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

Effects of dihydropyridine Ca2+ channel blockers on the discriminative stimulus and the motor impairing effects of (+/-)-Bay K 8644

Functional interactions between the dihydropyridine Ca2+ channel activator, (+/-)-Bay K 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethyphenyl )-pyridine-5-carboxylate), and several dihydropyridine L-type Ca2+ channel blockers were investigated on rotarod performance in mice and in rats trained to discriminate between (+/-)Bay K 8644 and saline. When administered alone, (+/-)-Bay K 8644 produced dose-dependent impairments of rotarod activity with an ED50 of 1.3 mg/kg. Pretreatment with nifedipine (10-30 mg/kg) produced dose-dependent rightward shifts of the (+/-)-Bay K 8644 dose-response curve. In contrast, pretreatment with several other dihydropyridine L-type Ca2+ channel blockers, including nicardipine, nimodipine, isradipine and nitrendipine, did not modify the (+/-)-Bay K 8644 dose-effect function. Rats learned to discriminate between (+/-)-Bay K 8644 (0.5 mg/kg) and saline in an average of 65 training sessions. In substitution tests, the Ca2+ channel activator engendered dose-related increases in the percentage of rats selecting the drug-associated lever with an ED50 of 0.19 mg/kg. Pretreatment with nifedipine (10 mg/kg) produced a rightward shift of the (+/-)-Bay K 8644 dose-response function. Pretreatment with nicardipine (2.5 mg/kg) only partially antagonised the training dose of (+/-)-Bay K 8644 whereas nimodipine (0.6-10 mg/kg) did not affect the (+/-)-Bay K 8644 discriminative stimulus. The results of the present study show that the behavioural effects of the dihydropyridine Ca2+ channel activator are differentially modified by dihydropyridine L-type Ca2+ channel blockers. These results may suggest that dihydropyridine blockers possess different intrinsic activities or act at different binding sites.

Molecular localization of regions in the L-type calcium channel critical for dihydropyridine action

Sensitivity to dihydropyridines (DHPs) is a distinct characteristic that differentiates L-type Ca2+ channels from T-, N-, and P-type Ca2+ channels. To identify regions necessary for the functional effects of DHPs, chimeric Ca2+ channels were constructed in which portions of motif III or motif IV of a DHP-insensitive brain Ca2+ channel, BI-2, were introduced into the DHP-sensitive cardiac L-type Ca2+ channel. The resultant chimeric Ca2+ channels were expressed in Xenopus oocytes, and the effects of a DHP agonist and antagonist were studied. The results show that the linker region between S5 and S6 in motif IV of the L-type Ca2+ channel is a major site for DHP action. The DHP agonist and antagonist molecules interact with distinct sites on the alpha 1 subunit of the L-type Ca2+ channel. The data further show that the SS2-S6 region of motif III is not involved in DHP action but may be an important structural component of inactivation.

Effects of (+)-S-12967 and (-)-S-12968, two enantiomers of a new slow-acting 1,4-dihydropyridine, on rat coronary resistance arteries

The action of (+)-S-12967 and (-)-S-12968, two isomers of a new 1,4-dihydropyridine molecule (2-(-7-amino-2,5-dioxaheptyl)-3-ethoxycarbonyl-4-(2,3-dichlorop hen yl)-5-methoxycarbonyl-6-methyl 1,4-dihydropyridine), was studied on responses of rat isolated coronary resistance arteries (i.d. about 230 microns) to K+, Ca2+, and 5-hydroxytryptamine (5-HT). Both isomers slowly relaxed coronary arteries contracted with 125 mM K+, reaching a maximal effect in about 2 h. In contrast, the maximal relaxing effect of nifedipine was obtained within 20 min. The response to 125 mM K+ did not recover within the 2-h washout period in vessels exposed to the isomers but returned to pre-drug levels within 40 min in vessels exposed to nifedipine. Nifedipine was 4 times more potent than the (-)-isomer which again was about 200 times more potent that the (+)-isomer. The IC50[M] values were approximately 1 nM, 4 nM and 0.8 microM, respectively. The relaxing effect of the isomers, which has a pKa of 8.6, was dependent on the extracellular pH being greater at high than low pH. Both isomers antagonized the vessel responses to K+ and Ca2+ and 5-HT. Higher concentrations of the isomers were required to antagonize responses to K+ and 5-HT than to Ca2+, probably due to the more depolarized state of the vascular smooth muscle in the latter experiments. In conclusion, the results demonstrate extracellular pH dependence as well as stereoselectivity regarding potency of (+)-S-12967 and (-)-S-12968 in rat coronary arteries.(ABSTRACT TRUNCATED AT 250 WORDS)

(+)-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.

(+)-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)

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.

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.

Calcium and activation of the release of endothelium-derived relaxing factor

Indirect and direct experimental evidence demonstrates that both the entry of extracellular calcium and the liberation of calcium from intracellular stores can contribute to an increase in free cytoplasmic calcium concentration in endothelial cells, which seems to be an essential step in the synthesis and/or release of endothelium-derived relaxing factors(s). A variety of Ca2+ transport mechanisms may be involved in the regulation of cytoplasmic calcium in endothelial cells. Ca2+ entry may occur via voltage-operated Ca2+ channels. If they do exist, these channels may have characteristics different from those in underlying vascular smooth muscle cells. Sustained activation of the release of EDRF by various receptor agonists (e.g., acetylcholine, adenine nucleotides, and bradykinin) is also dependent on Ca2+ entry, but it is insensitive to organic Ca2+ channel antagonists. These findings indicate that, when used clinically in various cardiovascular diseases, organic calcium channel antagonists are not expected to interfere with endothelium-dependent relaxation evoked by endogenous vasoactive substances (e.g., ADP, serotonin). Since amiloride and its analogues blocked endothelium-dependent relaxations in different arterial preparations, Na+ transport and Na+/Ca2+ exchange were suggested to play a role in calcium-dependent release of EDRF. The exact nature of Ca2+ transport mechanisms and also the calcium-sensitive cellular processes that lead to the synthesis/release of endothelium derived relaxing factor(s) remain to be determined. However, the available data suggest that calcium handling by the vascular smooth muscle and endothelial cells may be different, allowing potentially selective modulation of Ca2+ activation in these two cell types.

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.

Induction of seizures in mice by intracerebroventricular administration of the calcium channel agonist BAY k 8644

The calcium channel agonist BAY k 8644 was used to investigate the role of the calcium ion (Ca2+) in epileptogenesis. Intracerebroventricular administration of the compound induced murine seizures that were reversed by calcium channel inhibitors (CCIs) but not by anticonvulsants such as carbamazepine, pentobarbital, and diazepam. The seizures were exacerbated by phenytoin and valproic acid. Chronic administration of CCI's, previously shown to produce down-regulation of the binding of the CCI [3H]nitrendipine, resulted in augmentation of BAY k 8644-induced seizures.

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.

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.