Effects of dihydropyridine calcium channel modulators in the heart: pharmacological and radioligand binding correlations

Characteristics of L-type calcium channel blockade by lacidipine in guinea-pig ventricular myocytes

1. The Ca(2+)-antagonistic properties of lacidipine were investigated in patch-clamp guinea-pig ventricular myocytes. 2. In basal conditions, 0.1 microM lacidipine reduced the action potential duration, associated with a decrease in the L-type calcium current (ICa,L) to 66 +/- 4% of the control value, without a change in the current-voltage relationship. Sodium current and background potassium currents were not affected. All the effects reached a steady state within 2 min. 3. The Ca(2+)-antagonistic effect of lacidipine was voltage-dependent: a marked negative shift (about 20 mV) of the steady-state inactivation curve was observed with long (10 s) conditioning prepulses, but not with short (350 ms) prepulses. 4. The onset of and recovery from the voltage-dependent effect caused by 0.1 microM lacidipine were significantly slower when compared to those of equiactive concentrations of nimodipine (0.5 microM) and nisoldipine (0.1 microM). ICa,L measured after prepulses at -40 mV lasting 500 ms or less was unchanged (95 +/- 5% of maximum current value) while it was reduced to 49 +/- 10% by nimodipine and 43 +/- 9% by nisoldipine (P < 0.05 vs lacidipine for both). 5. Similarly, the recovery from block in the presence of lacidipine was slower than with nimodipine and nisoldipine. After a prepulse of 1 s at -80 mV, ICa,L recovered up to 54 +/- 2% of the maximum current value in the presence of lacidipine, and up to 91 +/- 3% and 93 +/- 5% in the presence of nimodipine and nisoldipine, respectively (P < 0.05 vs lacidipine). 6. Blockade of ICa,L by lacidipine was use-dependent. After ten 200 ms long pulses (1 Hz) from -80 mV, ICa,L was reduced to 55 +/- 7% of the current measured at the first pulse. In the presence of nimodipine and nisoldipine, ICa,L elicited by the tenth pulse amounted to 93 +/- 3% and 80 +/- 6% of the first pulse value, respectively (P < 0.05 vs lacidipine). Lacidipine did not cause use-dependent blockade of ICa,L in cells stimulated with 10 ms long pulses. 7. These results demonstrate that lacidipine selectively inhibits ICa,L in isolated cardiomyocytes and suggest that this effect occurs mainly through binding to the inactivated Ca2+ channels.

Investigation of the negative inotropic effects of 17 beta-oestradiol in human isolated myocardial tissues

1. The aim of the present study was to evaluate the effects of 17 beta-oestradiol in human myocardium. The effects of 17 beta-oestradiol, progesterone and testosterone on force of contraction were investigated in electrically driven isolated atrial trabeculae and ventricular papillary muscles from human hearts in the presence and absence of Bay K 8644, a calcium channel agonist. In addition, the effects of 17 beta-oestradiol, progesterone and testosterone on binding of [3H]-PN 200 110 were assessed in membranes prepared from human ventricular myocardium. 2. 17 beta-Oestradiol elicited a negative inotropic effect in atrial (IC50: 7.1 mumol 1(-1), confidence interval 3.8 to 13.4, n = 3) and ventricular preparations (IC50: 4.6 mumol 1(-1)), confidence interval 2.2 to 9.4, n = 3) as compared with solvent controls. There was no significant difference (P > 0.05) of IC50 values in the absence and presence of isoprenaline (0.0 mumol 1(-1)) in atrial (IC50: 10.8 mumol 1(-1), confidence interval 9.1 to 12.9, n = 6) and ventricular preparations (IC50: 9.4 mumol 1(-1), confidence interval 7.3 to 11.9, n = 8). 3. 17 beta-Oestradiol at 30 mumol 1(-1) induced a significant rightward shift of the concentration-response curves for the positive inotropic effect of Bay K 8644 in atrial preparations (EC50: 0.13 mumol 1(-1), confidence interval 0.08 to 0.19, n = 6; EC50 with 17 beta-oestradiol: 0.58 mumol 1(-1), confidence interval 0.33 to 0.83, n = 6, P < 0.05) and ventricular preparations (EC50: 0.07 mumol 1(-1), confidence interval 0.04 to 0.11, n = 8; EC50 with 17 beta-oestradiol: 0.3 mumol 1(-1), confidence interval 0.18 to 0.49, n = 8, P < 0.05). Testosterone, progesterone at 30 mumol 1(-1) and the solvent control had no significant effect on the concentration-response curves to Bay K 8644. 4. In membranes prepared from human ventricular myocardium the effect of 17 beta-oestradiol on binding of [3H]-PN 200 110, an antagonist at the 1,4 dihydropyridine binding site, was not different from that observed with progesterone, testosterone or solvent controls. 5. In myocardial membranes no specific oestrogen receptors were demonstrated by [3H]-oestradiol binding studies. 6. Thus, the calcium antagonistic property of 17 beta-oestradiol cannot be attributed to a direct interaction with 1, 4 dihydropyridine binding sites.

Effect of changes in aortic pressure and in coronary arterial pressure on left ventricular geometry and function Anrep vs. gardenhose effect

Sudden increases in aortic pressure (AoP, mm Hg) are associated with increases in left ventricular (LV) function which persist even after diastolic volume has returned to its initial value (Anrep effect). Likewise, increases in coronary arterial pressure (CAP, mm Hg) are associated with improved LV function (gardenhouse effect). In situ, increases in AoP are paralleled by increases in both CAP and coronary blood flow, i.e., oxygen supply. We investigated the individual contributions of AoP and CAP increases on function (peak systolic pressure: LVPmax, mm Hg; dP/dtmax, mm Hg/s; end-diastolic pressure: LVPed, mm Hg) and end-diastolic geometry (inner diameter: IDed, mm; wall thickness: WTed, mm; sonomicrometry). CAP-induced increases in coronary flow were prevented by admixing dextran to the perfusate. The experiments were performed on isolated, saline-perfused, working rabbit hearts. Increasing CAP from 60 to 80 mm Hg (n = 11) resulted in improved function: LVPmax 89 +/- 3 vs. 94 +/- 3, dP/dtmax 1160 +/- 50 vs. 1250 +/- 50, LVPed 17 +/- 1 vs. 16 +/- 1 (mean +/- SEM). IDed decreased from 9.96 +/- 0.25 to 9.64 +/- 0.33 and WTed increased from 6.02 +/- 0.16 to 6.15 +/- 0.17. In a second series, AoP was increased from 60 to 80 (n = 9). Both LVPmax, dP/dtmax and LVPed increased (90 +/- 4 vs. 97 +/- 3, 1170 +/- 70 vs. 1270 +/- 90 and 18 +/- 1 vs. 19 +/- 1). IDed increased from 9.76 +/- 0.39 to 9.99 +/- 0.37 and WTed decreased from 6.08 +/- 0.22 to 5.86 +/- 0.25. After additionally increasing CAP to 80, function further improved (LVPmax: 101 +/- 3, dP/dtmax: 1310 +/- 80) while LVPed decreased (18 +/- 1). This time, IDed decreased to 9.71 +/- 0.36 and WTed increased to 6.03 +/- 0.26. Increases in CAP improve LV function via the gardenhose effect and likely do not depend on simultaneous increases in coronary flow or oxygen supply. On the other hand, increases in AoP alone improve systolic function via the Frank-Starling mechanism. Increases in both pressures together amplify this effect. Increases in CAP and in AoP have opposing effects on IDed and WTed. In conclusion, the homeometric Anrep effect--at least in part--can be viewed as synergistic action of the Frank-Starling mechanism and the gardenhose effect for this experimental model.

Calcium modulatory properties of 2,6-dibutylbenzylamine (B25) in rat isolated vas deferens, cardiac and smooth muscle preparations

1. In rat isolated vas deferens the new compound 2,6-dibutylbenzylamine (B25) evoked a series of repeating rhythmic contractions. Concentration-response curves constructed for this effect were bell-shaped, indicating a biphasic effect for this compound. By contrast, B25 depressed heart contractility without any visible positive inotropic or chronotropic activity. 2. Experiments with tetrodotoxin, reserpine, capsaicin, alpha-adrenoceptor blocking compounds and other agents permit us to exclude a release of neuromediators or a direct stimulation of post-synaptic receptors to account for the rhythmic effect of B25 in the rat vas deferens. 3. In the same tissue, the increase in 45Ca2+ uptake, the voltage-dependency as well as the dependence of the B25-induced rhythmic activity upon the external calcium concentration indicate a direct activation of voltage-sensitive calcium channels (VSCC). 4. Verapamil paradoxically stimulated the rhythmic effect of B25 in the rat vas deferens. La3+ was inactive while nifedipine was a weak inhibitor. By contrast Ni2+ and Mn2+ ions were good inhibitors (IC50 < 10(-4) M), suggesting that a possible opening of T-type VSCC underlies rhythmic effect of B25. 5. In radioligand binding studies competition experiments with [3H]-nitrendipine indicated that only at high concentrations was B25 able to interact with dihydropyridine-sensitive binding sites of heart and vas deferens smooth muscle. 6. B25 (3-30 microM) counteracted the inhibitory effects of omega-conotoxin GVIA in field-stimulated rat vas deferens.

Effects of felodipine, nifedipine and verapamil on cytosolic Ca2+ and contraction in vascular smooth muscle

The inhibitory effects of felodipine, nifedipine and verapamil were compared in vascular smooth muscle. In rat aorta, these inhibitors attenuated the high K(+)-induced contraction with a parallel decrease in the cytosolic Ca2+ level ([Ca2+]i). Maximal inhibition was obtained with 10 nM felodipine, 100 nM nifedipine and 10 microM verapamil. The inhibitory effects were antagonized by an increase in external Ca2+ concentration to 6.5 mM and the addition of a Ca2+ channel activator, 100 nM Bay k 8644. These inhibitors also attenuated the contraction induced by norepinephrine although these effects were weaker than those on high K(+)-induced contraction. Furthermore, these inhibitors attenuated the norepinephrine-stimulated [Ca2+]i more strongly than contraction. In contrast, none of these inhibitors inhibited the transient increase in [Ca2+]i and muscle tension induced by norepinephrine in Ca(2+)-free solution and the Ca(2+)-induced contraction in permeabilized smooth muscle. These results suggest that felodipine, nifedipine and verapamil inhibit smooth muscle contraction by inhibiting Ca2+ channels at concentrations which do not change Ca2+ release or Ca2+ sensitivity of contractile elements.

Modulation of dihydropyridine-sensitive calcium channels in heart cells by fish oil fatty acids

The highly unsaturated n-3 fatty acids from fish oils, eicosapentaenoic acid [EPA; C20:5 (n-3)] and docosahexanoic acid [DHA; C22:6 (n-3)], prevent the toxicity of high concentrations of the cardiac glycoside ouabain to isolated neonatal rat cardiac myocytes. Arachidonic acid [C20:4 (n-6)] lacks such protective action. The protective effect of the n-3 fatty acids is associated with their ability to prevent high levels of cytosolic free calcium from occurring in response to the ouabain. This in turn results, at least in part, from a 30% reduction in calcium influx rate induced by the n-3 fatty acids. This protective effect is simulated by nitrendipine, a dihydropyridine inhibitor of the L-type calcium channels in cardiac myocytes. Nitrendipine (0.1 mM) alone, however, inhibits myocyte contractility, as do verapamil (10 microM) and diltiazem (1.0 microM). EPA or DHA (5 microM) blocks the inhibitory effects of nitrendipine but not those of verapamil or diltiazem. Bay K8644, a known dihydropyridine agonist of L-type calcium channels, produces a ouabain-like effect that is also prevented by EPA or DHA. Specific binding of [3H]nitrendipine to intact myocytes is noncompetitively inhibited by EPA or DHA in a manner that reduces the number of high- and low-affinity binding sites (Bmax) and increases their affinities. The fish oil fatty acids prevent calcium overload from ouabain and Bay K8644. They also prevent a calcium-depleted state in the myocytes caused by the L-type calcium channel blocker nitrendipine. The protective effects of the n-3 fatty acids appear to result from their modulatory effects on nitrendipine-sensitive L-type calcium channels.

(+)-S-12967 and (-)-S-12968: 1,4-dihydropyridine stereoisomers with calcium channel agonistic and antagonistic properties in rat resistance arteries

1. The actions of (+)-S-12967 and (-)-S-12968 two isomers of a new 1,4-dihydropyridine (DHP) derivative, were studied on 125 mM K(+)-, Ca(2+)- and noradrenaline-induced contractions in rat isolated mesenteric resistance arteries and compared to those of nifedipine. 2. The action of (+)-S-12967 and (-)-S-12968 was slow in onset in contrast to nifedipine. Both isomers had a dual contractile and relaxant action in arteries contracted with 125 mM K+; however, the (-)-isomer was about 300 times more potent than the (+)-isomer. The response to 125 mM K+, being depressed by 70%, recovered within 20 to 30 min for all DHP derivatives. All vessels were treated with 1 x 10(-6) M phenoxybenzamine thus excluding the possibility that the contraction is mediated by activation of amine-receptors. 3. Both (+)-S-12967 and (-)-S-12968 at low concentrations potentiated responses induced by Ca2+ in arteries activated by 125 mM K+ and inhibited the responses at higher concentrations. (+)-S-12967 and (-)-S-12968 had no contractile action in arteries kept in normal buffer. Nifedipine had only an inhibitory action on vessel responses to 125 mM K+ and Ca2+. 4. Both isomers and nifedipine depressed the maximal vessel response to noradrenaline by about 20% and 44%, respectively. 5. The results confirm that DHP calcium antagonists selectively inhibit vascular smooth muscle responses induced by high potassium and that the potency of 1,4-DHP isomers may vary considerably. Furthermore, since the agonistic/antagonistic properties on the calcium channel were shared by both stereoisomers of the 1,4-DHP molecule and apparently dependent on their concentration and the vascular smooth muscle membrane potential, it suggests that the agonistic action of 1,4-DHPs may be ascribed to functional characteristics of their binding site regulating the Ca2l -channel.

Dihydropyridine modulation of voltage-activated calcium channels in PC12 cells: effect of pertussis toxin pretreatment

In this study, we report the effect of pertussis toxin pretreatment on dihydropyridine modulation of voltage-sensitive calcium channels in PC12 cells. The rise in intracellular calcium concentration caused by potassium depolarization is not affected significantly by pertussis toxin pretreatment. Nicardipine, a dihydropyridine derivative, added either before or after potassium-induced depolarization, reduces the resultant elevation in cytosolic calcium level both in control and in pertussis toxin-treated cells. The dihydropyridine agonist Bay K 8644, when added before potassium, is able to enhance the potassium-induced spike of cytosolic calcium levels, an effect significantly reduced by pertussis toxin pretreatment. Moreover, the addition of Bay K 8644 after potassium holds the intracellular calcium concentration at a cytosolic sustained level during the slow inactivating phase of depolarization. This effect of Bay K 8644 is inhibited by nicardipine. Pertussis toxin pretreatment slightly weakens the effect of Bay K 8644 when added after potassium-induced depolarization, whereas it significantly reduces the nicardipine inhibition of cytosolic calcium rise stimulated by potassium and Bay K 8644, but not by potassium alone. In conclusion, our findings suggest that a pertussis toxin-sensitive guanine nucleotide regulatory protein could be involved in the interaction between dihydropyridine derivatives and voltage-dependent calcium channels.

(+)-isradipine but not (-)-Bay-K-8644 exhibits voltage-dependent effects on cat adrenal catecholamine release

1. Catecholamine release from cat adrenal glands perfused at a high rate (4 ml min-1) at 37 degrees C with polarizing (1.2 or 5.9 mM K+) or depolarizing (17.7, 35, 59 or 118 mM K+) solutions, was triggered by 5 or 10 s pulses of Ca2+ (0.5 or 2.5 mM) in the presence of various concentrations of K+. 2. In polarized glands, secretion was greater the higher the K+ concentration present during the secretory K+/Ca2+ test pulse. Thus, in 17.7 mM K+, catecholamine released was 162 +/- 27 ng per pulse, while in 118 mM K+ secretion rose to 1839 +/- 98 ng per pulse. In depolarized glands, secretion reached a peak of around 1000 ng per pulse in 35-59 mM K+; in 118 mM K+, secretion did not increase further, suggesting that voltage changes are implicated in the control of the secretory process. 3. Blockade of secretion by increased concentrations of (+)-isradipine was much more manifest in polarized glands. The higher the degree of depolarization was (35, 59 or 118 mM K+), the lower the IC50 s were. So, the ratios between the IC50 s in polarized and depolarized glands rose from 3.92 in 35 mM K+ to 26.7 in 118 mM K+. 4. In contrast, the Ca2+ channel activator (-)-Bay K 8644 potentiated catecholamine release evoked by K+/Ca2+ pulses equally well in polarized or depolarized glands. The ratios between EC50 s in polarized or depolarized glands were, respectively, 0.30, 0.59 and 0.69 for 17.7, 35 and 118 mM K+. 5. In simultaneous experiments, the two enantiomers of Bay K 8644 exhibited opposite effects on secretion. (+)-Bay K 8644 (a Ca21 channel blocker) inhibited secretion better in depolarized than in polarized glands, whilst (-)-Bay K 8644 potentiated secretion in a voltage-independent manner. 6. Potentiation of secretion by (-)-Bay K 8644 was concentration-dependent from 10-8 to 10-6M. At 10- 5M, such potentiation largely disappeared in both polarized and depolarized glands. However, this dual effect of (-)Bay K 8644 was better seen in depolarizing conditions, suggesting that using the same enantiomer, the voltage-dependence is only seen when blockade of secretion dominates. 7. In the presence of increasing concentrations of (-)Bay K 8644 (3 x 10-9, 3 x 10-8 and 3 x 10-7M), the concentration-response curves for (+)isradipine to inhibit secretion were displaced to the right. However, a Schild plot of (dose ratio - 1) against (-)-Bay K 8644 concentrations gave a slope of 0.6, suggesting that the interactions between (+)-isradipine and (-)Bay K 8644 were non-competitive in nature. The pA2 for (-)-Bay K 8644 was 9.13. 8. Overall, the results suggest that potentiation of secretion by (-)Bay K 8644 (a voltage-independent phenomenon), and blockade by (+)-isradipine or (+-Bay K 8644 (a voltage-dependent phenomenon) might be exerted through binding of the dihydropyridines activators and blockers to separate sites on chromaffin cell L-type Ca2 + channels.

Opposite cardiac actions of the enantiomers of Bay K 8644 at different membrane potentials in guinea-pig papillary muscles

The influence of membrane potential on the effects of the enantiomers and the racemate of Bay K 8644 [1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluor-methylphenyl)-p yri dine-5-carboxylate] on force of contraction and on action potentials were studied in guinea-pig papillary muscles in order to detect possible changes in the direction of drug action or in potency. Membrane potential was varied by changing the potassium concentration ([K+]o) in the bathing solution. At normal resting potential, (-)-Bay K 8644 enhanced force of contraction and prolonged the action potential duration measured at 50% of repolarization (APD) to the same extent as the racemate and with similar pD2 values. After membrane depolarization by raising [K+]o from 5.4 to 17.4 mmol/l, the (-)-enantiomer and the racemate prolonged the APD to a similar degree but enhanced force to a lesser extent. The maximum rate of depolarization of slow action potentials, Vmax, was increased at the highest concentrations (10(-5) mol/l). The effects of (+)-Bay K 8644 were more complicated. At high concentrations (10(-5) mol/l) it decreased force of contraction and APD, the pD2 values were one order of magnitude lower than for the (-)-enantiomer and the racemate. A high concentration (+)-Bay K 8644 (10(-5) mol/l) virtually abolished contractile activity at all membrane potentials, the extent of shortening in APD increased with membrane depolarization in elevated [K+]o. Vmax of slow action potentials was decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage- and use-dependent modulation of cardiac calcium channels by the dihydropyridine (+)-202-791

The modulation of L-type voltage sensitive calcium channels in isolated guinea pig ventricular myocytes by the dihydropyridine (+)-202-791 was examined with the whole-cell voltage-clamp technique with 1.8 mM Ba or Ca as the charge carrier. Striking voltage- and use-dependent effects of the dihydropyridine calcium channel "agonist" (+)-202-791 were revealed. From a holding potential of -60 mV, depolarizing test pulses in the presence of (+)-202-791 demonstrated a concentration-dependent (EC50, 177 nM) increase in the measured peak inward barium current compared to control. In contrast, more depolarized holding potentials (greater than or equal to -30 mV) (+)-202-791 caused a biphasic effect on the peak inward current resulting in a transient enhancement followed by a steady-state block. A saturable, concentration-dependent hyperpolarizing shift in the voltage dependence of current inactivation was observed in the presence of (+)-202-791 with an EC50 of 10.2 nM. The voltage dependence of current activation was also shifted in the hyperpolarizing direction in the presence of (+)-202-791. A use-dependent relative block by (+)-202-791 was observed after repetitive depolarizing test pulses at a frequency of 2 Hz. Thus, the single enantiomer (+)-202-791 can result in either an increase in the whole cell calcium channel current (favored by hyperpolarized holding potentials and low rates of stimulation) or block of calcium channel current (favored by depolarized holding potentials and high rates of stimulation). Various combinations of (-)-202-791, a reported calcium channel antagonist, and (+)-202-791 resulted in intermediate effects on voltage sensitive calcium or barium currents compared with the presence of either enantiomer alone, and no clear cooperative interactions between the enantiomers were observed in contrast to a previous single channel study (Kokuban S, Prod'ham B, Becker C, Porzig H, Reuter H: Studies on Ca channels in intact cardiac cells: Voltage-dependent effects and cooperative interaction of dihydropyridine enantiomers. Mol Pharmacol 1986;30:571-584). The results are discussed in relation to the possible presence of multiple dihydropyridine receptors associated with the voltage sensitive calcium channel.

Pharmacologic and radioligand binding analysis of the actions of 1,4-dihydropyridine activators related to Bay K 8644 in smooth muscle, cardiac muscle and neuronal preparations

The structure-activity relationships of a series of 1,4-dihydropyridine Ca2+ channel activators, including Bay K 8644, have been determined by pharmacologic and radioligand binding techniques. Pharmacologic techniques included tension responses and the measurement of pA2 values for nifedipine antagonism of Bay K 8644 responses in guinea pig ileal, rat femoral and rat atrial and papillary muscle preparations. Radioligand binding experiments employed competition against [3H]nitrendipine binding in ileal smooth muscle and rat ventricular membranes and rat brain synaptosomal preparations. The series of compounds was employed as the racemates. Binding affinities were not significantly different between smooth muscle, cardiac muscle and brain preparations and the same rank order of pharmacologic activities is observed in smooth and cardiac muscle, where the effects of the 4-phenyl substituents, o greater than or equal to m greater than p, parallel those observed for 1,4-dihydropyridine antagonists. In the ileal and femoral artery smooth muscle preparations a 1:1 correlation is observed between pharmacologic and radioligand binding affinities. However, in the cardiac muscle preparations, left atrium and papillary muscle, there is an approximately 10-fold difference between the binding affinities and the lower pharmacologic affinities. A similar difference between smooth and cardiac muscle is observed with the pA2 values of 6.97 and 7.06 in atrial and papillary muscle respectively, which are significantly lower than the values of 8.54 and 8.72 measured in ileal and femoral artery respectively. The structure-activity expressions measured for this small series of 1,4-dihydropyridine activators parallel those observed in the larger series of 1,4-dihydropyridine antagonists. This is consistent with proposals that activators and antagonists interact at common binding sites that are components of a voltage-dependent Ca2+ channel.

The dihydropyridine derivative 202-791: interpretation of the effects of the racemate considering inverse agonistic enantiomers

1. The influence of the enantiomers and the racemate of the dihydropyridine derivative 202-791 [isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-nitro-3- pyridinecarboxylate] on force of contraction and action potential was studied in guinea-pig isolated papillary muscles. The effects were investigated during regular stimulation (1 Hz) and after a period of rest (10 min). 2. The enantiomers of the dihydropyridine derivative 202-791 had opposite effects on the mechanical and bioelectrical parameters: the (+,S)-enantiomer enhanced contractility and prolonged action potential duration whereas the (-,R)-enantiomer reduced force and shortened action potential duration. Analogous to the effects during regular stimulation, the post-rest adaptation was modified adversely: in the presence of the (+,S)-enantiomer the pattern of adaptation was intensified while the (-,R)-enantiomer caused an attenuation. The term 'inverse agonism' seems more suitable than the commonly used comparison of agonist and antagonist, because each enantiomer possesses intrinsic activity, albeit in opposite directions. 3. The racemate of 202-791 acted like the (+,S)-enantiomer. In concentrations up to 1 microM, the racemate increased the force of contraction to the same extent as if the cardiodepressant (-,R)-enantiomer was not present. Only at the highest concentration (3 microM) did the counteracting effect of the (-,R)-enantiomer become evident. The racemate prolonged the action potential duration like the (+,S)-enantiomer although to a lesser extent. Moreover, the typical post-rest adaptation of contractile force and action potential duration was accentuated by the racemate as with the (+,S)-enantiomer. 4. The results demonstrate that in case of 202-791, the effects of the racemate do not reflect the opposite actions of the two enantiomers, but rather mimic that of the (+,S)-enantiomer. A prediction concerning the effects of the enantiomers which is based on findings obtained with the racemate is not possible.

Three components of the calcium current in cultured glomerulosa cells from rat adrenal gland

1. Ca2+ channels were studied in cultured glomerulosa cells from the rat adrenal gland. The whole-cell configuration of the patch-clamp technique was used. Cs+-filled pipettes were used in order to block K+ channels. 2. Three Ca2+ components were found, namely, T, L and N, according to the nomenclature proposed by Nowycky, Fox & Tsien (1985). The T-component was a fast transient component activated in the range -60 to -40 mV; the L-component did not inactivate for a sustained depolarization and activated at voltages around -30 mV; the third component, the N-component, was transient and was activated at voltages close to -20 mV. 3. A statistical analysis made on seventy-one experiments showed that the L-component was the most frequent (65% of the experiments), followed by the T- and finally the N- components (59 and 29% of the experiments, respectively). 4. The substitution of Ba2+ ions for Ca2+ ions greatly enhanced the L-component's amplitude (iBa/iCa = 4) while the N-component was unaffected and the T-component was reduced (iBa/iCa = 0.4). 5. A comparison of the voltage-dependent steady-state inactivation of the three components showed that the T-component was inactivated at -60 mV while the inactivation of the L- and N-components was complete at -25 and 0 mV, respectively. 6. A run-down effect was detected in some cells. The time stability of the L-component was lower than that of the T-component. The N-component seemed to be insensitive for at least 1 h. The results for the L- and T-components were obtained in cells which presented no run-down of the current or only a weak one. 7. Cd2+ ions (5 x 10(-5)M) completely blocked the long-lasting component (L-component) and slightly decreased the T-component. 8. Bay K 8644, a dihydropyridine agonist, enhanced the L-component at a concentration of 2.5 microM but decreased it for a higher concentration (5 microM). The T-component was decreased in a reversible way by 1 microM-Bay K 8644. Nifedipine, a well-known antagonist, blocked completely the L-component. This effect was reversed by the addition of Bay K 8644 to the perfusion medium. The T-component was also blocked by nifedipine, a result which is in keeping with the fact that Bay K 8644 has a weak effect on this current.

Positive inotropic effects of calcium channel antagonists are not necessarily caused by partial calcium channel agonism

Recently it has been reported that some dihydropyridine calcium channel antagonists (nifedipine, nimodipine, nitrendipine) are able to produce positive inotropic effects in isolated perfused guinea pig hearts. We studied the effects of nifedipine in isolated perfused paced rat hearts under constant pressure and constant flow perfusion conditions. We found that nifedipine is able to produce a positive inotropic effect under constant pressure conditions but not under constant flow conditions. We conclude that nifedipine does not have partial calcium channel agonistic properties and that the positive inotropic effect seen under constant pressure conditions is a result of the vasodilating properties of the drug. Positive inotropic effects caused by vasodilatation can be explained by the "garden-hose-effect".

Low affinity binding sites for 1,4-dihydropyridines in mitochondria and in guinea pig ventricular membranes

In this paper, we describe the occurrence of both high and low affinity sites for dihydropyridines in crude membrane preparations from guinea pig ventricular tissue. The physiological significance of the low affinity site (apparent dissociation constant = 76 +/- 9 nM) is not currently known; it has, however, a binding capacity which was 300-1000 times that of the high affinity site and was resistant to heat denaturation. The magnitude of the binding to the low affinity site was affected by both the ionic strength of the medium and by the presence of divalent ions. Both unlabeled nitrendipine and nimodipine inhibited [3H]nitrendipine binding at both sites, but verapamil and diltiazem only affected binding at the high affinity site. We also characterized, both kinetically and by equilibrium binding, a low affinity, heat-stable nitrendipine binding site in purified mitochondria. The Bmax for this site was also dependent on ionic strength. This suggests the possibility that the low affinity site in crude membranes is due to mitochondrial contaminants and hence not directly related to voltage-dependent calcium channels.

Toxins that affect voltage-dependent calcium channels

At this time, there are five potential candidates for calcium channel specific toxins. All five of these toxins appear to affect the function of voltage-dependent calcium channels. Atrotoxin, beta-leptinotarsin-h and maitotoxin activate channels, whereas both taicatoxin and omega-conotoxin are inhibitors. Neither maitotoxin nor omega-conotoxin alters the binding of dihydropyridines to membranes derived from the cells upon which the toxins exert their effects. In contrast, both atrotoxin and taicatoxin inhibit the binding of dihydropyridines to ventricular membranes. It is not currently known whether beta-leptinotarsin-h affects the binding. The effects of maitotoxin and atrotoxin are blocked by dihydropyridines and verapamil. Direct binding studies with radiolabeled toxins have been performed only with omega-conotoxin, and the binding site density for this toxin appears to be at least one order of magnitude greater than the density of dihydropyridine binding sites in synaptosomes. Studies to examine the third and fourth criteria which we have listed (i.e. that the effects are not via a second messenger, or an enzyme activity) have not been reported for either beta-leptinotarsin-h or omega-conotoxin. Atrotoxin and taicatoxin, added outside a patch pipette, have no effects on calcium channels within the patch and are, therefore, probably not affecting calcium channels via a second messenger. Maitotoxin, however, affects the formation of inositol phosphates and, hence, could be affecting the channel indirectly. The fractions containing the toxic components atrotoxin and taicatoxin have no phospholipase or protease activity, and this is presumably true also for omega-conotoxin since it has been purified to homogeneity. Although all of the toxins have the potential to be important tools with which to study calcium channel structure and function, a number of experiments remain to be done in order to establish conclusively that these five toxins bind specifically to the voltage-dependent calcium channel. In conclusion, we would like to briefly mention why so much effort is being devoted to the search for these calcium channel specific toxins. Such a toxin would provide a very valuable tool in the study of calcium channels for a number of reasons. First, the toxin would be another ligand for the channel and, as such, would provide an alternative to organic ligands such as the dihydropyridines which are lipophilic and, in many tissues, have more than one binding site.(ABSTRACT TRUNCATED AT 400 WORDS)

Paradoxical augmentation of (-)Bay K 8644-induced calcium influx by nitrendipine

(-)Bay K 8644 produced a concentration-dependent contraction of porcine coronary artery rings with the maximal contraction at 10(-6) M. Pretreatment of the rings with 10(-6) M nitrendipine inhibited (-)Bay K 8644-induced contraction, while pretreatment with 10(-8) M nitrendipine potentiated the contraction elicited by (-)Bay K 8644. (-)Bay K 8644 (10(-6) M) significantly stimulated Ca2+ influx. Although 10(-8) M nitrendipine never stimulated Ca2+ influx, Ca2+ influx induced by (-)Bay K 8644 was significantly potentiated by pretreatment with 10(-8) M nitrendipine. Pretreatment with 10(-6) M nitrendipine significantly decreased Ca2+ influx in tissues treated with (-)Bay K 8644. Our results suggest that the increased Ca2+ influx might be involved in the mechanisms by which (-)Bay K 8644-induced contraction was potentiated by pretreatment with nitrendipine.

Solubilization of calcium channel antagonist binding sites from rat brain

Calcium antagonist binding sites were solubilized from rat brain membranes using the detergent 3-[(3-cholamidopropyl)dimethylammonio] 1-propanesulfonate (CHAPS). CHAPS-solubilized [3H]nitrendipine binding sites are saturable over a range of 0.05-4 nM and Scatchard analysis reveals a single, high-affinity (KD = 0.49 +/- 0.10 nM), low-capacity (Bmax = 56 +/- 4 fmol/mg of protein) binding site. Reversible ligand competition experiments using solubilized binding sites demonstrated appropriate pharmacologic specificity, with dihydropyridines (nifedipine = nitrendipine greater than Bay K 8644) completely displacing binding, verapamil partially displacing binding, and diltiazem enhancing binding, as previously described in membrane preparations. Lyophilized Crotalus atrox venom was purified by ion exchange chromatography followed by gel filtration to a single peptide band on sodium dodecyl sulfatepolyacrylamide gel electrophoresis. This fraction of molecular weight 60,000 competitively inhibits [3H]nitrendipine binding to both membrane and soluble preparations with an IC50 of 5 micrograms/ml. This polypeptide should serve as a useful ligand for future efforts in purifying the dihydropyridine calcium channel binding site in brain.

Correlation of changes in cardiac calcium channels with hemodynamics in Syrian hamster cardiomyopathy and heart failure

We compared hemodynamics with [3H]nitrendipine (calcium channel) binding to cardiac membranes from Bio 14.6 cardiomyopathic Syrian hamsters at 4 and 10 months with their F1B controls. A 50% increase in the number (Bmax) of nitrendipine binding sites (calcium channels) was seen only in the 4 month old myopathic vs controls (Bmax = 468 +/- 11 vs 309 +/- 10 fmol/mg prot with no change in affinity (KD) (KD = .65 +/- .12 vs .75 +/- .14 nM), while no differences in Bmax or KD were seen at 10 months (Bmax = 375 +/- 9 vs 362 +/- 7 fmol/mg prot/KD = .82 +/- .18 vs .89 +/- .17 nM) myopathic vs control respectively. Hemodynamic studies revealed no significant differences in cardiac output, cardiac index, stroke volume, heart rate, mean arterial pressure, peripheral resistance, body weight, heart weight at 4 months, but a significant decrease in peripheral resistance (1120 +/- 360 vs 2080 +/- 240) increase in body weight (118 +/- 2 vs 94 +/- 2 grams) and heart weight (97 +/- 5 vs 78 +/- 2 gms/100 gms body weight) in 10 month myopathic vs control animals. We conclude that the onset of cardiomyopathy at 4 months is associated with a selective increase in calcium channel binding sites and heart failure at 10 months is associated with a relative decrease in these sites.

Evidence for distinct calcium channel agonist and antagonist binding sites in intact cultured embryonic chick ventricular cells

To determine whether calcium channel agonists and antagonists bind to distinct pharmacologically active sites, the binding of dihydropyridine calcium channel agonists and antagonists was related to calcium flux and contractile state in primary monolayer cultures of spontaneously contracting chick embryo ventricular cells. Equilibrium binding studies using the antagonist (+)-[3H]PN200-110 demonstrated equilibrium binding to intact, beating cells consistent with a single class of binding sites (KD, 1.1 nM; Bmax, 40 fmol/mg protein). Membrane depolarization of the intact cells by incubation in 30 mM potassium caused a 91% increase in the apparent number of (+)-PN200-110 binding sites (Bmax 76 fmol/mg protein), but no significant change in the KD (1.2 nM). The (+)-PN200-110 produced a concentration-dependent decrease in calcium uptake (IC50 2.2 nM) and contractile amplitude (IC50 5.6 nM). The calcium channel agonist, (+/-)-[3H]BAY k 8644, bound to two distinct binding sites with high affinity (KD 1.0 nM) and low affinity (KD 1.9 microM). The (+/-)-BAY k 8644 produced biphasic modulation of calcium flux and contractile state. At concentrations of 100 nM or less, (+/-)-BAY k 8644 increased calcium flux and contractile amplitude, consistent with drug interaction with the high affinity agonist site. However, at higher concentrations, the stimulatory effect of (+/-)-BAY k 8644 on calcium flux and contractile amplitude was abolished, a finding that is consistent with drug interaction with the low affinity antagonist site.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor pharmacology of calcium entry blocking agents

The mechanism of action of calcium channel modulators, a class of drugs that includes 3 chemical groups--1,4-dihydropyridines, phenylalkylamines and benzothiazepines--has been extensively reviewed. The best known representatives of these 3 groups are nifedipine, verapamil and diltiazem, respectively. These drugs bind reversibly, stereospecifically and with high affinity to both the membrane-bound and the purified receptor complex. Non-dihydropyridines allosterically regulate dihydropyridine binding. This has been shown by using (-) [3H]202-791 and (+) [3H]PN200-110 as labeled ligands. The purified receptor complex that possesses binding sites for all 3 chemical groups is likely to be related to the voltage-dependent calcium channel. As the result of a drug-receptor interaction, voltage-dependent calcium channels are either activated or inactivated. The drugs that activate channels act by promoting long-lasting channel openings. The drugs that inhibit calcium channels, the calcium entry-blocking agents, act by preventing channel openings upon membrane depolarization. A complex pharmacologic, electrophysiologic, biochemical, immunologic and molecular genetic approach is required to determine the molecular mechanism of action of calcium channel modulators. Clinically, calcium entry-blocking agents are recommended for the treatment of angina pectoris, hypertension, posthemorrhagic cerebral vasospasm, supraventricular tachycardia, migraine and asthma and the protection of the ischemic myocardium.

Calcium antagonists stimulate prostaglandin synthesis by cultured rat cardiac myocytes and prevent the effects of hypoxia

The effect of three calcium antagonists on the synthesis of prostacyclin (PGI2, assayed as 6-Keto-PGF1 alpha) and PGE2 by cultured rat cardiac myocytes and fibroblasts was investigated. In myocytes only, bepridil, diltiazem and verapamil (10(-9) to 10(-7) M) stimulated PGs synthesis by two- to three-fold, dose-dependently. At a concentration of 10(-6) or 10(-5) M the intensity of the stimulation of PGI2 and PGE2 decreased. Cobalt chloride (2 X 10(-3) M) did not change PGs synthesis (pg/mg of protein/30 min; means +/- SE, N = 10; PGE2: 365 +/- 59 and 463 +/- 89 treated vs controls; PGI2: 824 +/- 214 and 799 +/- 143 treated vs controls). After 30 min exposure of myocytes to hypoxic conditions (glucose-free medium and low PO2), the glycogen content was half that of the controls (P less than 0.001), ATP content did not change and PGI2 and PGE2 synthesis increased (X1.5, P less than 0.05). When applied to myocytes 30 min before inducing hypoxia, the three calcium antagonists stimulated PGs synthesis by three- to seven-fold at maximal effect, and bepridil (10(-8) M) or diltiazem (10(-7) M) prevented the hypoxia-induced decrease in glycogen content. With 10(-5) M drug concentration, the effect on PGs was not significant, except for the effect of bepridil on PGI2 (P less than 0.05). It is concluded that therapeutic concentrations of calcium antagonists simultaneously prevent the decrease in myocyte glycogen induced by hypoxia and stimulate PGs synthesis by myocytes.

The effects of the cardiotonic dihydropyridine derivatives Bay k 8644 and H160/51 on post-rest adaptation of guinea-pig papillary muscles

In isolated guinea-pig papillary muscle, the effect of the two dihydropyridine derivatives Bay k 8644 (0.03-10 mumol/l) and H160/51 (0.1-3 mumol/l) on transmembrane action potentials and force of contraction were investigated at regular stimulation (1 Hz) and after a period of rest (10 min). The following results were obtained: At regular stimulation of the preparations, Bay k 8644 and H160/51 enhanced force of contraction without affecting time-to-peak tension. The time required for relaxation and the action potential duration were prolonged. These effects were transient with exposure to high concentrations of Bay k 8644 (greater than 3 mumol/l). The amplitude of the post-rest contraction thought to depend entirely on transmembrane calcium influx was small under control conditions and increased because of prolongation in time-to-peak tension in the presence of either dihydropyridine derivative. Isoprenaline (30 nmol/l) - as opposed to Bay k 8644 and H160/51 - increased the rate of force development of post-rest contractions. Bay k 8644 and H160/51 prolonged the duration of the first action potential after 10 min of rest. In the course of adaptation to steady state stimulation this prolongation transiently increased further resulting in a biphasic pattern which was attenuated by addition of nifedipine. With isoprenaline the biphasic pattern changed into a monotonous adaptation to pre-rest control. Our results show that the small enhancement of the post-rest contraction in the presence of Bay k 8644 or H160/51 is due to prolonged action potential duration after rest, whereas isoprenaline enhances the intensity of post-rest activation.(ABSTRACT TRUNCATED AT 250 WORDS)

BAY k 8644, a calcium channel agonist, reverses hypotension in endotoxin-shocked rats

The hypotension and depressed myocardial function frequently observed in endotoxin-induced shock are difficult to overcome pharmacologically. In this paper we demonstrate that the calcium channel agonist BAY k 8644 potently elevates blood pressure in endotoxin-shocked rats. A one time dose as low as 10 micrograms/kg of BAY k 8644 significantly elevated mean arterial pressure (MAP) in endotoxin-treated hypotensive rats while having minimal effects in normal rats. The maximum BAY k-induced percentage increase in MAP was greater in endotoxin-treated rats when compared with saline-treated control (153% vs. 120% increase respectively). BAY k 8644 also caused a dose-dependent decrease in heart rate of 37% in endotoxin-treated rats and 39% in control rats (NS vs. control). No differences in the regulatory properties of [3H]nitrendipine binding sites were discerned comparing control and endotoxin-treated rats. Thus, the enhanced activity of BAY k 8644 in hypotensive rats was not due to augmented affinity for the cardiac dihydropyridine binding site. These results demonstrate that the use of calcium channel agonists might represent a unique pharmacologic approach in pathologic states characterized by hypotension and diminished cardiac function.

Dual effects of dihydropyridines on whole cell and unitary calcium currents in single ventricular cells of guinea-pig

We studied the effects of dihydropyridine Ca channel ligands (DHPs), mainly nitrendipine and Bay K8644, on whole cell and single channel Ca currents on single myocytes isolated from the adult guinea-pig ventricle. Nitrendipine had dual effects, stimulatory or inhibitory, depending upon the membrane potential. At low frequencies (less than 0.03 Hz) and negative holding potentials (-90 mV or more), nitrendipine increased the Ca currents in a dose-dependent manner. The dose-response curve was best fitted by a Langmuir adsorption isotherm model which was the sum of two independent one-to-one drug-receptor sites with median effective doses (ED50S) of 1.0 X 10(-9) M and 1.4 X 10(-6) M respectively. When the membrane potential was held at -30 mV or less, nitrendipine inhibited the Ca currents, also in a dose-dependent manner. The dose-response curve was fitted by a single binding site model having a median inhibitor concentration (IC50) of 1.5 X 10(-9) M. At holding potentials between -70 and -40 mV, nitrendipine produced mixed effects on Ca currents; an increase occurred initially and this was followed by a decrease. When rundown was excluded, Bay K8644 showed only stimulatory effects on the Ca currents between holding potentials of -120 and -30 mV. When the test potential was zero or +10 mV the Ca currents reached peak values and the dose-response curve was best fitted by a single binding site model having an ED50 of 3 X 10(-8) M. When the effects were measured at negative test potentials of -30 to -10 mV, the curve was best fitted by a two-site model with ED50S of 3 X 10(-9) and 9 X 10(-7) M. At the single Ca channel level the stimulatory effect of nitrendipine was due to an increased probability that a Ca channel which had opened once would reopen, a reduction in records without activity and an increase in the mean open time. There were no changes in unit conductance. Inhibitory effects were due to a large increase in nulls. At lower concentrations the main effect of Bay K8644 was an increase in the probability of opening. At doses above 10(-6) M, a pronounced increase in the open time was observed. The effects we observed are attributed to at least two sites for DHP related to Ca channels; one with high affinity and one with a lower affinity. The low affinity site mediates a stimulatory effect due to greatly prolonged openings.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-dependent effects of YC-170, a dihydropyridine calcium channel modulator, in cardiovascular tissues

Voltage-dependent effects of YC-170, a putative calcium channel activator, were examined and compared with those of Bay K 8644 in isolated guinea-pig cardiac tissues and rabbit aortae. In guinea-pig left atria superfused with a normal bathing solution (4 mmol/l K+), both YC-170 (10 mumol/l) and Bay K 8644 (1 mumol/l) produced a positive inotropic action accompanied by a prolongation of action potential durations (APDs). In normally-polarized guinea-pig papillary muscles Bay K 8644 increased force of contraction (fc) and APDs. However, YC-170 failed to increase fc in spite of a slight prolongation of APDs. In papillary muscles partially depolarized by 25 mmol/l K+ solution, Bay K 8644 enhanced the electrically-induced slow action potentials and contractile force. In contrast with Bay K 8644, YC-170 significantly depressed the slow action potentials and decreased fc. YC-170 also showed the depressant action on the slow action potentials induced by isoproterenol (0.1 mumol/l), histamine (3 mumol/l) and tetraethylammonium (10 mmol/l) plus high Ca2+ (4 mmol/l). In sinoatrial node cells of guinea-pig right atria Bay K 8644 produced a positive chronotropic action with increases in the maximum rate of rise (Vmax) and action potential amplitude (APA), whereas YC-170 produced a negative chronotropic action with decreases in Vmax and APA. In the rabbit aortic strips preincubated with bathing solution containing various concentrations of K+ (15, 20, 30 and 40 mmol/l), Bay K 8644 produced concentration-dependent contractions in a range of concentrations up to 0.3 mumol/l. However, when the concentration exceeded 1 mumol/l, Bay K 8644 caused a slight relaxation, irrespective of the K+ concentrations of bathing solution. YC-170 in concentrations of 10 and 30 mumol/l contracted the aortic strips placed in 5.9 or 15 mmol/l K+ bathing solution, but caused relaxation in 30 or 40 mmol/l K+ bathing solution. These results suggest that YC-170 is a dihydropyridine calcium channel modulator which behaves as a Ca2+ channel agonist in tissues of high membrane potentials, but as a Ca2+ channel antagonist in tissues of low membrane potentials.

Effects of BAY K-8644 on inotropic and arrhythmogenic actions of digoxin

Myocardial intracellular 'Ca2+ overload' may be involved in the direct arrhythmogenic actions of cardiotonic steroids. This proposal was examined by determining if the sensitivity of guinea-pig atrial muscle to digoxin-induced arrhythmias was affected by BAY K-8644, a 1,4-dihydropyridine derivative which promotes Ca2+ influx via slow channels. BAY K-8644 significantly reduced both the time required for a given concentration of digoxin to produce arrhythmias and the amount of digoxin bound to atrial muscle at the onset of arrhythmias. In addition, BAY K-8644 increased the maximum developed tension observed in the presence of digoxin before the onset of arrhythmias. Similar results were obtained with increasing concentrations of buffer Ca2+. In contrast, A23187, a Ca2+ ionophore, enhanced the sensitivity to digoxin-induced arrhythmias without affecting maximum developed tension. These results suggest that increases in intracellular Ca2+ enhance cardiac sensitivity to digoxin-induced arrhythmias and that the arrhythmogenic action may involve Ca2+ overload at a pool other than that which activates contractile proteins.

Modulation of calcium channel function by drugs

Calcium channel blocking drugs, or "calcium antagonists", have been increasingly used in the last decade, both as valuable cardiovascular drugs, and as tools to investigate the pharmacology of the calcium channels which play a vital role in the excitation-activation coupling of many excitable cells. Three important developments, "patch clamping" to investigate single calcium channels, ligand binding studies to investigate the calcium antagonist "receptor sites", and the introduction of novel calcium channel activators, or "calcium agonists", have recently led to greater understanding of the mechanism of action of drugs on the calcium channel. We show here how the calcium channel modulators interact with the binding sites to increase or decrease calcium flux, and hence to modulate the activity of many excitable tissues. We predict that these new developments will soon result in the isolation of purified calcium channels, and investigation of their subtypes and drug sensitivities. This information could lead to the introduction of novel, more selective calcium antagonists for a variety of indications such as atherosclerosis or neurological disorders. Of particular interest is the potential of tissue-selective calcium agonistic drugs to combat cardiac failure or endocrinological disorders.

Profile of the oppositely acting enantiomers of the dihydropyridine 202-791 in cardiac preparations: receptor binding, electrophysiological, and pharmacological studies

Receptor binding, electrophysiological, and inotropic effects of the pure dihydropyridine enantiomers (+)S202-791 and (-)R202-791 were studied in cardiac preparations. The KI for (+)S202-791 binding correlated with the ED50's for an increase in contractile force and an increase in calcium current, the latter effect occurring at depolarized as well as resting holding potentials. The KI for (-)R202-791 binding was much lower than the IC50's for inhibition of calcium current measured at holding potentials of -80 or -90 mV and a negative inotropic effect, but correlated closely with the IC50 for inhibition of calcium current measured at -30 mV. Thus, (+)S202-791, is a voltage independent calcium channel activator and (-)R202-791 is a voltage dependent calcium channel inhibitor.

Effect of calcium channel agonist Bay K 8644 on calcitonin secretion from a rat C-cell line

Bay K 8644, a novel dihydropyridine, stimulates calcitonin secretion in a dose-dependent manner from a rat medullary thyroid carcinoma cell line, rMTC 6-23, and causes an increase in cytosolic free calcium concentration, as measured by quin-2. These effects are competitively inhibited by nifedipine, and completely abolished in the absence of extracellular calcium. These data suggest that calcium influx via voltage-dependent calcium channels plays a crucial role in the regulation of cytosolic free calcium concentration and calcitonin secretion.

Characterization of the binding sites for nimodipine and (-)-desmethoxyverapamil in bovine cardiac sarcolemma

The bovine cardiac sarcolemmal binding sites for the dihydropyridine nimodipine and the phenylalkylamine (-)-desmethoxyverapamil were studied. The density of the nimodipine and (-)-desmethoxyverapamil binding sites increased 8.3-fold and 3.4-fold with the sarcolemma. The binding sites for both compounds were destroyed by trypsin. Nimodipine bound in the presence of 1 mM free calcium to a high-affinity and a low-affinity site with apparent Kd values of 0.35 +/- 0.09 nM (n = 9) and 33 +/- 6.0 nM (n = 9) and with apparent densities of 0.3 +/- 0.05 pmol/mg (n = 9) and 8.2 +/- 1.0 pmol/mg (n = 9). The binding to the high-affinity site was abolished by 1 mM EGTA. The binding sites were specific for dihydropyridines. The (-)-isomers of several phenylalkylamines inhibited nimodipine binding by an apparent allosteric mechanism. (-)-Desmethoxyverapamil bound in the presence of 5 mM EGTA to a high-affinity and a low-affinity site with apparent Kd values of 1.4 +/- 0.3 nM (n = 6) and 171 +/- 26 nM (n = 6) and with apparent densities of 0.16 +/- 0.02 pmol/mg (n = 6) and 13.6 +/- 2.7 pmol/mg (n = 6). The binding to both sites was inhibited by calcium with a half-maximal concentration of 4.3 mM. The binding sites were specific for the other phenylalkylamines and had a higher affinity for the (-)-isomers than for the (+)-isomers. Nimodipine inhibited the binding of (-)-desmethoxyverapamil by an apparent allosteric mechanism. d-cis-Diltiazem inhibited non-competitively the binding of (-)-[3H]desmethoxyverapamil with a Ki of 3.7 microM. Diltiazem up to concentrations of 10 microM did not affect the amount of nimodipine bound at equilibrium at 20 degrees C. However, but in agreement with this result, diltiazem decreased threefold at 20 degrees C the dissociation and association rates for the high-affinity nimodipine receptor. These rates were only marginally affected at 4 degrees C and 37 degrees C. d-cis-Diltiazem reversed in a competitive manner the inhibition of nimodipine binding elicited by the addition of (-)-desmethoxyverapamil with a Ka value of 1.6 microM. The amount of nimodipine bound was inhibited by 50% by the adenosine uptake inhibitors nitrobenzylthioinosine and hexobendine with apparent median inhibitory concentrations of 1 nM and 3 nM, respectively. Nitrobenzylthioinosine completely abolished binding of nimodipine to the low-affinity site, but did not affect binding to the high-affinity site.(ABSTRACT TRUNCATED AT 400 WORDS)

Dihydropyridine Ca2+ agonists and channel blockers interact in the opposite manner with photogenerated unpaired electrons

Interaction of Ca2+-channel antagonists (felodipine, ryocidil, verapamil, diltiazem) and agonists (dihydropyridine derivatives Bay K 8644 and CGP 28392) was studied by the methods of absorption spectroscopy. Ca2+-channel antagonists were found to act as electron donors, the agonists being electron acceptors in the interaction with dye free radicals in solution. Redox transitions in channel-forming protein were proposed as a possible mechanism of the modulation of channel activity by the compounds tested.

High and low affinity states of the dihydropyridine and phenylalkylamine receptors on the cardiac calcium channel and their interconversion by divalent cations

Drug receptors associated with Ca2+-channels in isolated chick heart membranes were found to exist in high and low affinity states. When assays were conducted in the presence of EDTA most of the receptors detected with the dihydropyridines (+)[3H]PN 200-110 or [3H]nitrendipine appeared to be in the lower affinity state. Inclusion of either Mg2+ or Ca2+ in the binding reactions resulted in the disappearance of the lower affinity state and the conversion of the receptors to a single high affinity state. Similar results were obtained with the phenylalkylamine derivative [3H]desmethoxyverapamil (D888). The results suggest that both the dihydropyridine and phenylalkylamine receptors on the cardiac Ca2+-channel can exist in interconvertible high and low affinity states in vitro, and that the proportion of receptors in each affinity state can be altered by the absence or presence of divalent cations.

The calcium channel blocker nitrendipine blocks sodium channels in neonatal rat cardiac myocytes

The dihydropyridine calcium channel blocker, nitrendipine, was studied for its effects on the sodium current of single cultured ventricular cells from neonatal rats. The patch-clamp method of recording whole cell currents was used, and sodium currents were isolated by suppressing potassium and calcium currents. Potassium currents were blocked by replacing potassium with cesium in the internal and external solutions and by adding tetraethylammonium chloride and 4-aminopyridine in the external solution; calcium current was blocked by replacing calcium with cobalt in the external solution. At low frequencies (0.1 Hz), nitrendipine reduced sodium currents without any significant change in the current-voltage relation. The block was dose dependent, and assuming a single occupancy model with complete block, had a half-maximum value of 3 X 10(-6) M at a holding potential of -80 mV where half the sodium channels are activatable. This value is within the range of the Kd's that have been reported for low-affinity dihydropyridine-binding sites found in cardiac sarcolemmal vesicles. In the presence of nitrendipine, the inactivation curve was shifted to hyperpolarized potentials. The block was greater with pulse intervals shorter than 1000 msec, and repriming was prolonged in the presence of the drug. These effects are similar to those of local anesthetics of the tertiary amine class, such as lidocaine. The block was relieved by the dihydropyridine agonist Bay K8644. The results are interpreted as indicating that dihydropyridines react with sodium channels.

Nitrendipine potentiates Bay k 8644-induced contraction of isolated porcine coronary artery: evidence for functionally distinct dihydropyridine receptor subtypes

Bay k 8644 and nitrendipine, dihydropyridines classified as calcium channel agonist and antagonist, respectively, produced concentration-dependent biphasic responses (contraction and relaxation) in porcine coronary artery rings. Nitrendipine relaxed rings (IC50 = 60 nM) that were contracted with 100 nM Bay k 8644. Pretreatment of rings with 60 nM nitrendipine caused paradoxical potentiation of Bay k 8644-induced contraction. The data are consistent with a model that consists of two functionally-distinct dihydropyridine "receptors" with which Bay k 8644 and nitrendipine interact as partial agonists. We propose that these excitatory and inhibitory dihydropyridine receptor subtypes mediate contraction and relaxation, respectively, by dihydropyridines.