Correlation of nitrendipine and Bay k 8644 binding to isolated canine heart sarcolemma with their pharmacological effects on the canine heart

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)

The binding of (+)-isradipine by guinea-pig intact atria

1. The accumulation of [3H]-(+)-isradipine (PN 200-110) was measured in quiescent guinea-pig left atria with normal (K+ 2.7 mM) or lowered (K+ 40 mM) membrane potential. 2. Under control conditions (2.7 mM K+) a high affinity binding of (+)-isradipine could not be detected. If, however, the atria were partially depolarized to about -30 mV by 40 mM K+, high affinity binding became evident displaying a dissociation constant of 4.2 x 10(-11) M and a capacity of 9.7 nmol kg-1 wet wt. 3. The depolarization-induced binding was reversible upon repolarization of the atria although isradipine was still present in the medium. This indicates that the high affinity binding sites disappear as soon as the cell membranes become polarized. 4. Isradipine belongs to the less hydrophobic dihydropyridines, but nevertheless the unsaturable binding led to an accumulation of about 84 fold. At a concentration of 2 x 10(-8) M (+)-isradipine, which reduces the contractile force by 50%, the cellular concentration will rise to more than 10(-6) M.

Pharmacological analysis of the effects of Bay K 8644 and organic calcium antagonists on the mouse isolated distal colon

1. Bay K 8644 (10(-8) to 10(-6) M) induced concentration-related contractions of the longitudinal muscle of the mouse distal colon. The maximal responses were enhanced and the EC50 was lowered in the presence of tetrodotoxin (TTX; 1.5 x 10(-7) M). The responses were not affected by atropine (10(-7) M), mepyramine (2.5 x 10(-7) M), methysergide (5 x 10(-7) M), propranolol (10(-6) M), phentolamine (10(-6) M) or naloxone (4 x 10(-7) M). By contrast, the contractile responses were inhibited by Ca2+ entry blockers (verapamil, nifedipine) and abolished in Ca2+-free EGTA solution. These observations indicate that the contractile effects of Bay K 8644 are dependent on its ability to promote Ca2+ influx. 2. At 10(-4) M, Bay K 8644 provoked a slow relaxation of the preparation. Moreover, from 10(-5) M, Bay K 8644 markedly reduced the contractile responses to ACh and K+ depolarization. These inhibitory effects were comparable with those produced by nifedipine. Such data suggest that, at high concentrations, Bay K 8644 could act in part as a dihydropyridine Ca2+ channel antagonist. 3. Bay K 8644 (10(-9) M) preferentially enhanced, while nifedipine (10(-10) to 10(-8) M) as well as verapamil (3 x 10(-9) to 10(-6) M) preferentially inhibited, the tonic component of the contractile response evoked by K+ depolarizing solution. This may indicate that different populations of voltage-sensitive Ca2+ channels are involved in the biphasic response to K+ depolarization. 4. The biphasic contractile activity induced by ACh was barely enhanced by Bay K 8644 (10-9M) and was less sensitive to Ca2+ entry blockers than the responses to KCl. These findings are discussed in terms of receptor-operated channels and mobilization of bound calcium.

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.

Analysis of the properties of binding of calcium-channel activators and inhibitors to dihydropyridine receptors in chick heart membranes

The interaction of 1,4-dihydropyridine derivatives with their receptors on voltage-dependent calcium channels in cardiac membranes was studied to determine if there are basic differences in the binding properties of ligands that cause inhibition or activation of calcium channels. The binding characteristics of 6 pure stereoisomers, (-) and (+)202-791, (-) and (+)Bay k 8644, (-) and (+)PN 200-110, as well as racemic Bay k 8644 and nitrendipine, were compared. Competition studies using the cold ligands and 3 different radiolabelled dihydropyridines, (+)[3H]PN 200-110, (+/-)[3H]nitrendipine, and (+/-)[3H]Bay k 8644, showed that, for each combination tested, the labelled dihydropyridine could be displaced by the cold dihydropyridine. The binding reactions were markedly affected by temperature. The Kd values for most compounds were significantly higher (5-19 times) at 0 degrees than at 37 degrees C. In contrast, the affinity of (+)PN 200-110 was similar at 0 degrees and 37 degrees C, but slightly higher at 25 degrees C. A thermodynamic analysis indicated that the binding of the two pure isomers that are Ca2+-channel activators ("agonists"), (-)Bay k 8644 and (+)202-791, was driven entirely by enthalpy and was associated with an unfavorable decrease in entropy. This was in marked contrast to the binding of the inhibitors ("antagonists"). The binding of (+)PN 200-110 and nitrendipine at low temperatures was driven largely or entirely by entropy. Other antagonist-binding reactions were driven mainly by enthalpy but were associated with favorable increases in entropy. The affinity of the three radiolabelled ligands for the dihydropyridine receptor differed 100 times and appeared to be due to large differences in dissociation rate constants for each of the ligands. The rates of dissociation of (+)[3H]PN 200-110 and (+/-)[3H]nitrendipine, but not of (+/-)[3H]Bay k 8644, were significantly slowed by diltiazem, a calcium-channel inhibitor that binds to another receptor on the calcium channel. The results show that there were marked differences in the binding of the various dihydropyridines and suggest that the energetics of binding of Ca2+-channel activators and inhibitors may be fundamentally different.

High and concentration-proportional accumulation of [3H]-nitrendipine by intact cardiac tissue

The binding of [3H]-nitrendipine to intact, electrically driven isolated left atria of guinea-pigs was investigated over the concentration range 10(-10) M to 3 X 10(-5) M. A high affinity binding site saturable in the nM range as found in ventricular homogenates could not be detected. Instead the accumulation of nitrendipine in intact atria was found to be proportional to the concentration from 10(-10) M to 10(-6) M; beyond 10(-6) M the binding started to become saturated. Nitrendipine was highly accumulated in atrial tissue. The cell:medium ratios amounted to about 120 in the range from 10(-10) M to 10(-6) M. The concentration-response curve for the negative inotropic action of nitrendipine yielded an ED50 of 3 X 10(-7) M, thus lying within the range of concentration-proportional accumulation. The reduction of the contractile force proceeded faster (t1/2 less than 10 min) than the uptake process t1/2 approximately 40 min) suggesting that it is the binding of nitrendipine into a superficial compartment, which interferes with the excitation-contraction coupling. The results suggest that the high concentration of nitrendipine present in hydrophobic cellular compartments such as the plasmalemma might be involved in its pharmacological action.

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)

Effects of Bay K 8644 on contraction of the human isolated bronchus and guinea-pig isolated trachea

The effects of Bay K 8644, a dihydropyridine which increases calcium flux through the potential-operated channels were studied on the contractions induced by histamine, acetylcholine, KCl and Ca2+ on human isolated bronchial strips and the results were compared to those obtained on guinea-pig isolated tracheal spirals. Subsequently the contractant effects of Bay K 8644 in K+-enriched medium and in the presence of Ca2+ 0.03 mM were investigated. In Krebs normal calcium medium, Bay K 8644 did not significantly modify the EC50 of acetylcholine or histamine on the human bronchus, but in concentrations of 10(-7)-10(-6)M it potentiated the effects of KCl on that preparation. It did not modify the EC50 of acetylcholine, histamine or KCl on the guinea-pig trachea. In Ca2+-free Krebs medium with additional K+ (30 mM), Ca2+ concentration-response curves were displaced to the left by Bay K 8644 in the two preparations. Shifts were 0.52 +/- 0.11 and 0.72 +/- 0.16 log units respectively with Bay K 8644 10(-8) and 10(-7) M on human bronchus (n = 4) and 0.67 +/- 0.16 and 1.06 +/- 0.19 log units respectively with Bay K 8644 10(-7) and 10(-6) M on the guinea-pig trachea (n = 5). In Krebs medium with Ca2+ 0.03 mM and K+ 30 mM, Bay K 8644 (10(-8) to 10(-6) M) contracted both the human bronchus and the guinea-pig isolated trachea. This effect was competitively antagonized by nicardipine. 5 These results demonstrate the presence of dihydropyridine sites of action on human bronchus and confirm the minor role played by Ca2+ influx through potential-operated channels in the contractile effects of acetylcholine or histamine. They also demonstrate the similar reactivity of human bronchus and guinea-pig isolated trachea to Bay K 8644.

Dietary intake of sodium chloride in the rat influences [3H]nitrendipine binding to adrenal glomerulosa cell membranes but does not alter binding to vascular smooth muscle membranes

Angiotensin II-stimulated secretion by adrenal glomerulosa cells and contraction by vascular smooth muscle (VSM) are dependent on calcium influx through membrane calcium channels. We have examined the hypothesis that the altered responsiveness of adrenal glomerulosa cells and VSM to angiotensin II during NaCl restriction may be associated with a change in membrane calcium channel number. To test this hypothesis, female rats were placed on a high or low NaCl diet. On the 14th day, membranes were prepared from the zona glomerulosa, aorta, mesenteric artery, and uterus. [3H]Nitrendipine binding was used to monitor calcium channel number. The [3H]nitrendipine binding capacity was observed to be higher in the zona glomerulosa during NaCl restriction than during high NaCl intake (83 +/- 18 vs. 49 +/- 9 fmol/mg protein, P less than 0.025, n = 6 paired experiments). The binding capacities of [3H]nitrendipine on the low and high NaCl diet were similar in the mesenteric artery (10 +/- 1 vs. 9 +/- 1 fmol/mg protein, n = 8), aorta (33 +/- 5 vs. 35 +/- 8 fmol/mg protein, n = 5), or uterus (87 +/- 15 vs. 85 +/- 16 fmol/mg protein, n = 4), respectively. The dissociation constants of [3H]nitrendipine binding did not differ on a low or high NaCl intake in the zona glomerulosa (0.84 +/- .12 vs. 0.79 +/- .10 nM), mesenteric artery (0.82 +/- .06 vs. 83 +/- .05 nM), aorta (0.90 +/- .11 vs. 0.92 +/- .12 nM), or uterus (0.55 +/- .12 vs. 0.56 +/- .10 nM), respectively. We conclude that the blunted response of VSM to angiotensin II during NaCl restriction is best explained by the previously reported lower number of angiotensin II receptors since calcium channel number does not change. In the adrenal glomerulosa cell, NaCl restriction is associated with a higher number of membrane calcium channels and angiotensin II receptors. The increase in calcium channel number may reflect the influence of an unknown factor(s) believed to be necessary for the full expression of the adrenal glomerulosa cell response to NaCl restriction.

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.

Slow channel calcium activators, a new group of pharmacological agents

Specific calcium channels in myocardium and vascular smooth muscle and pharmacologic agents which possess the ability to block them have been the subject of intense research over the past several years. Many studies have utilized dihydropyridine derivatives (e.g. nifedipine, nitrendipine, nisoldipine) which have been shown to be efficacious inhibitors of calcium influx through voltage sensitive slow channels. Administration of these agents results in vascular smooth muscle relaxation and negative inotropic effects. Recently, novel dihydropyridines such as Bay k 8644, CGP 28 392 and YC-170, with actions diametrically opposed to those of compounds typified by nifedipine have been synthesized. These agents demonstrate vaso-constrictor and positive inotropic effects - actions which might be expected of compounds capable of stimulating the transmembrane influx of calcium into vascular smooth muscle and myocardium. Actions of Bay k 8644 and CGP 28 392 studied in vitro and in vivo have also shown that pharmacological blockade of beta or alpha adrenergic receptors does not influence the direct effects of these agents. Future analogs, with similar but more selective actions on myocardial calcium channels, may prove useful in the management of pathologic states characterized by insufficient contractile function of the heart.

The calcium channel activator, Bay K 8644, enhances K+-evoked efflux of acetylcholine and noradrenaline from rat brain slices

Slices of rat brain were incubated with either (3H) choline (hippocampus) or (3H) noradrenaline (hypothalamus) and superfused with Krebs buffer. The release of (3H) acetylcholine and (3H) noradrenaline after inhibition of monoamine oxidase by pargyline was induced by a short exposure to Krebs buffer containing elevated K+ ions (25 mmol/l). Nifedipine (1 mumol/l) caused only a slight inhibition of noradrenaline efflux and was without effect on acetylcholine overflow. The calcium channel activator, Bay K 8644 (0.1-1 mumol/l), increased the K+-evoked efflux of both neurotransmitters. The additional efflux evoked by Bay K 8644 (0.3 mumol/l) was blocked by nifedipine (1 mumol/l). The results from the present study thus extend the earlier findings with the neurotransmitter 5-hydroxytryptamine to include noradrenaline and acetylcholine. The functional correlates for voltage operated calcium channels concerned with transmitter release are clearly a widespread phenomenon in the CNS.

Stimulation rate modulates effects of the dihydropyridine CGP 28 392 on cardiac calcium-dependent action potentials

Calcium (Ca2+)-dependent action potentials were recorded from 22 mM potassium (K+)-depolarized guinea-pig papillary muscle at several different pacing frequencies in the absence and presence of CGP 28 392 (10 microM), a Ca2+ channel agonist. The maximum upstroke velocity (Vmax) of the slow response action potential was measured to determine relative changes in Ca2+ current as a function of pacing frequency. CGP 28 392 increased Vmax more than two fold at low rates of stimulation (1 or 12 pulses min-1), but had no significant effect on Vmax during rapid pulsing (200 pulses min-1). The enhancement of Vmax was dependent upon extracellular [K+]. Increasing extracellular [K+] from 22 mM to 27 mM suppressed the frequency-dependent agonist effects and increased the antagonist effects on Vmax. These results indicate that CGP 28 392 is a partial Ca2+-channel agonist and suggest that its effects on Ca2+ current are voltage-dependent.

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.

[3H]diltiazem binding to calcium channel antagonists recognition sites in rat cerebral cortex

The presence of a diltiazem recognition site within the macromolecular complex of the calcium channel in the brain had been hypothesized on the basis of binding studies with [3H]dihydropyridine calcium channel antagonists. In the present study, we therefore characterized [3H]diltiazem binding sites in the rat cerebral cortex. Saturable high affinity (Kd = 50-170 nM) [3H]diltiazem binding to the rat cerebral cortex was stereospecifically inhibited by the enantiomers of diltiazem according to their activity as calcium channel antagonists and modulators of [3H]dihydropyridine binding. An association between the [3H]diltiazem binding site and the calcium channel was further corroborated by the effects of chemically heterogeneous calcium channel antagonists on [3H]diltiazem binding. Dihydropyridines appeared to allosterically affect [3H]diltiazem binding according to their pharmacological effects; e.g. at 37 degrees C nitrendipine enhanced whereas the calcium agonist Bay K 8644 failed to affect [3H]diltiazem binding at concentrations fully inhibiting [3H]nitrendipine binding. The effect of nitrendipine may, at least in part, be explained by an increase in the affinity of [3H]diltiazem.

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

Bay k 8644 produced a concentration-dependent positive inotropic effect followed by a negative inotropic effect in isolated and intact cardiac preparations. Nimodipine in low concentrations produced slight positive inotropy and in higher concentrations, the usual negative inotropic action. Radioligand binding experiments revealed equilibrium dissociation constants that, taken together with the pharmacological data, suggest that dihydropyridines bind to receptor subtypes and have varying intrinsic activities.