SYNTHESES, CALCIUM-CHANNEL AGONIST-ANTAGONIST MODULATION ACTIVITIES, AND VOLTAGE-CLAMP STUDIES OF ISOPROPYL DIMETHYL-3-NITRO-4-PYRIDINYLPYRIDINE-5-CARBOXYLATE RACEMATES AND ENANTIOMERS

A novel group of racemic isopropyl dimethyl-3-nitro-4-pyridinylpyridin e-5-carboxylate isomers [(+/-)-12-14] were prepared using a modified H antzsch reaction that involved the condensation of nitroacetone with i sopropyl 3-aminocrotonate and 2-, 3-, or 4-pyridinecarboxaldehyde. Det ermination of their in vitro calcium channel-modulating activities usi ng guinea pig ileum longitudinal smooth muscle (GPILSM) and guinea pig left atrium (GPLA) assays showed that the 2-pyridinyl isomer (+/-)-12 acted as a dual cardioselective calcium channel agonist (GPLA)/smooth muscle selective calcium channel antagonist (GPILSM). In contrast, th e 3-pyridinyl [(+/-)-13] and 4-pyridinyl [(+/-)-14] isomers acted as c alcium channel agonists on both GPLA and GPILSM. The agonist effect ex hibited by (+/-)-12 on GPLA was inhibited by nifedipine and partially reversed by addition of extracellular Ca2+. In anesthetized rabbits, t he 4-pyridinyl isomer (+/-)-14 exhibited a hypertensive effect that wa s qualitatively similar to that exhibited by the nonselective agonist Bay K 8644 and the 3-pyridinyl isomer (+/-)-13, whereas the 2-pyridiny l isomer (+/-)-12 induced a hypotensive effect similar to that of the calcium channel antagonist nifedipine. Similar results were obtained i n a spontaneously hypertensive rat model. In vitro studies showed that the (+)-2-pyridinyl enantiomer (+)-12A exhibited agonist activity an both GPILSM and GPLA, but that the (-)-2-pyridinyl enantiomer (-)-12B exhibited agonist activity on GPLA and antagonist activity on GPILSM. Whole-cell voltage-clamp studies using isolated guinea pig ventricular myocytes indicated that (-)-12B inhibited the calcium current (I-Ca), that (+)-12A increased slightly I-Ca; and that (+/-)-12 inhibited I-C a but the latter inhibition was less than that for (-)-12B. (-)-12B ef fectively inhibited I-Ca at all membrane potentials examined (-40-50 m V), whereas (+)-12A exhibited a weak agonist effect near the peak of t he I-V curve. The 2-pyridinyl isomers (enantiomers) 12 represent a nov el type of 1,4-dihydropyridine calcium channel modulator that could pr ovide a potentially new approach to drug discovery targeted toward the treatment of congestive heart failure and probes to study the structu re-function relationships of calcium channels.