The activities of menthol and peppermint oil were determined in guineapig 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 lnhibitor of K+ depolarization-induced and electrically stimulated responses in ileum and electrically stimulated atrial and papillary muscles. IC,, values in the ileal preparation ranged from 7.7 to 28.1 pg ml-' and in the cardiac preparations from 10.1 to 68.5 pg ml-I.Similar potencies were demonstrated against K+ depolarizationinduced "Ca2+ uptake in synaptosomes and against K+ depolarization and Bay K 8644-induced uptake in chick retinal neurons. IC,, values for menthol inhibition of K+ and Bay K 8644 responses in the retinal neurons were 1.1 x M (26.6 pg ml-I), respectively, and for peppermint oil were 20.3 and 41.7 pg ml-'respectively. Both menthol and peppermint oil inhibited specific M (17.2 pg ml-') and 1.75 x

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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.

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 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.