Effect of an homologous series of aliphatic alcohols on neuronal and smooth muscle voltage-dependent Ca2+ channels.

The acute inhibitory actions of alcohol on K(+)-stimulated 45Ca2+ uptake into synaptosomes shows regional variation in sensitivity throughout the brain, suggesting the possibility of a selective action on a specific Ca2+ channel subtype. This was examined by comparing the effects of a homologous series of aliphatic alcohols on synaptosomal Ca2+ channels with their actions on K(+)-stimulated Ca2+ channels in guinea-pig intestinal longitudinal muscle, which have been demonstrated to be of the L-type. K(+)-stimulated contraction of and [3H]nitrendipine binding to smooth muscle were both inhibited by the alcohols at similar concentrations, with the potency increasing with chain length. In synaptosomes, however, K(+)-stimulated 45Ca2+ uptake was 5-30 times more sensitive to the inhibitory actions of alcohol than were [3H]nitrendipine and [125I]omega-conotoxin binding. These observations suggest that K(+)-stimulated 45Ca2+ uptake is mediated by a non-L non-N type channel which is more sensitive to the acute effects of alcohols. This is supported by the observation that K(+)-stimulated 45Ca2+ uptake which is insensitive to L- and N-channel antagonists was inhibited by funnel web spider venom.

1,4-Dihydropyridine activators in the tiamdipine series.

The pharmacologic and radioligand binding properties of 5-nitro analogs of the 1,4-dihydropyridine Ca2+ channel antagonist, tiamdipine (2-(2-aminoethylthio)methyl-3-carbomethoxy-5-carbomethoxy-6-m ethyl-4-(3-nitrophenyl)-1,4-dihydropyridine) and its N-formyl derivative have been measured in rat tail artery, guinea pig ileum and rat heart. The enantiomers of both analogs showed activator and antagonist properties, the latter being exhibited at lower concentrations.

Characterization of ion movements and their relationship to muscarinic receptor binding and excitation-contraction coupling in guinea pig ileal longitudinal smooth muscle.

The relationship between ion movements (sodium uptake and potassium release) and agonist-induced contractile responses or muscarinic receptor binding was investigated in the guinea pig ileal longitudinal muscle (GPLM). Sodium uptake and potassium release were agonist-dependent, concentration-dependent, and stereoselective, with the following rank order of maximum ion movement: muscarinic agonists greater than histamine greater than substance P = serotonin. Potassium depolarization did not initiate sodium uptake or potassium release. Sodium uptake was rapid and monophasic, preceding potassium release which was biphasic in nature. Full muscarinic agonists produced equal maximal increases in sodium uptake, while maximal potassium release varied for all muscarinic agonists and in addition differed from sodium uptake in the following ways: time course, stereoselectivity, sensitivity to calcium antagonists, modulation by the guanylyl nucleotide derivative, 5'-guanylylimidodiphosphate (Gpp(NH)p), and inhibition by muscarinic receptor blockade with benzilylcholine mustard. The calcium ionophores A23187 and ionomycin (SQ23377) did not produce any sodium uptake; A23187 but not ionomycin produced potassium release comparable to that evoked by muscarinic agonists. Ion movement in response to combinations of agonists were not additive. Muscarinic agonist binding as measured by competition for [3H]quinuclidinyl benzilate ([3H]QNB) binding, was best described by multiple sites and was regulated by Gpp(NH)p. Excellent correlations were observed between the dissociation constants for binding and sodium uptake, potassium release, and contraction. The best correlations were those between the pharmacologic responses and the high affinity binding site in the absence, and the low affinity site in the presence, of Gpp(NH)p, respectively. Furthermore, the potencies of muscarinic agonists to evoke ion movements and to inhibit [3H]QNB binding were similar, and from one to two orders of magnitude less than those for contraction. It is suggested that contraction and potassium release were mediated by the high affinity, and sodium uptake by the low and average affinity muscarinic agonist binding sites, respectively. These findings suggest an agonist-activated receptor-effector coupling model in GPLM that leads to the activation of sodium uptake, potassium release, and subsequently, contraction.

Binding of A 1,4-dihydropyridine calcium channel activator, (-) S Bay K 8644, to cardiac preparations.

The 1,4-dihydropyridine Ca2+ channel activator, (-) [3H]Bay K 8644, binds to cardiac membranes and polarized [5 mM K+] and depolarized [50 mM K+] cardiac cells. Binding to microsomal membranes at 25 degrees C indicates a single set of binding sites, KD = 2.9 x 10(-9) M and a site density, 337 fmoles/mg protein, not different from that measured by antagonist 1,4-dihydropyridines. Binding to neonatal rat myocytes at 37 degrees C was independent of membrane potential with a KD value of 5 x 10(-8)M and a site density, 63 fmoles/mg protein, not significantly different from that measured by PN 200 110. These results indicate that 1,4-dihydropyridine activators and antagonists label the same number of binding sites in cardiac tissue, but that activator binding to intact myocytes is voltage-independent.

Dimeric 1,4-dihydropyridines as calcium channel antagonists.

A series of 1,n-alkanediylbis(1,4-dihydropyridines) (n = 2, 4, 6, 8, 10, 12) bridged at C3 of 2,6-dimethyl-3-carboxy-5-carbethoxy-4-(3-nitrophenyl)-1,4-dihydropyridin e were synthesized and evaluated in a radioligand binding assay, [3H]nitrendipine in intestinal smooth muscle, as Ca2+ channel ligands. Binding activity was comparable to that of nitrendipine itself but independent of chain length, suggesting the lack of a major binding contribution by the second 1,4-dihydropyridine group. Analogues lacking the second 1,4-dihydropyridine nucleus or possessing an inactive function (4-nitrophenyl) were no less active, confirming that this series of ligands likely does not bridge adjacent 1,4-dihydropyridine receptors of the Ca2+ channel.

The actions of peppermint oil and menthol on calcium channel dependent processes in intestinal, neuronal and cardiac preparations.

The activities of menthol and peppermint oil were determined in guinea-pig ileal smooth muscle, in rat and guinea-pig atrial and papillary muscle, in rat brain synaptosomes and in chick retinal neurones by pharmacological 45Ca2+ uptake and radioligand binding assays. Menthol is a major constituent of peppermint oil and is approximately twice as potent as peppermint oil as an inhibitor of K+ depolarization-induced and electrically stimulated responses in ileum and electrically stimulated atrial and papillary muscles. IC50 values in the ileal preparation ranged from 7.7 to 28.1 micrograms ml-1 and in the cardiac preparations from 10.1 to 68.5 micrograms ml-1. Similar potencies were demonstrated against K+ depolarization-induced 45Ca2+ uptake in synaptosomes and against K+ depolarization and Bay K 8644-induced uptake in chick retinal neurons. IC50 values for menthol inhibition of K+ and Bay K 8644 responses in the retinal neurons were 1.1 x 10(-4) M (17.2 micrograms ml-1) and 1.75 x 10(-4) M (26.6 micrograms ml-1), respectively, and for peppermint oil were 20.3 and 41.7 micrograms ml-1 respectively. Both menthol and peppermint oil inhibited specific [3H]nitrendipine and [3H]PN 200-110 binding to smooth and cardiac muscle and neuronal preparations with potencies comparable to, but slightly lower than, those measured in the pharmacological and 45Ca2+ uptake experiments. Binding of menthol and peppermint oil, studied at 78 micrograms ml-1, was competitive against [3H]nitrendipine in both smooth muscle and synaptosome preparations. The data indicate that both menthol and peppermint oil exert Ca2+ channel blocking properties which may underlie their use in irritable bowel syndrome. Ca2+ channel antagonism may not be the only pharmacological effect of menthol and peppermint oil contributing to intestinal smooth muscle relaxation.

Crystal structures and pharmacologic activities of 1,4-dihydropyridine calcium channel antagonists of the isobutyl methyl 2,6-dimethyl-4-(substituted phenyl)-1,4-dihydropyridine-3,5-dicarboxylate (nisoldipine) series.

A series of isobutyl methyl 2,6-dimethyl-4-(X-substituted phenyl)-1,4-dihydropyridine-3,5-dicarboxylates (X = H, 2-NO2, 3-NO2, 3-CN, 3-MeO, 4-F, 2-CF3, 3-CF3, and 4-Cl) related to and including nisoldipine (X = 2-NO2) has been synthesized, their solid-state structures determined by X-ray analysis (X = H, 2-NO2, 3-NO2, 3-CN, 3-MeO, and 4-F), and their pharmacologic activities determined, as the racemic compounds, against [3H]nitrendipine binding and K+-depolarization-induced tension responses in intestinal smooth muscle as measures of Ca2+ channel antagonist activity. Comparisons of structure are presented to previously analyzed 1,4-dihydropyridines. The degree of 1,4-dihydropyridine ring puckering is dependent on the nature and position of the phenyl ring substituent and the adopted interring conformation. Different ester substituents affect 1,4-dihydropyridine ring puckering to a small extent in most cases. Pharmacologic and radioligand binding activities for the nine compounds studied show a parallel dependence on phenyl ring substituent, but the compounds are approximately 10-fold more active in the radioligand binding assay than in the pharmacologic assay. Consistent with a previous report for the nifedipine series (Fossheim et al. J. Med. Chem. 1982, 25, 126), pharmacologic activity increases with increasing 1,4-dihydropyridine ring planarity.

Comparative aspects and temperature dependence of [3H]1,4-dihydropyridine Ca2+ channel antagonist and activator binding to neuronal and muscle membranes.

Binding of [3H]nitrendipine, [3H]nimodipine, and (+)[3H]PN 200-110 to microsomal preparations of guinea pig smooth and cardiac muscle and brain synaptosomes revealed high affinity interaction with KD values in the sequence, (+)PN 200-110 greater than nitrendipine greater than nimodipine. Bmax values for a particular tissue were independent of the 1,4-dihydropyridine employed in radioligand binding at 25 degrees C. The temperature dependence of [3H]nitrendipine binding in cardiac and smooth muscle microsomal preparations and brain synaptosomes was measured from 0 degrees to 37 degrees C and for skeletal muscle preparations from 0 degrees to 30 degrees C. Bmax values increased with temperature for cardiac membranes, but did not vary in other tissues. van't Hoff plots were nonlinear in all tissues, enthalpy and entropy changes becoming increasingly negative with increasing temperature. Competition binding of the activator-antagonist enantiomeric 1,4-dihydropyridine pairs of Bay k 8644 and PN 202-791 for [3H]nitrendipine in smooth muscle did not reveal significant thermodynamic differences between activator and antagonist molecules.

Pharmacologic and radioligand binding analysis of the actions of 1,4-dihydropyridine activator-antagonist pairs in smooth muscle.

The actions of the enantiomers of Bay K 8644 and 202-791 were studied in rat tail artery and guinea pig ileal longitudinal smooth muscle using pharmacologic and radioligand binding assays. (-)-(S)-Bay K 8644 (below 10(-7) M in rat tail artery and 3 X 10(-7) M in guinea pig ileum) and (+)-(S)-202-791 (below 10(-6) M) induced contractions and potentiated the responses to KCl depolarization in both smooth muscle preparations. In contrast, (+)-(R)-Bay K 8644 and (-)-(R)-202-791 inhibited the responses to KCl-induced depolarization. At higher concentrations, (-)-(S)-Bay K 8644 (10(-7) to 10(-6) M) and (+)-(S)-202-791 (10(-6) to 3 X 10(-5) M) in rat tail artery, and (-)-(S)-Bay K 8644 (3 X 10(-7) to 3 X 10(-6) M) in guinea pig ileal smooth muscle relaxed the tissues contracted maximally at lower concentrations of the same drug. Cross antagonism between 1,4-dihydropyridine activators was observed when (-)-(S)-Bay K 8644 (10(-7) to 10(-6) M) relaxed the maximum contraction in response to (+)-(S)-202-791. [3H]Nitrendipine bound in a tail arterial microsomal preparation to a single class of site, with KD of 3.63 X 10(-10) M and maximum binding of 552.7 fmol mg-1 protein. In both rat tail artery and guinea pig ileal smooth muscle, (-)-(S)-Bay K 8644, (+)-(R)-Bay K 8644, (+)-(S)-202-791 and (-)-(R)-202-791 inhibited specific [3H]nitrendipine binding competitively; the Kl values correlate well to the pharmacologic EC50 (for activators) or IC50 (for antagonists, measured against 80 mM KCl depolarization) values. The biphasic response to (-)-(S)-Bay K 8644 and (+)-(S)-202-791 suggests that the properties of Ca++ channel activation and antagonism may reside within a single 1,4-dihydropyridine molecule.

Effects of metal cations and calmodulin antagonists on [3H] nitrendipine binding in smooth and cardiac muscle.

It was previously reported that [3H]nitrendipine binding to a microsomal fraction from intestinal smooth muscle was dependent upon the presence of divalent metal cations (Bolger et al., J. Pharmacol. Exp. Ther. 225: 291-309, 1983). The effects of cations and calmodulin antagonists on [3H]nitrendipine binding in smooth and cardiac muscle have been studied further. Treatment of ileal and aortic smooth muscle and cardiac muscle with EDTA reduced specific [3H]nitrendipine binding by 70 to 95%. Microsomes from rabbit ventricle were more resistant to EDTA treatment than were those from ileal smooth muscle, but low concentrations of Ca++ (less than 10(-5) M) produced half-maximal restoration of binding in both tissues. The ability of cations at a concentration of 10(-3) M to restore binding to membranes from guinea-pig ileum was in the sequence, Ca++ = Sr++ greater than Mg++ = Mn++ = Co++ greater than Ba++ = Ni++ greater than Zn++ = Cd++ greater than La = Sm = Tm . In contrast to the activation of calmodulin-dependent processes, the ability of these cations to restore [3H]nitrendipine binding did not correlate linearly with ionic radius. However, calmodulin antagonists were found to inhibit [3H]nitrendipine binding with the order of potency: pimozide greater than less than calmidazolium (R 24571) greater than trifluoperazine greater than chlorpromazine greater than promethazine greater than chlorpromazine sulfoxide, that correlates quite well with the potency of these drugs as inhibitors of calmodulin-dependent processes. The results suggest that calmodulin antagonists bind to a protein associated with the [3H]nitrendipine binding site that has a hydrophobic domain similar to that exposed on calmodulin by Ca++, but that this protein is not calmodulin itself.

Subcellular distribution of [3H]nitrendipine binding in smooth muscle.

Distribution of specific binding sites for [3H]nitrendipine was studied in subcellular fractions isolated from rat gastric fundus smooth muscle and from rat myometrium. There was an excellent correlation between the distribution of [3H]nitrendipine binding determined at the nitrendipine concentrations of 0.138 and 1.38 nM, and the distribution of the plasma membrane markers K+-activated ouabain-sensitive p-nitrophenylphosphatase, 5'-nucleotidase, phosphodiesterase I, and Mg-ATPase, but not between the mitochondrial markers cytochrome c, oxidase, succinate-dependent cytochrome c reductase, or rotenone-insensitive NADH-dependent cytochrome c reductase or the putative endoplasmic reticulum marker NADPH-dependent cytochrome c reductase. The binding occurred with high affinity and with a similar (0.097-0.146 nM) equilibrium dissociation constant to all the fractions, even though the density of binding sites varied and was highest in the plasma membrane marker-enriched fractions. The maximal binding in the plasma membrane-enriched fraction from the rat gastric fundus smooth muscle was 0.43 +/- 0.04 pmol/mg, and in that from rat myometrium was 0.72 +/- 0.09 pmol/mg. Thus in the two smooth muscles studied the plasma membrane is the locus of the high affinity nitrendipine binding.

Characterization of binding of the Ca++ channel antagonist, [3H]nitrendipine, to guinea-pig ileal smooth muscle.

A chemically heterogeneous group of compounds, the Ca++ channel antagonists, which includes verapamil, diltiazem and nifedipine inhibits excitation-contraction coupling in smooth and cardiac muscle by blocking Ca+ entry at a specific class of Ca++ channels. The binding of the nifedipine analog, [3H]nitrendipine, to a microsomal fraction from guinea-pig longitudinal smooth muscle has been characterized. Specific binding was saturable, linear with protein concentration and reversible. The apparent equilibrium dissociation constant was 1.63 +/- 0.06 X 10(-10)M and the maximum site density was 1.13 +/- 0.03 pmol/mg of protein determined from Scatchard analysis of equilibrium binding at 25 degrees C. Inhibition of binding was specific and stereoselective for Ca++ channel antagonist drugs and was unaffected by a variety of receptor active ligands. Correlations between binding and inhibition of mechanical response to methylfurmethide- and K+-stimulation in a series of nifedipine analogs were determined. A 1:1 correlation was found for the K+ tonic response, but for the phasic component of the K+ response and for both components of the methylfurmethide response the antagonists were more active as inhibitors of [3H]nitrendipine binding than as inhibitors of mechanical response. [3H]Nitrendipine binding was sensitive to other Ca++ channel antagonists including verapamil, D-600, diltiazem, flunarizine, lidoflazine and bepridil. Interaction with these agents suggests, consistent with previous reports, that more than one binding site for Ca++ antagonists exists. A variety of inorganic divalent and trivalent cations (Mn++, Co++, Ni++, Pb++, UO2++, Zn++, Cd++, Cu++, Tm+++ and La+++) inhibit specific [3H]nitrendipine binding. The data suggest that [3H]nitrendipine binding in smooth muscle is to a site which mediates the pharmacologic response.

Calcium-channel antagonist binding to isolated vascular smooth muscle membranes.

The binding characteristics of the calcium-channel antagonist nitrendipine, an analog of nifedipine, have been measured in plasma membrane enriched microsomal fractions from the canine thoracic aorta, canine mesenteric artery, and rat mesenteric artery. The dissociation constants, KD, for the high-affinity binding sites were, respectively, 0.308, 0.254, and 0.101 nM and Bmax values for binding capacity were 20.3, 25.0, and 18.0 fmol/mg of microsomal protein. Studies with isolated tissues, in which the sensitivity of the high potassium mechanical response to nitrendipine was determined, indicate that the apparent KD for nitrendipine in the canine mesenteric artery, as reflected by the IC50 (mean inhibition constant) value, is 4.5-5.3 nM and for the rat mesenteric artery 2.5-6.6 nM.