation was undertaken to determine its effects on pial arteries and cerebral blood flow and its therapeutic value in transient focal cerebral ischemia.

METHODS

This experiment was divided into two protocols. In the first, 200 or 400 micrograms/kg per hour TA3090 was administered continuously for 3 hours in cats without ischemic insult (n = 6 for each group). The effects on pial arteries and cerebral blood flow were estimated. In the second protocol, 400 micrograms/kg per hour TA3090 (treated group, n = 14) or physiological saline (control group, n = 10) was administered 5 minutes before 1 hour of middle cerebral artery occlusion in cats. The effects on the pial arteries and cerebral blood flow were observed continuously, followed by autoradiography for a quantitative measurement of cerebral blood flow 5 hours after middle cerebral artery recirculation. The volumes of the cerebral edema and infarct were estimated by planimetry from cerebral preparations made for histological examination.

RESULTS

Pial arteries dilated by up to approximately 10% in the 400-micrograms group and 3% in the 200-micrograms group 30 minutes after administration of TA3090. Increases in cerebral blood flow of about 10% in the 400-micrograms group and 2% in the 200-micrograms group were demonstrated with laser Doppler flowmetry. In the second protocol, dilatation of pial arteries was significantly smaller during and after the ischemic insult in the treated group compared with the control group (p < 0.01). Cerebral blood flow decreased less significantly during ischemia (p < 0.01 at the end of ischemia) and increased less significantly after ischemia (p < 0.01 at the end) in the treated group compared with the control group. Autoradiography showed a more remarkable increase in cerebral blood flow due to luxury perfusion in the cerebral cortex, which was mainly perfused by the middle cerebral artery on the affected side in the control group (p < 0.01). Cerebral blood flow in the cerebral cortex, thalamus, and caudate nucleus on the contralateral side of the treated group increased by about 20% more than that of the control group (p < 0.05). Cerebral edema and infarction were much smaller in the treated group than in the control group (p < 0.01).

CONCLUSIONS

1) TA3090 dilates pial arteries and increases cerebral blood flow in normal brain regions in a dose-response fashion; 2) in ischemic regions compared with those in untreated animals, TA3090 results in a lesser reduction of cerebral blood flow during ischemia and in a lesser degree of hyperemia during reperfusion; 3) TA3090 is associated with less pial artery dilatation during ischemia, presumably due to improved collateral flow; and 4) the improved hemodynamic state with TA3090 is associated with significant reduction of cerebral edema and infarct size.

Calcium-induced vasodilation due to increase in nitric oxide formation in the vascular bed of rabbit ear preparation

Participation of calcium-induced vasodilation (due to an increase in synthesized nitric oxide (NO) content in endothelial cells) in the arterio-venous circulation, including the vascular bed was investigated by the vessel perfusion method in the isolated rabbit ear preparation. The perfusion medium used was a tris-buffered solution. When CaCl2 (6.25, 12.5 and 25 mg) was injected in the perfused vessel of the rabbit ear preparation, dose-dependent vasocontraction was observed when vascular tone was kept at a normal level. However, CaCl2 dose-dependently induced vasodilation of the vessel when it was continuously contracted by norepinephrine (1.2 x 10(-7) M). This calcium-induced vasodilation was inhibited in the presence of NG-nitro-L-arginine (5 x 10(-5) M), a selective inhibitor of NO synthesis, and methylene blue, a guanylate cyclase inhibitor, although it was rarely affected by indomethacin (10(-5) M), a cyclooxygenase inhibitor. Calcium-induced vasodilation was also obtained in the in situ circulation containing vascular bed, and this suggests that the vasodilation was due to a Ca(2+)-induced increase in the synthesis of NO derived from endothelial cells.

Role of the endothelial system in Bay K 8644 enantiomer and nifedipine vasomodulator action in rat aorta

The potential importance of the endothelial system in regulating the effects of (-)-Bay K 8644 (0.1 microM), (+)-Bay K 8644 (0.1 microM) and nifedipine (10 nM) on resting tension, on contractile responses to noradrenaline (NA) and Ca2+ (in a Ca(2+)-free high-K+ solution), and on basal, NA-induced and K(+)-induced 45Ca2+ uptake, was investigated in rat aorta rings. Mechanical removal of endothelium considerably potentiated the contractile response induced by NA in standard medium and by Ca2+ in Ca(2+)-free high-K+ (15 mM) medium, but did not modify the response induced by Ca2+ in Ca(2+)-free high-K+ (55 mM) medium or by NA in Ca(2+)-free medium. Furthermore, the basal 45Ca2+ uptake and that induced by NA (10 microM) or KCl (15 and 55 mM) were similar in endothelium-rubbed and intact rings. (-)-Bay K 8644 (0.1 microM) shifted the NA and Ca2+ concentration-response curves to the left with potentiation of the maximal contraction. However, (+)-Bay K 8644 (0.1 microM) and nifedipine (10 nM) caused a shift to the right, with depression of the maximal contraction. The NA concentration-response curves, and those of Ca2+ in Ca(2+)-free high-K+ (55 mM) medium, were affected by the drugs to similar extents, and were not modified by the presence or absence of endothelial cells. The drugs tested did not affect resting tension. Basal 45Ca2+ uptake was not modified by either nifedipine or the Bay K 8644 enantiomers.(ABSTRACT TRUNCATED AT 250 WORDS)