Effects of vasodilators on cyclic contraction induced by 3,4-diaminopyridine in isolated porcine coronary arteries

3,4-Diaminopyridine (3,4-DAP), which is known to decrease K conductance, produced spontaneous repetitive phasic contractions of a regular (28/60) and an irregular (15/60) cycle or tonic contraction (16/60) following a latent period of 5-100 min in isolated porcine coronary arteries. Effects of pinacidil, a newly-synthesized vasodilator, were investigated using the preparation in which 3,4-DAP produced phasic contractions of the regular cycle in comparison with those of various vasodilators. Pinacidil produced dose-dependent prolongation of the cycle and reduced the peak tension and the tension at the relaxation phase, a mode of action that closely resembles that of nicorandil, suggesting the increase in K conductance and hyperpolarization. Nifedipine (10(-8) M) and dilazep (10(-4) M) markedly reduced the peak tension, while adenosine, dipyridamole and nitroglycerin did not produce such effects. The latter three drugs produced a prolongation of the cycle and reduced the tension of the relaxation phase. These data suggest that reduction of K conductance and activation of the voltage-dependent Ca channel may play an important role in initiation of the spontaneous repetitive phasic contraction in porcine coronary artery.     

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.     

Coronary vasodilator versus cardiac effects of MCI-176, a novel quinazolinone calcium antagonist, in the dog heart

We compared the coronary vasodilator and cardiac effects of MCI-176, a novel quinazolinone calcium antagonist, in isolated, blood-perfused sinoatrial (SA) node, atrioventricular (AV) node, and papillary muscle preparations of dogs. The drug was administered intraarterially. In SA node preparations MCI-176 reduced sinus rate and produced atrial standstill in large doses. In AV node preparations MCI-176 prolonged AV conduction time and produced second- or third-degree AV block in large doses only when administered into the artery supplying the AV node, but failed to affect AV conduction when administered into the artery supplying the His-Purkinje-ventricular system. In paced papillary muscle preparations MCI-176 reduced the force of contraction. In spontaneously beating papillary muscles MCI-176 failed to change the beating rate. MCI-176 increased blood flow in all preparations. The dose that doubled blood flow was slightly larger than the dose that produced a 15% increase in AV conduction time, but about one-third the dose that produced a 15% decrease in sinus rate. The dose estimated to reduce the force of contraction by half was more than approximately 10 times the dose that doubled blood flow. The results indicate that MCI-176 can be classified as a nonvasoselective calcium antagonist but that it differs from others.     

Effect of amlodipine, a new calcium antagonist on isolated blood vessels]

Amlodipine (AML), a new 1,4-dihydropyridine derivative, similar to nifedipine (NIF), dose-dependently relaxed or inhibited the KCL-contraction (cont.) of dog coronary artery (DCA) and rat aorta (RA) and the spontaneous motility of rat portal vein (RPV). However, in contrast to the case of NIF, the maximum effect was obtained at 1-2 hr following an addition of AML, and the recovery of KCl-cont. after removal of AML was very slow. Similar effects of AML on KCl-cont. were observed in dog femoral artery (DFA), dog basilar artery (DBA) and rabbit aorta (RBA). The IC50 values of AML for producing the half-maximal effect were 6.5 x 10(-9) M, 6.5 x 10(-9) M, 1.1 x 10(-8) M, 1.7 x 10(-8) M and 4.8 x 10(-8) M in DCA, DFA, RPV, DBA and RBA, respectively. In these vessels, the potency of AML was slightly less than the potency of NIF. On the other hand, AML (10(-7), 10(-6) M) only slightly attenuated norepinephrine (NE)-induced cont. in DFA and RA. Based on the ratio of IC50 values in DFA, AML exhibited 846-fold higher selectivity toward KCl-cont. than NE-cont., and it was much higher than the selectivity of NIF. In calcium-free medium, AML failed to inhibit the NE-cont. in RA, but it markedly inhibited CaCl2-induced cont. These results indicate that AML significantly inhibits KCl-cont. in blood vessels at lower doses with slightly less potency than NIF, but with a higher selectivity than NIF in comparison with the potency against NE-cont., and they also confirmed that the effect of AML is of slow onset and long-lasting.     

Voltage- and use-dependent block of the inward calcium current by MCI-176, a new non-dihydropyridine calcium antagonist, in canine ventricular muscles and single ventricular cells of the guinea-pig

Effects of MCI-176, 2-(2,5-dimethoxyphenylmethyl)-3-(2-dimethylaminoethyl)-6-isopro pox y-4(3H)- quinazolinone hydrochloride, on action potentials of canine ventricular muscles and on membrane currents of single ventricular cells of the guinea-pig heart were studied with the microelectrode and the patch-clamp ("whole-cell recording") methods. In canine ventricular trabeculae, MCI-176 (10(-5)-10(-4) M) decreased the plateau potential, the action potential duration at 30%-repolarization and the maximum rate of rise of the action potential; and it also decreased the amplitude and the duration of the slow response action potential in a concentration-dependent manner. Those effects were much more apparent at higher stimulus frequency. Under voltage clamp condition of single ventricular cells of the guinea-pig heart, MCI-176 (3 x 10(-5) M) decreased the inward calcium current (ICa) by 25-30% when the membrane potential was held at the resting membrane potential, and the drug abolished it when the membrane potential was held at -30 mV. MCI-176 added at rest decreased ICa ("initial block") and reduced it further with repetitive depolarizations in a beat-to-beat fashion. MCI-176 facilitated the reduction of ICa by increasing the clamp pulse frequency. These results indicate that MCI-176 decreases ICa of mammalian ventricular muscles in a voltage- and use-dependent manner.     

Relaxant effect of trans-resveratrol on isolated porcine coronary arteries

Recent studies provided evidence that trans-resveratrol (3,4',5-trihydroxystilbene, found in high concentrations in some red wines, may possibly decrease the risk of coronary heart disease mortality. The aim of this study, performed with large epicardial porcine coronary arteries (PCA) strips, was to investigate the relaxant effect of trans-resveratrol on these main conductance vessels, which have been described to be pathologically prone for vasospastic contractions. The data show that the tonic component of the biphasic contractions induced by histamine, as well as the contractions induced by F- ions (10 mmol/l), which activate G proteins downstream of the receptors, could dose-dependently be inhibited by trans-resveratrol (0.1-100 mumol/l). The EC50 values of the dose-response curves established for the inhibition of the sustained component of histamine-induced contractions were very similar to those obtained for the relaxations of fluoride-induced contractions: 0.45 +/- 0.08 and 0.29 +/- 0.05 mumol/l, resp. (n = 6). Ouabain (10 mumol/l)-induced contractions and rhythmic contractions elicited by tetraethylammonium (12 mmol/l) were also strongly inhibited by trans-resveratrol (20 mumol/l). It may be inferred from the results obtained in this study, that the relaxation of the coronary conductance vessels induced by trans-resveratrol is possibly based on a nongenomic interaction with steroid-like receptors located on the cell membrane.     

Pharmacological activity of the new calcium antagonist,lacidipine, on isolated preparations

• 1.1. The calcium-channel blocking activity of lacidipine has been studied compared with that of nifedipine and verapamil on the isolated rabbit heart and aorta.
• 2.2. All the compounds induced a dose-dependent negative inotropic effect (10⁻⁸-10⁻⁵ M); although lacidipine showed less, but longer lasting, activity.
• 3.3. Lacidipine showed a weak negative chronotropic effect and nifedipine was ineffective. Only verapamil strongly decreased the heart rate.
• 4.4. The three calcium antagonists abolished vasopressin-induced coronary spasm and inhibited partially metoxamine-induced coronary spasm. Only lacidipine reduced basal coronary pressure.
• 5.5. In the aortic strips, all the compounds antagonized KC1-induced contractions, and they exerted a partial effect on noradrenaline-and angiotensin II-induced contractions.

     

Effects of semotiadil fumarate, a novel Ca2+ antagonist, on cytosolic Ca2+ level and force of contraction in porcine coronary arteries.

1. The mechanisms of action of semotiadil fumarate, a novel Ca2+ antagonist, were examined by measuring the cytosolic Ca2+ level ([Ca2+]i) and force of contraction in porcine coronary arteries, and by determining [3H]-pyrilamine binding to bovine cerebellar membranes. 2. Semotiadil or verapamil (0.1 and 1 microM) inhibited both the high KCl-induced increases in [Ca2+]i and force in a concentration-dependent manner. 3. Histamine (30 microM) produced transient increases followed by sustained increases in [Ca2+]i and force, which were inhibited by semotiadil and verapamil (1 and 10 microM). The agents were different in that semotiadil reduced the maximum [Ca2+]i and force responses to histamine, but not pD2 values, whereas verapamil did reduce the pD2 values for histamine, but not the maximum responses. 4. Verapamil (10 microM), but not semotiadil, inhibited histamine-induced increases in [Ca2+]i and force in Ca(2+)-free solution. Neither semotiadil nor verapamil affected the increases in [Ca2+]i and force induced by caffeine. Semotiadil even at the higher concentration (10 microM) did not displace specific binding of [3H]-pyrilamine to bovine cerebellar membranes. 5. These results suggest that semotiadil inhibits both KCl- and histamine-induced contractions mainly by blocking voltage-dependent L-type Ca2+ channels.     

Effect of somatostatin on rabbit isolated coronary arteries

Somatostatin analogues are capable of inhibiting vascular smooth muscle and endothelial cell proliferation. However, little is known about the effect of somatostatin on vascular responses in endothelium-denuded coronary arteries in vitro. The aim of this work was to determine whether or not somatostatin prevented the contractile response induced by 5-hydroxytryptamine and acetylcholine in endothelium-denuded rabbit coronary arteries. Somatostatin attenuated the contraction produced by 5-hydroxytryptamine in both proximal (PC) and distal coronary (DC) arteries (contraction induced by 10(-4) M 5-hydroxytryptamine was inhibited by 10(-6) M somatostatin by 90.8 +/- 11.0% (P < 0.001, n = 9) and by 46.2 +/- 14.0% (P < 0.05, n = 9) in DC and PC, respectively), but concentration-dependently decreased the contraction induced by U46619 (11alpha-epoxy-methanoprostaglandin F2alpha) only in PC arteries, suggesting that the response of PC and DC arteries to somatostatin were qualitatively different. Furthermore, we suggest that somatostatin may enhance acetylcholine-induced relaxation by combination of increasing endothelium-dependent relaxation (by a NO-dependent mechanism) and blocking contraction at the muscle level.     

Effect of calcium antagonist darodipine on the isolated human heart

Darodipine is a new light stable dihydropyridine calcium antagonist. In isolated large conductance vessels from experimental animals, darodipine blocks the potential-dependent but not the receptor-operated calcium channel. We studied the effects of darodipine in isolated human epicardial coronary arteries and in isolated right ventricular trabecula obtained from the explanted hearts of patients undergoing cardiac transplantation. In the coronary artery, the PD'2 for calcium was 9.7 and for carbachol was 9.5. In the cardiac muscle, the PD'2 for calcium was 6.5. Using 45Ca2+, darodipine blocked uptake in presence of potassium and histamine. We conclude that darodipine is (1) a potent calcium antagonist in the isolated human coronary artery, but not in ventricular myocardium, thus showing very pronounced target tissue selectivity, and (2) in contrast to findings obtained in rabbit aorta, it inhibits contraction and 45Ca2+ uptake mediated by both the potential-dependent and receptor-operated channels in the human epicardial coronary artery.     

Effects of SAS 1310, a new calcium antagonist, on isolated myocardial and smooth muscle preparations

Some features of the effects of the diltiazem derivative SAS 1310 on in vitro myocardial and smooth muscle preparations were compared to those of the effects of diltiazem. Left atria, aortic strips and taenia coli of guinea-pigs were used. SAS 1310 induced a negative inotropic response of the left atria driven at 1 Hz similar to the response to diltiazem (IC50 values: SAS 1310 1.34 microM, diltiazem 0.8 microM). The inotropic effect of diltiazem (5 microM) was clearly rate-dependent whereas the reduction of left atria contractility induced by SAS 1310 (5 microM) was not modified by changes of the stimulation rate (the range of frequencies used was 0.5-2 Hz, with stepwise changes of 0.5 Hz). Diltiazem (0.1-0.5 microM) was more effective than SAS 1310 (0.1-5 microM) in inhibiting the contractile response to calcium of taenia coli depolarized by high K+ as well as in relaxing the aortic strips contracted by high K+ (IC50 SAS 1310 12.3 microM, diltiazem 0.41 microM). The response of aortic strips to norepinephrine (50 microM) in Ca2+-free medium was inhibited by SAS 1310 (50 microM) and was not affected by diltiazem (2 microM). The drug concentrations used were equiactive in inhibiting the high K+-induced contraction of the aortic strips. The different effects of diltiazem and its derivative on left atria contraction at different force-frequency ratios and on aortic strip contraction induced by norepinephrine in a Ca2+-free medium suggest that the actions of the two drugs differ qualitatively.     

Cardiac versus coronary vasodilator actions of KB-944, a new calcium antagonist, assessed in isolated, blood-perfused heart preparations of dogs

We compared the cardiac and coronary vasodilator actions of a new calcium-antagonistic vasodilator, KB-944, in isolated, blood-perfused heart preparations of dogs. In all preparations KB-944 injected intra-arterially produced an increase in blood flow. In sinoatrial (SA) node preparations KB-944 decreased sinus rate and in large doses produced atrial standstill. In atrioventricular (AV) node preparations KB-944 increased AV conduction time and in large doses produced second- or third-degree AV block only when injected into the artery supplying the AV node. In the same preparations KB-944 had virtually no effect on AV conduction when injected into the artery supplying the His-Purkinje-ventricular system. In papillary muscle preparations KB-944 in medium and large doses depressed force of contraction, the depressant action being greater at high rates of contraction. In the same kind of preparations KB-944 affected automaticity slightly and inconsistently. Depression by KB-944 of SA nodal automaticity and AV nodal conduction occurred pari passu with coronary vasodilation, whereas force of contraction was depressed to a lesser extent in coronary vasodilator doses. In these respects, KB-944 resembles verapamil and diltiazem rather than nifedipine and nicardipine.     

A new experimental approach in endothelium-dependent pharmacological investigations on isolated porcine coronary arteries mounted for impedance planimetry.

1. The aim of this study was to investigate whether the balloon-based impedance planimetry technique could be a useful tool in endothelium-dependent investigations. 2. Porcine large coronary arteries contracted with prostaglandin F2alpha (PGF2alpha, 10 microM) did not relax to bradykinin (0.1 nM - 0.1 microM), but did relax to sodium nitroprusside (SNP, 10 microM). However, after eversion of the segments, bradykinin induced relaxations with pD2 values and maximal responses of 8.78+/-0.09 and 75+/-2% (n=6), respectively. 3. Incubation with captopril (1 microM) did not reveal a relaxation to bradykinin in the normal vessel configuration and had no influence on the concentration-relaxation relationship in everted segments. 4. Lowering the luminal pressure in contracted segments from 131+/-5 mmHg (isometric, n=5) to 60 mmHg (isobaric, n=5) did not facilitate the action of bradykinin. 5. Eversion of segments did not influence the concentration-response relationship for K+ (4.7 - 125 mM), PGF2alpha (0.3 - 30 microM), and SNP (30 nM - 30 microM), although the time-courses of responses were faster when the agents were added from the intimal compared to the adventitial side of the preparation. 6. In the same everted segment contracted with PGF2alpha, the concentration-response relationship for bradykinin was not different under isometric and isobaric conditions. 7. These results indicate that, (1) reduced endothelium-dependent relaxations to adventitially administered substances can be ascribed to a diffusion barrier in the vessel wall, while enzymatic degradation, luminal pressure and precontractile responses seem not to play a role, (2) impedance planimetry applied to everted cylindrical segments could be a useful experimental approach in pharmacological studies of endothelium-dependent responses under isobaric and isometric conditions.     

Effect of FR 7534, a new calcium antagonist, on myocardial oxygen demand.

The actions of FR 7534, a new calcium antagonist, nitroglycerin, and dipyridamole on myocardial oxygen demand have been compared. Six anaesthetized dogs received two infusion levels of each drug which produced equivalent arterial hypotension. All three compounds reduced the tension-time index (TTI). FR 7534 and nitroglycerin but not dipyridamole produced significant decreases in myocardial oxygen consumption (MVO2). FR 7534 demonstrated oxygen-sparing actions comparable to nitroglycerin. FR 7534 may prove beneficial in alleviating myocardial ischemia.     

Effect of a new calcium antagonist, tiapamil, in hypertension of the elderly.

The antihypertensive effect of a single oral dose of tiapamil (450 mg) and placebo were compared in a single blind randomized cross-over study in 10 71-86 year old hypertensive patients. Blood pressure (BP) and heart rate (HR) were recorded every 15 min for 12 h by an automatic device. Tiapamil led to a decrease in mean daytime systolic (SBP) and diastolic (DBP) BP from 171 +/- 12/98 +/- 10 mm Hg to 159 +/- 11/90 +/- 9 mm Hg (P less than 0.001) without significant variation in HR. Thereafter patients received tiapamil 450 twice daily; by the seventh day of treatment mean daytime SBP and DBP were 155 +/- 13/85 +/- 14 mm Hg (P less than 0.001 vs placebo). The hourly mean values of SBP recorded for 8/12 h (first tiapamil day) and 10/12 h (seventh tiapamil day) were significantly lower than the corresponding values after placebo. We conclude that tiapamil in the elderly exerts a sustained antihypertensive effect lasting 12 h or more, with only minor variations in HR. This effect predominates on systolic pressure and is significant from the first dose.     

Hemodynamic effects of a new calcium antagonist, bepridil, in patients with coronary artery disease

Hemodynamic Effects of the New Calcium Antagonist Bepridil in Patients with Coronary Artery Disease. For the evaluation of the hemodynamic effects of the new calcium antagonist bepridil 11 patients with coronary artery disease were studied. The following parameters were measured before, immediately after and 20 min after injection of 3 mg/kg bepridil i.v.: pulmonary capillary wedge pressure, pulmonary artery pressure, cardiac output, mean aortic pressure, peripheral vascular resistance, prolongation of the QT-interval in the ECG. Bepridil led acutely to a slight deterioration of the left ventricular function with a significant increase of the preload, but this negative effect was neutralized by the concomitant decrease of the afterload. These results indicate a favorable hemodynamic profile of this new agent.     

Effects of S-312, a new calcium antagonist, on the mechanical and electrophysiological responses of isolated cardiovascular preparations

S-312, a new calcium antagonist with a bicyclic dihydrothienopyridine structure, potently relaxed the helical strips of various isolated rabbit arteries precontracted with high K+-depolarization, serotonin (5-HT) and U46619 (thromboxane A2 analogue), and it competitively inhibited Ca++-induced contractions in depolarized basilar and femoral arteries. These effects of S-312 were more potent than nifedipine and almost comparable to or slightly more potent than those of nicardipine. In comparison with nifedipine and nicardipine, the calcium antagonistic effect and the relaxant effect on 5-HT-induced contractions of S-312 were most prominent in the basilar artery. The potent vasodilating action of S-312 in the high K+-depolarized basilar artery was not easily reversed by washing. S-312 did not affect Ca++-induced contraction in the skinned fiber of guinea pig taenia caecum. The negative inotropic effect of S-312 in isolated guinea pig left atria was much less potent than those of nifedipine and nicardipine. S-312 above 10(-7) M preferentially increased AV nodal conduction time in Langendorff-perfused isolated rabbit hearts; and above 3 x 10(-8) M, it mainly decreased the maximum upstroke velocity of the action potential in isolated rabbit sinus node preparations. In summary, the present results indicate that S-312 is a potent new calcium antagonist possessing vasculoselectivity, especially for cerebral vessels.     

Comparison of the effects of KRN2391 and other coronary dilators on porcine isolated coronary arteries of different sizes

Abstract— The present study was performed to determine whether KRN2391 (N-cyano-N′-(2-nitroxyethyl)-3-pyridinecarboximidamide monomethanesulphonate), a novel vasodilator, shows different effects on porcine isolated coronary arteries of different sizes. The vasodilating effects of KRN2391 on porcine large (2·5–3·0 mm outer diam.) and small (0·8–1·0 mm) coronary arteries were also compared with those of cromakalim, nicorandil, nifedipine, nitroglycerin and adenosine. The relaxant effects of these drugs were examined in coronary arteries contracted by 25 Mm KCl. Nitroglycerin caused greater relaxation in large vessels than in small vessels. In contrast, adenosine, nifedipine and cromakalim caused greater relaxation in small vessels. However, there was no difference between large and small vessels in the relaxing effects of KRN2391 and nicorandil. These unique features of KRN2391 and nicorandil appear to be beneficial in ischaemic heart disease.     

钙增敏剂MCI-154对内毒素血症大鼠心肌收缩系统钙敏感性的影响(英文)

目的:研究MCI-154对内毒素血症大鼠心肌收缩系统钙敏感性的影响.方法:利用皂素500 mg/L制备内毒素血症大鼠蜕膜右心室乳头状肌标本,用不同钙浓度的含或不含强心药物的激活液进行顺序激活,记录Ca~(2 )激活张力.结果:同正常对照组相比,内毒素血症大鼠蜕膜乳头状肌钙最大激活张力(T_(max))降低,pCa_(50)值下降.甲腈吡酮50μmol/L对上述异常无明显的改善作用.内毒素血症大鼠蜕膜乳头状肌经含MCI-154 10 μmol/L的激活液处理后,T_(max)和pCa_(50)值明显增加,与假休克组类似,明显高于内毒素血症组和内毒素血症 甲腈吡酮组值.MCI-154的上述作用还具有剂量依赖性.结论:大鼠内毒素血症后,心肌收缩系统对Ca~(2 )敏感性降低,MCI-154可明显增加内毒素血症大鼠心肌收缩蛋白对钙的敏感性,增加心肌钙最大激活张力.     

Effect of calcium on the vascular contraction induced by cobra venom cardiotoxin

1. The cytotoxic effects of cardiotoxin (CTX) purified from Cobra venom were tested in endothelium-denuded rat aortic ring preparations in tissue organ baths and the effect of extracellular Ca2+ on the cytotoxic effect of CTX was investigated using a digital dynamic calcium imaging technique. 2. At 10 micromol/L, CTX induced a slowly developing and sustained contraction that amounted to approximately 50% of the maximal contraction induced by 80 mmol/L KCl. At high concentrations (> 15 micromol/L), CTX caused irreversible damage to the smooth muscle contractile function. However, washout of CTX at its peak contraction did not affect the subsequent contraction to either KCl or phenylephrine. 3. Contraction induced by CTX was dependent on the Ca2+ concentration in the external solution. A maximal contractile response to CTX was obtained in medium containing 1-2.5 mmol/L Ca2+. This contractile response induced by CTX decreased with higher Ca2+ concentrations and was completely diminished when 7 mmol/L Ca2+, 3 mmol/L Ni2+ or 30 micromol/L tetrandrine (a non-selective calcium channel blocker) was present in the external solution before addition of CTX to the bath. 4. The above observations were supported by the calcium imaging work performed with cultured aortic smooth muscle cells from Wistar-Kyoto rats, in which CTX was shown to induce the elevation of cytosolic Ca2+ in the presence, but not in the absence, of 2.5 mmol/L extracellular Ca2+. Increasing the extracellular Ca2+ concentration to 7 mmol/L, the addition of 3 mmol/L Ni2+ or inclusion of 30 micro mol/L tetrandrine inhibited the elevation of cytosolic Ca2+ induced by CTX. 5. These results suggest that: (i) a CTX-sensitive internal calcium store does not exist in rat aortic smooth muscle; (ii) the contractile effect CTX is associated with a Ca2+ influx process; and (iii) CTX interacts extracellularly with the plasma membrane at the level of the calcium channels, as well as anionic sites to which Ca2+ and other inorganic cations bind.