Selective modulation by membrane potential of the interaction of some calcium entry blockers with calcium channels in rat mesenteric artery

Endothelin-1 and endothelin-2 initiate and maintain contractile responses by different mechanisms in rat mesenteric and cerebral arteries

BACKGROUND AND PURPOSE

Endothelin (ET)-1 and ET-2 cause potent long-lasting vasoconstrictions by tight binding to smooth muscle ETA receptors. We tested the hypotheses that different mechanisms mediate initiation and maintenance of arterial contractile responses to ET-1 and ET-2 and that this differs among vascular beds.

EXPERIMENTAL APPROACH

Segments of rat mesenteric resistance artery (MRA) and basilar artery (BA) were studied in wire myographs with and without functional antagonists.

KEY RESULTS

Sensitivity and maximum of MRA contractile responses to ET-1 were not, or only moderately, reduced by stimulation of soluble GC, AC or K(+) -channels and by an inhibitor of receptor-operated ion channels. However, each of these reduced maintenance of ET-1 effects and relaxed ET-1-induced contractions in MRA. A calcium channel antagonist did not alter sensitivity, maximum and maintenance of ET-1 effects, but relaxed ET-1-induced contractions in MRA. A PLC inhibitor prevented contractile responses to ET-1 and ET-2 in MRA and BA, and relaxed ET-1- and ET-2-induced responses in MRA and ET-1 effects in BA. A Rho-kinase inhibitor did not modify sensitivity, maximum and maintenance of responses to both peptides in both arteries but relaxed ET-2, but not ET-1, effects in MRA and ET-1 effects in BA.

CONCLUSIONS AND IMPLICATIONS

PLC played a key role in arterial contractile responses to ETs, but ET-1 and ET-2 initiated and maintained vasoconstriction through different mechanisms, and these differed between MRA and BA. Selective functional antagonism may be considered for agonist- and vascular bed selective pharmacotherapy of ET-related diseases.

Antioxidant effects and the therapeutic mode of action of calcium channel blockers in hypertension and atherosclerosis

Drugs currently known as calcium channel blockers (CCB) were initially called calcium antagonists because of their ability to inhibit calcium-evoked contractions in depolarized smooth muscles. Blocking the entry of calcium reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological property has been the basis for the use of CCBs in the management of hypertension and coronary heart disease. A major question is whether drugs reducing blood pressure have other effects that help prevent the main complications of hypertension, such as atherosclerosis, stroke, peripheral arterial disease, heart failure and end-state renal disease. Experimental studies that focus on this question are reviewed in the present paper.

Dihydropyridine enantiomers block recombinant L-type Ca2+ channels by two different mechanisms

1. The molecular basis of the state-dependent block of L-type Ca2+ channels by dihydropyridines is still poorly understood. Therefore, we studied the enantioselectivity of Ca2+ channel block by isradipine enantiomers at three holding potentials (-80, -60 and -40 mV) in Chinese hamster ovary (CHO) cells stably expressing the rabbit lung alpha1C-b-subunit. 2. The extent of enantioselectivity did not markedly change with the holding potential (IC50 ratios of 104-138), whereas the potency of both isradipine enantiomers increased with depolarisation of the holding potential. 3. In addition to its block of the peak Ca2+ channel current, Ipeak, (-)-isradipine inhibited the relative current at the end of the test pulse, the so-called Ilate, normalised to Ipeak (Ilate/Ipeak). This effect was unaffected by the holding potential and revealed distinct kinetics compared to the development of conventional block of Ipeak. 4. When these effects were studied using an alpha1C-b-mutant lacking the high-affinity dihydropyridine binding site, expressed in human embryonic kidney (HEK 293) cells, both enantiomers blocked Ilate/Ipeak to a similar degree. 5. Our data are discussed within the framework of the 'guarded receptor' and the 'modulated receptor' hypotheses. The very different properties of the block of Ilate/Ipeak compared to those of the conventional high-affinity block of Ipeak suggest the existence of an additional mechanism possibly mediated via a second, distinct binding site.

Pharmacological characterisation of Ca2+ channels of the L-type in human peripheral blood lymphocytes

Ca2+ channels of the L-type were characterised in intact human peripheral blood lymphocytes using a radioligand binding technique and the dihydropyridine-type Ca2+ channel antagonist [3H](+)-PN 200-110 (isopropyl-4-(2,1,3-benzoxadiazol-4-yl)1,4-dihydro-5-methoxycarbon yl-2, 6-dimethyl-3-pyridine carboxylate) as a ligand. [3H](+)-PN 200-110 binding to human peripheral blood lymphocytes was time-, temperature-, concentration-dependent and of high affinity. The dissociation constant (Kd) value was 0.4 +/- 0.02 nM and the maximum binding capacity (Bmax) was 33.5 +/- 1.6 fmol/10(6) cells. Pharmacological analysis of [3H](+)-PN 200-110 binding to human peripheral blood lymphocytes was consistent with the labelling of a Ca2+ channel of the L-type. In fact, dihydropyridine derivatives were the most potent competitors of [3H](+)-PN 200-110 binding, whereas phenylalkylamine and benzothiazepine compounds or non-selective Ca2+ channel modulators were weak or ineffective displacers. These findings are the first observation that human peripheral blood lymphocytes express Ca2+ channels of the L-type. The possibility that Ca2+ channel antagonists may interfere with immune system function is discussed.

Localization of calcium channels of the L-type in human epicardial arteries: a light microscope autoradiographic study

The anatomical localization of Ca2+ channels of the L-type was analyzed in sections of the human right and anterior interventricular coronary arteries by using in vitro light microscope autoradiography associated with radioligand binding techniques. [3H]Nicardipine was utilised as a ligand. Binding of the radioligand to sections of the two coronary arteries was time-, temperature- and concentration-dependent. Analysis of binding isotherms revealed a dissociation constant value of about 0.5 nM in the two arteries and maximum binding capacities of 139 +/- 6.4 fmol/mg tissue for the right coronary artery and of 173 +/- 9.5 for the anterior interventricular branch. The pharmacological profile of [3H]nicardipine binding to sections of human coronary arteries was consistent with the labelling of Ca2+ channels of the L-type. Dihydropyridine derivatives were the most powerful competitors of [3H]nicardipine binding, whereas phenylalkylamines, benzothiazepine or non-selective channel modulators were weak competitors or ineffective. Light microscope autoradiography revealed the highest density of [3H]nicardipine binding sites in the tunica media of the coronary arteries. In this layer Ca2+ channels of the L-type are located within smooth muscle cells. A lower accumulation of the radioligand occurred in the tunica adventitia, whereas no specific binding was found in the tunica intima. Study of the localization of Ca2+ channels in sections of human coronary arteries may contribute to a better understanding of the mechanism of the marked coronary dilatory activity elicited by Ca2+ antagonists demonstrable in both in vitro preparations and in vivo.

Effects of nifedipine and Bay K 8644 on myotropic responses in aortic rings of pregnant rats

The hypothesis that Ca2+ channel function is altered during pregnancy was tested by comparing responses to potassium chloride (KCl) and phenylephrine in aortic rings of virgin and term-pregnant rats under the influence of nifedipine and Bay K 8644. Maximum response to KCl was progressively reduced by increasing nifedipine concentrations (1.0-100 nM) in both groups of tissues. Nifedipine produced a smaller inhibition of KCl-induced contraction in aortic rings of pregnant than of virgin rats. It exerted little inhibition on the concentration-response curve to phenylephrine. The Ca2+ channel antagonist (100 nM) reduced the maximum response to the alpha-adrenoceptor agonist in rings from virgin rats, but had no effect in pregnant rats. Bay K 8644, a Ca2+ channel activator, potentiated the responses to low concentrations of both phenylephrine and KCl in the tissues of both virgin and pregnant rats, but did not affect maximum responses. It also induced concentration-dependent contractions in rings of virgin but not of pregnant rats. The effects of Bay K 8644 were markedly potentiated by precontracting the aorta with 10mM KCl. Nevertheless tissues from pregnant rats were still less responsive to Bay K 8644. However, when the strips were precontracted to the same level by different concentrations of KCl, the concentration-response curves to Bay K 8644 were identical in both groups. [3H]Nitrendipine binding to membrane preparations of the thoracic aorta was similar in virgin and pregnant rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Why is nisoldipine a specific agent in ischemic left ventricular dysfunction?

Nisoldipine is a dihydropyridine calcium entry blocker that inhibits contraction of vascular smooth muscle with a potency that is 2-3 times greater than its impact on myocardial contractility. In isolated human coronary arteries, tonic contractions induced by serotonin are inhibited by nisoldipine with a potency 10 times greater than that in internal mammary arteries and 1,000 times greater than that in electrically driven myocardium. In contrast, nifedipine had little effect and verapamil and diltiazem had none. In this article an hypothesis is reviewed that relates vascular smooth muscle selectivity to membrane potential sensitivity. Nisoldipine's effect on calcium channel binding and blocking is enhanced by the degree of depolarization of the cell membrane. Verapamil and diltiazem are not membrane-potential sensitive. Vascular smooth muscle cells are more depolarized than myocardial cells, and human coronary arteries have a particularly small membrane potential. Thus, the potency of nisoldipine in these organs parallels the degree of membrane depolarization. This may then suggest ischemia selectivity, since membrane depolarization occurs in ischemic tissue. Nisoldipine might therefore have a potent negative inotropic effect and an enhanced vasodilator action in ischemic myocardium, yet leave normoxic regions functionally intact. Some experimental evidence is discussed.

Calcium antagonists and vasodilatation

Calcium antagonists comprise a diverse group of chemically unrelated agents that interact with voltage-operated calcium channels (L-type) and thereby inhibit smooth muscle contractility. They are used to treat several major cardiovascular disorders, including hypertension and angina pectoris; they are also studied in congestive heart failure and in atherosclerosis. The current view is that their therapeutic action is related to vasodilatation. This view is an oversimplification, as will be shown in this review. It will also be illustrated that all calcium antagonists are not identical pharmacological agents.

Cardioselectivity of calcium antagonists

Calcium antagonists comprise a diverse group of chemically unrelated agents that interact with voltage-operated calcium channels (L-type) and thereby inhibit smooth muscle and cardiac contractility. Although they interact with the alpha 1 subunit of voltage-operated calcium channels, all calcium antagonists are not identical pharmacological agents. They are not only different from a chemical point of view, but also because some of them exhibit tissue selectivity, being more powerful blockers of the contraction of arteries than of cardiac muscle. The current view that their major therapeutic action is related to vasodilation is an oversimplification, as their action is more complex and may be related to factors other than hemodynamic ones.

Autoradiographic localization of [3H]nicardipine binding sites in the human renal artery

In the present study the pharmacological profile and the anatomical distribution of dihydropyridine-type Ca2+ channels were analyzed in sections of the human renal artery by the use of combined radioligand binding and autoradiographic techniques with [3H]nicardipine as a ligand. The binding of [3H]nicardipine to sections of renal artery was time-, temperature- and concentration-dependent belonging, at least in the range of radioligand concentrations used, to a single class of high-affinity binding sites. The dissociation constant (KD) value was 0.3 nM and the maximum density of binding sites (Bmax) was 248 +/- 16 fmol/mg tissue. The pharmacological profile of [3H]nicardipine binding to sections of human renal artery was consistent with the labeling of dihydropyridine-type Ca2+ channels. In fact, dihydropyridine derivatives were the most powerful competitors of [3H]nicardipine binding, whereas phenylalkilamine, benzothiazepine or non-selective channel modulators were weak or ineffective competitors. Light microscope autoradiography revealed the highest density of [3H]nicardipine binding sites in the tunica media of the renal artery, probably within smooth muscle cells. A smaller accumulation of the radioligand occurred in the tunica adventitia, whereas the tunica intima did not show specific binding. These results indicate that light microscope autoradiography techniques associated with radioligand binding may represent a useful tool for analyzing the localization of receptors or targets of drug action within the arterial wall.

Radioligand and functional estimates of the interaction of the 1,4-dihydropyridines, isradipine and lacidipine, with calcium channels in smooth muscle

1. The present experiments were undertaken in order to characterize further the apparently irreversible inhibition of the contraction of depolarized rat aorta caused by lacidipine, a 1,4-dihydropyridine calcium antagonist. 2. We studied the effect of lacidipine on contraction evoked by 100 mM KCl solution in rat aorta, treated by N omega-nitro-L-arginine (0.1 mM), an inhibitor of nitric oxide (NO) synthesis. We compared the effect of prolonged depolarization on lacidipine and (+)-isradipine inhibition and the reversal of this inhibition after washout in the absence of dihydropyridines. Assuming that the onset of lacidipine-evoked inhibition was a pseudo-first order association kinetics, we estimated the dissociation rate constant (k-1 = 0.031 min-1), the association rate constant (k1 = 2.70 x 10(8) M-1 min-1) and the dissociation constant (KD = k-1/k1 = 115 pM) which was close to the IC50 value in steady-state conditions (160 pM). 3. The inhibitory effects of lacidipine and (+)-isradipine on rat aorta contraction were reversibly enhanced after preincubation with the drug in a 40 mM KCl-solution. Washout with drug-free 40 mM K(+)-depolarizing solution reversed inhibition in the (+)-isradipine-treated preparations, but not in the lacidipine-treated ones. 4. Radioligand binding studies were performed with [3H]-lacidipine and [3H]-isradipine in microsomes from rat aorta and rat ileum. Both ligands bound to a homogeneous population of binding sites (for[3H]-lacidipine: KD = 23 +/- 2.6 pM, Bmax = 380 +/- 21 fmol mg-1 protein in membranes from aorta; KD =23 +/- 3.1 pM, Bmax = 790 +/- 60 fmol mg-1 protein in membranes from ileum; for [3H]-isradipine:KD = 140 +/- 46 pM, Bmax = 350 +/- 64 fmol mg-1 protein in membrane from aorta; KD = 68 +/- 14 pM,Bmax = 760 +/- 75 fmol mg-1 protein in membranes from ileum). After isotopic dilution, [3H]-lacidipine and [3H]-isradipine dissociated according to a monoexponential kinetics. In membranes from ileum, the calculated dissociation rate constants (kappa_ 1) were 0.0257 min-1 and 0.0595 min-1, for [3H]-lacidipine and[3H]-isradipine, respectively.5. The non specific binding of [3H]-lacidipine and [3H]-isradipine, was measured in intact rat aorta preparations incubated under the conditions of the functional experiments, in the presence of nifedipine(1 microM). After incubation with [3H]-lacidipine 77.6 +/- 1.9 pM for 2 h the concentration of drug in the tissue was 15.15 +/- 1.18 fmol mg-1 w.wt. and still amounted to 7.24 +/- 0.61 fmol mg-1 w.wt. after 3.5 h washout in drug-free solution. After incubation with [3H]-isradipine 47.2 +/- 0.4 pM for 2 h it was 2.26 +/-0.07 fmol mg-1 w.wt. and was undetectable after 3.5 h washout in a drug-free solution.6. It is concluded that lacidipine interacts reversibly with dihydropyridine binding sites and that the apparent irreversible inhibition of contraction in depolarized preparations could be related to a nonspecific binding in a tissue compartment different from the plasma membrane.

Effects of (S)-nafenodone on 45Ca2+ fluxes and contractions in rat isolated vascular smooth muscle

The effects of (S)-nafenodone, a new antidepressant, were studied on contraction and 45Ca2+ fluxes in rat vascular smooth muscle. In isolated rat aorta (S)-nafenodone, 10(-7) - 10(-4) M, inhibited the contractions induced by 80 mM KCl (IC50 = 1.4 +/- 0.4 x 10(-5) M) and 10(-5) M noradrenaline (IC50 = 1.2 +/- 0.2 x 10(-5) M). (S)-Nafenodone relaxed the contractions induced by both high K+ and noradrenaline, this effect being independent of the presence of functional endothelium. It also inhibited the contractions induced by addition of CaCl2 to Ca(2+)-free high-K+ solution IC50 = 2.5 +/- 0.9 x 10(-6) M) and the phasic contractions induced by noradrenaline in Ca(2+)-free medium, but was a very weak relaxant of the contractions induced by phorbol 12-myristate-13-acetate in Ca2+-free medium. In addition, (S)-nafenodone inhibited the spontaneous mechanical activity in portal vein segments (IC50 = 1.4 +/- 0.8 x 10(-6) M). (S)-Nafenodone inhibited the 45Ca2+ uptake stimulated by high KCl or noradrenaline without altering 45Ca2+ uptake in resting strips and decreased the net 45Ca2+ content in aortic strips non-stimulated as well as stimulated by noradrenaline. In conclusions, (S)-nafenodone inhibited voltage- and agonist-stimulated Ca2+ entry in isolated rat aortas. In addition, it decreased Ca2+ content in both resting and noradrenaline-stimulated muscles, suggesting that it may deplete noradrenaline-sensitive intracellular Ca2+ stores. As a consequence, (S)-nafenodone would reduce the concentration of intracellular free Ca2+ available at the contractile apparatus for vascular smooth muscle contraction.

Activity of dihydrothienopyridine S312 enantiomers on L-type Ca2+ channels in isolated rat aorta and cerebral microvessels

The activity of the two enantiomers of the dihydrothienopyridine S312 was characterized in isolated rat aorta and cerebral microvessels. The interaction of S312 with 1,4-dihydropyridine and phenylalkylamine binding sites was also investigated in depolarized rat cerebral microvessels and in membranes from rat ileum. Both S-(+)-S312 and R-(-)-S312 dose dependently inhibited KCl-evoked contraction of the rat aorta, with IC50 values of 0.14 (0.13-0.16) and 2.98 (2.67-3.33) nM, respectively. When the aorta was preincubated with S-(+)-S312 in a depolarizing medium, the inhibitory effect was significantly increased, but this increased inhibition was not reversed by incubation in physiological medium. The effect of R-(-)-S312 was not affected by preincubation in a depolarizing medium. In rat cerebral microvessels, S-(+)-S312 inhibited the KCl-induced contraction and KCl-stimulated Ca2+ influx with similar potency. [3H](+)-PN 200-110 specific binding was competitively displaced by the two enantiomers in depolarized cerebral microvessels. The calculated Ki values were 0.12 nM for S-(+)-S312 and 2.4 nM for R-(-)-S312. Only 20% of [3H]D888 specific binding in rat ileum membranes was displaced by S-(+)-S312. The dissociation rate of [3H]D888 was markedly decreased by S-(+)-S312, and this allosteric interaction was significantly more marked than with nitrendipine. It is concluded that the dihydrothienopyridine S312 could interact with Ca2+ channels in a manner different to that of genuine dihydropyridines.

Binding sites for 1,4-dihydropyridine Ca(2+)-channel modulators in rat intestinal smooth muscle

The contractile response of intestinal smooth muscle to depolarization is characterized by a phasic and a tonic component which are differently sensitive to blockade by 1,4-dihydropyridines. As this difference in sensitivity could be related to different binding sites associated with distinct calcium channels, we analyzed the binding of the calciumantagonist 1,4-dihydropyridine (+)PN 200-110 [isopropyl-4-(2,1,3-benzodiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- methoxycarbonyl-pyridine-3-carboxylate] in longitudinal smooth muscle of the rat ileum. We carried out saturation binding experiments on intact tissue exposed to physiological and depolarizing (100 mmol l-1 K+) solution, and on different membrane fractions: the total microsomal fraction, the light microsomal fraction (enriched with plasma membranes) and the mitochondrial fraction. Binding of 3H(+)PN 200-110 to the intact longitudinal smooth muscle of rat ileum appeared to be voltage-dependent, KD decreased in depolarized tissue whereas Bmax was unchanged (change in membrane potential was assessed by measuring the distribution of 3H-tetraphenylphosphonium bromide). In membrane fractions two binding sites were detected, a high-affinity site associated with plasma membrane and a low-affinity site presumably associated with mitochondria (abundant in the fractions where the cytochrome c oxidase activity was high, and undetectable in the light microsomes poor in cytochrome c oxidase activity). The KD value of the high-affinity binding in isolated membrane fractions was similar to the KD value measured in intact depolarized tissue. The low affinity binding increased at high ionic strength and did not display any stereoselectivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of analogs of the 1,4-dihydropyridine tiamdipine in cardiac and smooth muscle

Two series of 1,4-dihydropyridines related to tiamdipine, 2-(2-aminoethylthio)methyl-3-carboethoxy-5-carbomethoxy-6- methyl-4-(3-nitrophenyl)-1,4-dihydropyridine, have been evaluated for their pharmacologic and radioligand binding properties in smooth and cardiac muscle. In the tiamdipine series the influence of phenyl ring substitution, 3-Cl, 3-MeO and 3-CF3, was greatly reduced relative to the N-formyl and neutral nifedipine derivatives. Consistent with our previous observations onset and offset of action were greatly reduced by the presence of the amine side chain. In tiamdipine analogs also bearing an asymmetric substituent at C-2, chirality at C-4 was determinant for activity.

Mechanism of blockade by (+)isradipine of adrenal catecholamine release

Cat adrenal glands were perfused at a high rate with various modified Krebs solutions containing different concentrations of K+ but no Ca2+. Catecholamine release was tested by applying brief Ca2+ pulses (10 s of a solution containing 120 mM K+ and 2.5 mM Ca2+). Under polarizing conditions (10 min perfusion with 1.4 mM K+ with no Ca2+), the total catecholamines released by the Ca2+ pulse amounted to 5 micrograms; in depolarizing conditions (10 min perfusion with a solution containing 70 mM K+ but no Ca2+), secretion was somewhat less (4-4.5 micrograms). (+)Isradipine, a 1,4-dihydropyridine Ca2+ channel blocker, did not affect the secretory response under polarizing conditions at 10(-8) M; at 10(-6) M, the secretory response was halved. When present under depolarizing conditions (70 mM K+ in 0 Ca2+), (+)isradipine (10(-8) M) blocked catecholamine release by 90%. In contrast, the inorganic Ca2+ channel blocker, Co2+, inhibited secretion equally well under polarizing or depolarizing conditions. Since 45Ca2+ uptake into adrenal medullary chromaffin cells was also inhibited by (+)isradipine (10(-8) M) in a voltage-dependent manner, it seems likely that blocking effects of the drug on catecholamine release are associated with inhibition of Ca2+ entry into cells through L-type Ca2+ channels. The association of (+)isradipine to its receptor is very rapid under polarizing conditions; dissociation is very slow in depolarized cells and very rapid upon polarization of such cells. Since chromaffin cells are being depolarized during stressful situations to secrete catecholamines into the circulation, (+)isradipine is likely to bind better to dihydropyridine receptors in this state; in this manner, the ensuing blockade of adrenal secretion could serve as a protective mechanism of cardiovascular tissues against massive increases in circulating catecholamines. If this suggestion is correct this mechanism could have additional therapeutic value in the treatment of hypertensive patients with (+)isradipine.

The selective potentiation of noradrenaline-induced tone by Bay K 8644 in the rabbit basilar artery

A number of studies indicate that relative to the maximal tone possible, for example, to histamine, noradrenaline produces only weak contractile responses in the rabbit basilar artery. Various factors, including a limited number of alpha-adrenoceptors, have been proposed to account for the reduced response to noradrenaline. We examined the effect of the Ca2+-channel activator, Bay K 8644 (0.1 and 1.0 nM) on dose-response curves to noradrenaline, histamine, calcium (Ca2+) and potassium (K+) in ring preparations of rabbit basilar artery and central ear artery. These concentrations of Bay K 8644 (0.1 and 1.0 nM) increased the magnitude of tension developed by noradrenaline (contractility) in the basilar artery, but did not alter its sensitivity (ED50) to the adrenergic vasoconstrictor. Bay K 8644 (0.1 and 1.0 nM) did not alter the contractility or sensitivity to histamine or K+ of the rabbit basilar artery. When dose-response curves to Ca2+ were made in K+-depolarized rabbit basilar artery rings, Bay K 8644 (0.1 and 1.0 nM) dose-dependently augmented tone generated by readmission of Ca2+. Bay K 8644 (0.1 and 1.0 nM) did not alter responses to noradrenaline, histamine, or K+ in rabbit central ear artery preparations. These results are compatible with a voltage-dependent mechanism of action of Bay K 8644 in the rabbit basilar artery, which may be partially depolarized in the resting state. We propose that in addition to other factors, the contractile response of rabbit basilar arteries is limited by a weak or inefficient coupling of alpha-adrenoceptors to Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by amlodipine of activity evoked in isolated human coronary arteries by endothelin, prostaglandin F2 alpha and depolarization

Amlodipine, a dihydropyridine calcium antagonist has been examined on the rhythmic activity of isolated human coronary arteries. Amlodipine inhibited both the spontaneous rhythmic activity and the rhythmic activity evoked by prostaglandin F2 alpha and endothelin in isolated human coronary arteries. It also inhibited the contraction evoked by potassium depolarization. The action of amlodipine was characterized by slow onset and voltage dependency.

Interactions between forskolin, calcium and Bay K 8644 in K+-depolarized rat uterine muscle

1. Forskolin (FORS) (0.03 to 10 microM) caused a concentration-dependent rightward shifts of the dose-response curves to CaCl2 in the isolated K+-depolarized rat uterus, with apparent, pA2 value of 8.3, but the slope was different from unity (0.55). 2. In presence of Bay K 8644 (10 and 100 nM), the pA2 values for FORS against CaCl2 contractions were reduced in a concentration-dependent manner and increased the slopes of Schild plots to values no longer different from unity. 3. In presence of low Ca2+ concentration (0.22 nM), the potency of FORS in antagonizing CaCl2 contractions was reduced in about 2-fold. 4. FORS (0.3-3 microM) caused a dose- and time-dependent relaxation of sustained contraction induced by CaCl2 (0.5 mM). 5. FORS (0.3 microM) like nifedipine (NIF, 30 pM) caused a concentration-dependent displacement to the right of the dose-response curve to Bay K 8644 (0.1-100 nM) in K+-depolarized preparations. 6. Higher concentration of both FORS (1 and 3 microM) and NIF (0.01 and 0.03 nM) also reduced the maximal response of Bay K 8644. 7. Both drugs caused a concentration-dependent relaxation of uterine muscle contracted by Bay K 8644 (100 nM) and K+ (80 nM), but neither FORS nor NIF discriminated the two types of contraction.