Diltiazem potentiates the negative inotropic action of nimodipine in heart

Effects of diltiazem and verapamil on (+)-PN 200-110 binding kinetics in dog cardiac membranes

The effects of d-cis-diltiazem (diltiazem) and verapamil on 1,4-dihydropyridine binding to dog cardiac membranes were studied in competition, saturation and kinetic binding experiments with [3H](+)-PN200-110. Diltiazem increased [3H](+)-PN200-110 binding with an observed maximal effect at 50 microM, while verapamil decreased [3H](+)-PN200-110 binding in a dose-dependent manner. Scatchard analysis of saturation binding data revealed that diltiazem (50 microM) increased the maximal binding site density and verapamil (100 microM) increased the dissociation constant (KD) of [3H](+)-PN200-110 binding. The kinetic experiments demonstrated that diltiazem significantly reduced both the association and the dissociation rate of [3H](+)-PN200-110 binding, resulting in no significant change in the apparent KD. In contrast, verapamil accelerated dissociation and slowed down association of [3H](+)-PN200-110 binding. Diltiazem appears to alter both the number of [3H](+)-PN200-110 binding sites and the characteristics of [3H](+)-PN200-110 binding.

Relaxation of potassium chloride-induced contractions by amlodipine and its interaction with the 1,4-dihydropyridine-binding site in pig coronary artery

Amlodipine is a light-insensitive and water soluble 1,4-dihydropyridine with prolonged vasodilatory action and plasma half-life. To determine whether the vasodilatory action of amlodipine is due to inhibition of voltage-dependent Ca2+ influx through the calcium channel, its effects on KCl-induced contraction of pig coronary artery rings and its interaction with the 1,4-dihydropyridine binding site in isolated sarcolemmal membranes were studied. The contractile function of artery rings was studied in an organ bath system, and the interaction with the 1,4-dihydropyridine binding site was studied in isolated sarcolemmal membranes of pig coronary artery using [3H](+)PN200-110 as a radioligand. Amlodipine, (+)PN200-110 and (-)PN200-110 inhibited KCl-induced contractions of arterial rings in a dose-dependent manner. The half-maximal inhibition (IC50) was observed at 0.46 +/- 0.02, 36 +/- 8 and 55 +/- 9 nM of (+)PN200-110, (-)PN200-110 and amlodipine, respectively. [3H](+)PN200-110 was found to bind to isolated sarcolemmal membranes with high affinity (KD = 0.04 nM, Bmax = 312 fmoles/mg protein) and stereospecifically, (+)PN200-110 (Ki = 0.05 nM) having about 130-fold higher affinity than (-)PN200-110 (Ki = 6.61 nM). Amlodipine also inhibited [3H](+)PN200-110 binding with high affinity (Ki = 4.41 nM). The inhibition was characterized by an increase in KD of binding of [3H](+)PN200-110 with very little effect on Bmax. The order of relative potency of inhibition of KCl-induced contraction was almost identical to the order of relative affinity for the 1,4-dihydropyridine binding site, as indicated by Ki.(ABSTRACT TRUNCATED AT 250 WORDS)

Lactamimides: a novel chemical class of calcium antagonists with diltiazem-like properties

The effects of a series of lactamimides on [3H]d-cis-diltiazem binding to rat brain membranes, on [3H]nitrendipine binding to cardiac membranes, and on calcium-induced contractions in depolarized guinea pig taenia and ileum preparations were examined. Several of the lactamimides examined displaced [3H]d-cis-diltiazem binding and antagonized, in a competitive fashion, calcium-induced contractions. Over the series of lactamimides, there was a highly significant, positive linear correlation (r = 0.87, P less than 0.001) between their potency to displace [3H]d-cis-diltiazem and their potency to antagonize calcium-induced contractions in the depolarized taenia and ileum preparations. Of the lactamimides examined, MDL 16,582A [N-(2,2-diphenylpentyl)azacyclotridecan-2-imine. hydrochloride] had potency equivalent to d-cis-diltiazem with pA2 values of 7.27 and 7.38, respectively, against calcium-induced contractions in the guinea pig ileum. These lactamimides are a novel chemical class displaying diltiazem-like calcium antagonist properties.

Diltiazem enhances and flunarizine inhibits nimodipine's antiseizure effects

The dihydropyridine calcium channel antagonist, nimodipine has antiepileptic and anticonvulsive properties that are thought to be mediated through neuronal calcium channel blockade. The dihydropyridine binding site can be positively and negatively allosterically regulated by the benzothiazepines and the phenylalkylamines/piperazines, respectively. We investigated this binding interaction at the physiologic level by examining the effects of diltiazem (a benzothiazepine) and flunarizine (a piperazine) on the antiseizure activity of nimodipine. Seizures were induced with pentylenetetrazole in awake rats with chronically implanted EEG electrodes. Calcium channel antagonists were administered intracerebroventricularly 30 min after pentylenetetrazole at doses given at 15 min intervals. Diltiazem and flunarizine alone lacked antiseizure properties. The calculated ED50 values for nimodipine were: nimodipine alone = 135 micrograms; nimodipine + diltiazem (100 micrograms) = 67 micrograms. Nimodipine + flunarizine (10 micrograms) completely suppressed nimodipine's antiseizure activity. These findings may reflect the interaction observed among these agents at binding sites associated with the calcium channel and supports the idea that dihydropyridines mediate their antiseizure actions through neuronal calcium channel antagonism.

Variable, voltage-dependent, blocking effects of nitrendipine, verapamil, diltiazem, cinnarizine and cadmium on adrenomedullary secretion

1. Catecholamine release from cat adrenal glands perfused at a high rate (4 ml min-1) at 37 degrees C with modified Krebs solutions lacking Ca and containing 1.2 mM K (hyperpolarizing solution) or 118 mM K (depolarizing solution) was triggered by 10-s pulses of Ca (0.5 mM) in the presence of 118 mM K. Hyperpolarized glands released 1280 +/- 135 ng per pulse and depolarized glands 831 +/- 98 ng per pulse (n = 29). 2. While the dihydropyridine Ca channel blocker nitrendipine inhibited secretion in hyperpolarized glands with an IC50 of 214 nM, in depolarizing conditions the drug was much more potent (IC50 = 0.99 nM). In contrast, the inorganic Ca channel blocker cadmium inhibited secretion with the same potency both in hyperpolarized or depolarized glands. 3. Cinnarizine, diltiazem and verapamil exhibited intermediate degrees of voltage-dependence in blocking secretion. The IC50 ratios between hyperpolarized and depolarized glands were 215, 36, 19, 8 and 0.76 respectively for nitrendipine, cinnarizine, diltiazem, verapamil and cadmium. Because the experimental design (strong depolarization in the absence of Ca) favours the highest opening probability of Ca channels, it seems that these drugs bind preferentially to their receptors when these channels are in their open state. 4. Variable voltage-dependent effects of the five Ca channel blockers on adrenomedullary catecholamine release suggests different sites and mechanisms of action on, or near L-type Ca channels in chromaffin cells. In addition, these findings might help to explain why these drugs exhibit tissue selectivity and why they act differently in normal polarized as compared to ischaemic depolarized cells.

A new Ca-antagonist, CD-349, binding to the Ca-channel of rat myocardium and brain and hog coronary artery

The binding to a Ca-channel of a novel 1,4-dihydropyridine (DHP) Ca-antagonist, 2-nitratopropyl 3-nitratopropyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate (CD-349), was studied in rat myocardium and brain and hog coronary artery, and the binding was compared with that of other DHPs. In rat myocardium, the binding reaction of [3H]CD-349 was faster than that of nitrendipine (NTD). The association rate constant of [3H]CD-349 was about 10 times higher than that of [3H]NTD. The dissociation rate was also higher than that of [3H]NTD. Scatchard plot analysis of [3H]CD-349 binding showed one high affinity site for CD-349. The dissociation constant (Kd) and maximum number of binding sites (Bmax) were 333 pM and 286 fmoles/mg protein. They were almost the same as those for [3H]NTD. [3H]CD-349 and [3H]NTD bindings were dose dependently inhibited by 1,4-DHPs: nifedipine, nimodipine, nicardipine and CD-349. The order of the inhibitory potency of these drugs was CD-349 greater than nicardipine greater than nimodipine greater than nifedipine, when [3H]CD-349 and [3H]NTD were used as the ligands. Similar results were obtained for rat brain and hog coronary artery. [3H]CD-349 binding was not changed by alpha- or beta-adrenergic, cholinergic, histaminergic or serotonergic agents in rat myocardium. From these results, it is suggested that CD-349 binds to the Ca-channel reversibly with high affinity due to its high association rate for the site.

Combination therapy with diltiazem and nifedipine in patients with effort angina pectoris

The antianginal effects of diltiazem and nifedipine alone and in combination were evaluated in a double-blind, randomized, placebo-controlled trial in 11 patients (nine men and two women, 57 +/- 8 years old) with stable effort angina. Each patient received placebo, 30 mg of diltiazem, 10 mg of nifedipine, and 30 mg of diltiazem plus 10 mg of nifedipine four times daily for 1 week each. Antianginal efficacy was assessed by means of a treadmill exercise test. The exercise tolerance time was significantly prolonged from 235.1 +/- 52 (placebo period) to 342.2 +/- 101 sec by diltiazem (p less than .01) and to 325.6 +/- 73 sec by nifedipine (p less than .01). The drug combination further prolonged exercise time to 451.1 +/- 103 sec, which was significantly longer than the interval attained with either diltiazem (p less than .01) or nifedipine (p less than .01) alone. The plasma concentration of diltiazem was unaffected by the addition of nifedipine, whereas the plasma nifedipine concentration was significantly increased from 34.8 +/- 11 to 106.4 +/- 37 ng/ml (p less than .001) by the concomitant administration of diltiazem. These data suggest that exercise tolerance in patients with effort angina is increased by the concomitant administration of diltiazem and nifedipine associated with an increase in the nifedipine plasma concentration.

Inhibition of radioligand binding to A1 adenosine receptors by Bay K8644 and nifedipine

Two dihydropyridine compounds, Bay K8644 (a calcium entry activator) and nifedipine (a calcium entry blocker), were found to inhibit the binding of [3H]phenylisopropyladenosine ([3H]PIA) to A1 adenosine receptors in rat cerebral cortex membranes with comparable potencies (IC50 10-30 microM). Scatchard analyses indicated that both Bay K8644 and nifedipine inhibited the binding of [3H]PIA by increasing the KD but without significant effect on the Bmax. When tested at 100 microM, neither Bay K8644 nor nifedipine showed a significant effect on [3H]-p-aminoclonidine ([3H]PAC; alpha 2-adrenergic receptor), [3H]dihydroalprenolol ([3H]DHA; beta-adrenergic receptor), [3H]spiperone (dopamine receptor), and [3H]nitrobenzylthioinosine [( 3H]NBMPR; nucleoside transporter) binding. In the presence of 10 mM Mg2+, the ability of 2-chloroadenosine (2-Cl-Ad, an A1 adenosine receptor agonist) to displace [3H]PIA binding was increased. Conversely, the potencies of 1,3-diethyl-8-phenylxanthine (DPX; an A1 receptor antagonist), Bay K8644 and nifedipine in inhibiting [3H]PIA binding were unchanged. It is suggested that both Bay K8644 and nifedipine may act as antagonists of adenosine A1 receptors, in addition to their well-known effects on calcium channels.

Comparison of diltiazem and nifedipine alone and in combination in patients with coronary artery spasm

Fifteen patients with coronary artery spasm completed a double-blind placebo-controlled trial comparing diltiazem and nifedipine. Increasingly, higher daily doses (diltiazem, 90 to 360 mg; nifedipine, 30 to 120 mg) were administered to achieve optimal clinical effects. Daily diaries and ambulatory electrocardiographic recordings were used to assess efficacy and side effects. Both drugs significantly decreased angina frequency compared with that in the preceding placebo period (diltiazem 1.4 +/- 0.4 [mean +/- SEM] to 0.4 +/- 0.2 episodes per day; nifedipine 1.4 +/- 0.3 to 0.4 +/- 0.1 episodes per day; both p less than 0.05). Ambulatory electrocardiographic recordings showed fewer ST shifts than were expected during all treatment periods (0.02/h recorded during placebo, none during diltiazem and 0.02/h during nifedipine therapy). Although some patients responded better to one drug than the other, neither drug resulted in a clearly superior clinical response. Diltiazem was discontinued in one patient because of urticaria, but the total number of side effects was higher with nifedipine (12 of 15 patients) than with diltiazem (5 of 15, p less than 0.01). Nine patients remained symptomatic on single drug treatment and entered open label treatment with the combination of diltiazem and nifedipine. Three patients did not tolerate the combination because of important side effects; the other six also had side effects, but these were relatively minor. Four patients received no more benefit from the combination than from a single agent; the condition of two patients improved. Both diltiazem and nifedipine provide effective antianginal therapy for coronary spasm, but diltiazem has fewer side effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Classification of calcium antagonists

Drugs of several chemical families have been identified as calcium antagonists. This article examines some pharmacologic properties of these drugs to clarify their terminology and their classification and to provide a rationale for their clinical use. Studies with nifedipine show quantitatively that the therapeutic effect in angina is related to the interaction of this drug with membrane calcium channels in human coronary arteries. This gives support to a classification based on studies at the molecular, tissue and organ levels. Among calcium antagonists, calcium entry blockers are defined as agents able to block calcium inward fluxes evoked by various stimuli. They may be subdivided in 2 groups. Group I is the group of selective calcium entry blockers. Group IA consists of those agents selective for slow calcium channels in myocardium (slow channel blockers); the leading agents are verapamil, nifedipine and diltiazem. Group IB contains agents without action on slow calcium channels in myocardium but with selective action on arteries; the leading agents are cinnarizine and flunarizine. Group II is the group of nonselective calcium entry blockers. Group IIA contains agents acting at similar concentration on calcium and on fast sodium channels. Group IIB consists of agents interacting with calcium channels while having another primary site of action. Other agents modulate calcium movements by an action on sodium-calcium exchange and by an action within the cell. Their identification requires the use of cell biology. The actual clinical uses of these drugs are consistent with this pharmacologic classification.

Correlation between the negative inotropic potency and binding parameters of 1,4-dihydropyridine and phenylalkylamine calcium channel blockers in cat heart

Partially purified plasma membranes were prepared from cat ventricle. The purification factors for the calcium channel ligands (+)-3H-PN 200-110, 3H-nimodipine (1,4-dihydropyridines) and (-)-3-H-desmethoxyverapamil (a phenylalkylamine) were 3.1-, 3.4- and 2.9-fold, respectively, whilst beta-adrenoceptors labelled with (-)-3H-dihydroalprenolol were purified 3.0-fold. (+)-3H-PN 200-110 bound to 930 +/- 140 fmol/mg of membrane protein with a dissociation constant of 70 pmol/l at 25 degrees C. Under the same conditions 3H-nimodipine bound to 490 +/- 24 fmol/mg of sites with a KD of 120 pmol/l. (-)-3-H-desmethoxyverapamil bound to 530 +/- 55 fmol/mg of sites with a KD of 2.47 nmol/l. Twelve 1,4-dihydropyridines were evaluated for binding inhibition constants (Ki) with (+)-3H-PN 200-110 and 13 phenylalkylamines with (-)-3-H-desmethoxyverapamil in radioligand binding assays. Of the twelve 1,4-dihydropyridines evaluated (+/-)-nitrendipine was the most potent with a Ki-value of 280 pmol/l, nifedipine had a Ki-value of 500 pmol/l and the weakest drug tested, (+/-)-Bay b 4328, had a Ki-value of 14.3 nmol/l. The EC50-values of the same 1,4-dihydropyridines to inhibit the electrically driven cat papillary muscle were 77- to 3,450-fold higher and little correlation existed between Ki and EC50-values. Thirteen phenylalkylamines were tested for their potency to inhibit (-)-3-H-desmethoxyverapamil binding. The most potent phenylalkylamine with respect to negative inotropy was (+/-)-D 595 with an EC50-value of 794 nmol/l, the least potent substance was (+/-)-Sz 45 with an EC50-value of 39.8 mumol/l. The binding inhibition constants for the phenylalkylamines were 13- to 322-fold lower than the values for negative inotropy, but a significant positive correlation between the Ki and EC50-values (n = 12, r = 0.84) was observed. The absolute differences may reflect the state-dependent binding of phenylalkylamines to the channel. QSAR analysis revealed nearly identical correlations between physicochemical substituent properties on the one hand and binding affinities or functional potency on the other hand. In both cases the electronic properties (F-constant) of ring substituents mainly determine the variance in potency.

Effects of calcium antagonists on KCl-evoked calcium uptake by rat cortical synaptosomes

A series of calcium antagonists were used to study their blocking effect on high potassium-induced calcium uptake into rat cortical synaptosomes; these antagonists were classified into five groups: dihydropyridine group (i.e. nifedipine and nitrendipine), benzothiazepine group (i.e. diltiazem), phenylalkylamine group (i.e. verapamil and D600), phenothiazine group (i.e. trifluoperazine) and diphenylpiperazine group (i.e. flunarizine and cinnarizine). Voltage-dependent 45Ca2+-uptake into this fraction was measured after 20 sec KCl-induced depolarization. The ID30 values of the above-mentioned antagonists affecting 45Ca2+-uptake were calculated to be nitrendipine (80 microM), nifedipine (100 microM), verapamil (50 microM), D600 (15 microM), diltiazem (70 microM), trifluoperazine (7 microM), cinnarizine (1.2 microM) and flunarizine (0.7 microM). Our results reveal that in rat brain synaptosomal fractions, calcium influx via the voltage-gated calcium channel appears to be more sensitive to diphenylpiperazine and phenothiazine groups; whereas, phenylalkylamine, benzothiazepine and dihydropyridine groups were relatively insensitive. This contrasts with the well known data obtained from vascular smooth muscle, in which the dihydropyridine group is the most sensitive of all the groups studied. Our results suggest that calcium channels in neuronal tissue are most likely different from those in non-neuronal tissue.

Nimodipine's interactions with other drugs: II. Diazepam

Adult Binghamton Heterogeneous (HET) stock mice were administered one of three doses of diazepam (0.1, 2.5, or 5.0 mg/kg) immediately followed by a second injection of either the slow calcium channel blocker, nimodipine (Bay e 9736), or its vehicle. Hypothermic responses and muscular incoordination were measured twenty and sixty minutes later as assessed by changes in rectal temperature and motoric activity on a rotating rod. Nimodipine (5 mg/kg) alone did not significantly affect body temperature or motor coordination. However, when administered in combination with the two highest doses of diazepam, nimodipine significantly potentiated the hypothermic response produced by these doses both twenty minutes and sixty minutes post-injection. Administration of high doses of diazepam (2.5 and 5.0 mg/kg) resulted in significant motor incoordination at both observation periods, but this effect was not potentiated by nimodipine.

Effect of neuraminidase on diltiazem-mediated alteration of nitrendipine binding in the hog coronary artery

The dissociation constant (Kd) and the maximum number of binding sites (Bmax) of 3H-nitrendipine (3H-NTD) were not altered by neuraminidase (NUase) treatment. The Kd and Bmax values were approximately 0.2 nM and 70 fmoles/mg protein, respectively. The influences of diltiazem on 3H-NTD bindings differed between NUase treated and untreated preparations. Diltiazem increased 3H-NTD (100 pM) binding dose-dependently with a 40% increase at about 10(-6) M in the untreated preparation. After treatment with NUase, a low dose of diltiazem decreased 3H-NTD (100 pM) binding, and the greatest effect was observed at 10(-9) M with a 50% decrease, while at a high dose of diltiazem (10(-6) M), 80% increase of the binding was observed. Scatchard plot analysis indicated that a high dose of diltiazem (10(-6) M) increased only the affinity of 3H-NTD to the binding site in both the NUase-treated and untreated preparations. In the untreated preparation, a low dose of diltiazem (10(-9) M) had no effect on 3H-NTD binding, but in the NUase-treated preparation, diltiazem (10(-9) M) showed opposing effects, namely, an increase and decrease of the binding at low and high concentrations of 3H-NTD, respectively. No straight line was obtained when a hill plot analysis of the effect of diltiazem on 3H-NTD binding was made. These results suggested that diltiazem allosterically regulates dihydropyridine derivative binding, and in this regulation, the concentration ratios between diltiazem and the dihydropyridine derivative is the most important factor. Furthermore, it is suggested that the sialic acid of the proteoglycan is involved in these interactions, especially in the effect of diltiazem on dihydropyridine binding.

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)

Relationship between the stereoselective negative inotropic effects of verapamil enantiomers and their binding to putative calcium channels in human heart

Ventricular preparations from patients with mitral disease and hypertrophic obstructive cardiomyopathy (HOCM) were set up to contract isometrically. Ventricular membrane particles were also prepared and putative calcium channels were labelled with [3H]-nimodipine. Positive staircase was induced by varying the rate of stimulation of isolated strips from 6 min-1 to 120 min-1 in the presence of 6-60 microM (-)-adrenaline or (-)-noradrenaline. (-)-Verapamil 3-5 microM or (+)-verapamil 20-30 microM reversed the force-frequency relationship (i.e. caused negative staircase) in preparations from patients with mitral disease or HOCM. In subendocardial strips of ventricular septum from 5 patients with HOCM paced at 60 min-1, both (-)-verapamil and (+)-verapamil caused cardiodepression. Half-maximal cardiodepression was observed with 0.4 microM (-)-verapamil and with 3 microM (+)-verapamil. [3H]-nimodipine bound to ventricular membrane particles in a saturable, reversible fashion to a high affinity site with an equilibrium dissociation constant of 0.23 nM. The density of these sites was 95 fmol mg-1 of membrane protein. Binding of the tritiated 1,4-dihydropyridine was stereoselectively inhibited by 1,4-dihydropyridine enantiomers and nifedipine. (-)-Verapamil and (+)-verapamil inhibited high affinity [3H]-nimodipine binding in a negative heterotropic allosteric manner with (-)-verapamil being 5 times more potent than (+)-verapamil on an IC50 basis. At a given [3H]-nimodipine concentration, (+)-verapamil inhibited a greater fraction of specific [3H]-nimodipine binding. The allosteric mode of (+)-verapamil inhibition of [3H]-nimodipine binding was confirmed by kinetic studies. (-)-Verapamil shifted (+)-verapamil-binding inhibition curves to the right in an apparently competitive fashion. The inversion of staircase caused by both verapamil enantiomers suggests that they cause a use-dependent channel blockade. The similar potency ratios for binding and for cardiodepression are indicative of a common locus of action for both verapamil enantiomers within the calcium channel.

Effect of membrane depolarization on binding of [3H]nitrendipine to rat cardiac myocytes

Binding of the dihydropyridine calcium antagonist [3H]nitrendipine was studied in cardiac myocytes incubated in normal and high potassium buffer so that we might examine the voltage dependence of dihydropyridine binding. Hearts were obtained from adult male Wistar rats, and isolated calcium tolerant myocytes were dissociated by enzymatic dispersion. Cells in 5.6 mM extracellular potassium showed specific binding of [3H]nitrendipine with KD 587 +/- 50 (mean +/- SE) pM and maximum receptor density, Bmax, 10.8 +/- 1.3 fmol/mg wet weight. Cells depolarized in 50 mM potassium showed no change in KD, 661 +/- 77 pM, but approximate doubling of Bmax 25.6 +/- 3.7 fmol/mg wet weight. Binding equilibrium was reached within 5 minutes at 37 degrees C, and the KD determined by kinetic analysis was in good agreement with KD determined by saturation experiments. Unlabeled nitrendipine and nifedipine completely inhibited [3H]nitrendipine binding with slope factors less than one, whereas verapamil and diltiazem only partially inhibited binding with slope factors substantially less than one. As a function of increasing extracellular potassium concentration from 2.4-54.1 mM, the number of nitrendipine-binding sites increased gradually 115%. Aconitine also produced a 58% increase in binding sites over a concentration range of 1-30 micrograms/ml. The potassium-induced changes in number of binding sites occurred rapidly and were rapidly reversible with changes in extracellular potassium concentration. There was no change in the number of [3H]nitrendipine-binding sites as a function of potassium or aconitine concentration in dead or digitonin-treated myocytes. We conclude that nitrendipine receptor density in viable myocytes is voltage dependent, but we detect no change in KD as a function of voltage.

The molecular pharmacology and structural features of calcium channels

Images

BAY K 8644, a 1,4-dihydropyridine Ca2+ channel activator: dissociation of binding and functional effects in brain synaptosomes

K+-stimulated 45Ca2+ uptake into rat brain and guinea pig cerebral cortex synaptosomes was measured at 10 s and 90 s at K+ concentrations of 5-75 mM. Net increases in 45Ca2+ uptake were observed in rat and guinea pig brain synaptosomes. 45Ca2+ uptake under resting or depolarizing conditions was not increased by the 1,4-dihydropyridine BAY K 8644, which has been shown to activate Ca2+ channels in smooth and cardiac muscle. High-affinity [3H]nitrendipine binding in guinea pig synaptosomes (KD = 1.2 X 10(-10) M, Bmax = 0.56 pmol mg-1 protein) was competitively displaced with high affinity (IC50 2.3 X 10(-9) M) by BAY K 8644. Thus high-affinity Ca2+ channel antagonist and activator binding sites exist in synaptosome preparations, but their relationship to functional Ca2+ channels is not clear.

Effects of Bay k 8644, a dihydropyridine analog, on [3H]nitrendipine binding to canine cardiac sarcolemma and the relationship to a positive inotropic effect

Equilibrium dissociation constants of Bay k 8644, a calcium agonist, and nitrendipine, a calcium antagonist, were determined in canine cardiac sarcolemma. The equilibrium dissociation constant for Bay k 8644 was compared to the concentration that produced a fifty percent increase, and the equilibrium dissociation constant for nitrendipine was compared to the concentration that produced a fifty percent decrease, in contractile force in canine heart trabecular muscle. Both saturation and inhibition binding data suggest that Bay k 8644 and nitrendipine bind to and compete for a high affinity dihydropyridine-binding site present in isolated cardiac sarcolemma preparations. The equilibrium dissociation constant (7-10 nM) and concentration that produced a fifty percent increase in contractile force in the canine trabecular muscle (30 +/- 8 nM) of Bay k 8644 were in a similar concentration range, but the equilibrium dissociation constant (0.29 +/- 0.025 nM) of nitrendipine binding was more than a thousand-fold lower than the concentration that produced a fifty percent decrease in contractile force in canine trabecular muscle (613 +/- 109 nM). These data suggest that binding of Bay k 8644 to high affinity binding sites is pharmacologically relevant, and is related to a positive inotropic effect.

Solubilization and affinity labeling of a dihydropyridine binding site from skeletal muscle: effects of temperature and diltiazem on [3H]dihydropyridine binding to transverse tubules

Effects of temperature and d-cis-diltiazem (DTZ) on [3H]nitrendipine (NTD) and [3H]nimodipine (NIM) binding to skeletal muscle t-tubular membranes were studied. A decrease in temperature from 37 degrees C to 10 degrees C decreased KD and increased Bmax slightly. DTZ increased binding by increasing Bmax under all conditions and also decreased KD for NTD at 37 degrees C. The binding protein labeled with [3H]isothiocyanate dihydropyridine revealed a molecular weight of 36,000. The binding site for NTD was solubilized by deoxycholate and dihydropyridine binding was still stimulated by DTZ in the solubilized form.

High affinity specific [3H](+)PN 200-110 binding to dihydropyridine receptors associated with calcium channels in rat cerebral cortex and heart

The binding properties of the 1,4-dihydropyridine calcium channel antagonist, [3H](+)PN 200-110, were studied in rat cerebral cortical and cardiac homogenates (37 degrees C, Krebs phosphate buffer). Specific binding of [3H](+)PN 200-110 was saturable, reversible, and of high affinity (Kd values are 35 and 64 pM for the cerebral cortex and heart, respectively). In parallel studies with [3H](+)PN 200-110, the dissociation constant of [3H]nitrendipine was 10-12 times higher. Substituted dihydropyridine calcium channel antagonists and agonists competitively inhibited specific [3H](+)PN 200-110 binding, but d-cis diltiazem enhanced and verapamil incompletely inhibited [3H](+)PN 200-110 binding in both the cerebral cortex and the heart. The effects of diltiazem and verapamil on [3H](+)PN 200-110 binding were due mainly to alterations in the dissociation constant (Kd), without alterations in the binding density (Bmax). The new [3H](+)PN 200-110 receptor binding assay is remarkable for its low degree of nonspecific binding as compared to [3H]nitrendipine at physiological temperatures. [3H](+)PN 200-110 is a useful ligand for the further analysis of the dihydropyridine binding sites associated with calcium channels.

Solubilization of the nitrendipine receptor from skeletal muscle transverse tubule membranes. Interactions with specific inhibitors of the voltage-dependent Ca2+ channel

The nitrendipine receptor associated with the voltage-dependent calcium channel from rabbit skeletal muscle transverse tubule membranes has been solubilized by detergent extraction. A highly stable solubilized receptor preparation was obtained using 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate as detergent with phospholipids or glycerol present as stabilizing agents. Binding of [3H]nitrendipine to the solubilized receptor was reversible and saturable. At 4 degrees C the equilibrium dissociation constant of the [3H]nitrendipine X receptor complex was 7 +/- 3 nM and was close to that determined from the rate constants of association (k1 = 1.3 10(5) M-1 s-1) and dissociation (k-1 = 1.10 X 10(-3) s-1) of 8.4nM. The nitrendipine concentration that gave a half-maximal inhibition of [3H]nitrendipine binding to the solubilized receptor was 10 nM, which was similar to the values for the dissociation constant determined for the radiolabelled ligand. [3H]Nitrendipine binding to its solubilized receptor was also inhibited by other antiarrythmic drugs, such as bepridil and verapamil, and enhanced by d-cis-diltiazem. Since these drugs are apparent non-competitive inhibitors of [3H]nitrendipine binding it was concluded that these different binding sites are tightly coupled. Sucrose density sedimentation of solubilized nitrendipine receptor resulted in the separation of three [3H]nitrendipine binding activities with apparent sedimentation coefficients of 11.4 S, 14.4 S and 21 S.