A novel group of racemic isopropyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-pyridinylpyridine-5-carboxylate isomers [(+/-)-12-14] were prepared using a modified Hantzsch reaction that involved the condensation of nitroacetone with isopropyl 3-aminocrotonate and 2-, 3-, or 4-pyridinecarboxaldehyde. Determination of their in vitro calcium channel-modulating activities using 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, the 3-pyridinyl [(+/-)-13] and 4-pyridinyl [(+/-)-14] isomers acted as calcium channel agonists on both GPLA and GPILSM. The agonist effect exhibited by (+/-)-12 on GPLA was inhibited by nifedipine and partially reversed by addition of extracellular Ca2+. In anesthetized rabbits, the 4-pyridinyl isomer (+/-)-14 exhibited a hypertensive effect that was qualitatively similar to that exhibited by the nonselective agonist Bay K 8644 and the 3-pyridinyl isomer (+/)-13, whereas the 2-pyridinyl isomer (+/-)-12 induced a hypotensive effect similar to that of the calcium channel antagonist nifedipine. Similar results were obtained in a spontaneously hypertensive rat model. In vitro studies showed that the (+)-2-pyridinyl enantiomer (+)-12A exhibited agonist activity on 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 (ICa), that (+)-12A increased slightly ICa, and that (+/-)-12 inhibited ICa but the latter inhibition was less than that for (-)-12B. (-)-12B effectively inhibited ICa at all membrane potentials examined (-40-50 mV), whereas (+)-12A exhibited a weak agonist effect near the peak of the I-V curve. The 2-pyridinyl isomers (enantiomers) 12 represent a novel type of 1,4-dihydropyridine calcium channel modulator that could provide a potentially new approach to drug discovery targeted toward the treatment of congestive heart failure and probes to study the structure-function relationships of calcium channels.

Taurine plays an important role in cell volume regulation in both vertebrates and invertebrates. Erythrocytes from two euryhaline fish species, the eel (Anguilla japonica) and the starry flounder (Platichthys stellatus) were found to contain high intracellular concentrations of this amino acid (approximately equal to 30 mmol per liter of cell water). Kinetic studies established that the cells possessed a saturable high-affinity Na+-dependent beta-amino-acid transport system which also required Cl- for activity (apparent Km (taurine) 75 and 80 microM; Vmax 0.85 and 0.29 mumol/g Hb per hr for eel (20 degrees C) and flounder cells (10 degrees C), respectively. This beta-system operated with an apparent Na+/Cl-/taurine coupling ratio of 2:1:1. A reduction in extracellular osmolarity, leading to an increase in cell volume, reversibly decreased the activity of the transporter. In contrast, low medium osmolarity stimulated the activity of a Na+-independent nonsaturable transport route selective for taurine, gamma-amino-n-butyric acid and small neutral amino acids, producing a net efflux of taurine from the cells. Neither component of taurine transport was detected in human erythrocytes. It is suggested that these functionally distinct transport routes participate in the osmotic regulation of intracellular taurine levels and hence contribute to the homeostatic regulation of cell volume. Volume-induced increases in Na+-independent taurine transport activity were suppressed by noradrenaline and 8-bromoadenosine-3', 5'-cyclic monophosphate, but unaffected by the anticalmodulin drug, pimozide.

Torpor in mammals and birds is characterized by a periodic lowering of the set point for body temperature regulation to achieve a hypometabolic state for energy and water conservation. Torpor may be seasonal (hibernation, estivation) or non-seasonal (nocturnal hypothermia, daily torpor) and, depending on the depth and duration of the torpor, energy savings from 10 to 88% may be achieved. Despite the different patterns of torpor, recent studies have demonstrated a physiological homology between sleep and torpor, suggesting a continuum in energy conservation in times of inactivity. The physiological and biochemical adaptations for torpor are many, including both species-specific and seasonally dependent aspects. Two examples are discussed. In terms of cardiac function, differences exist among species in the maintenance of resting and action potentials at low temperatures, whereas seasonal differences exist for dependence of trans-sarcolemmal calcium flux in excitation–contraction coupling. With respect to ionic regulation, the ability of red blood cells to maintain high intracellular K+ and low intracellular Na+ and Ca2+ at low temperatures is species-specific and not seasonally dependent. The competence for intracellular Ca2+ regulation (i.e., active calcium pump) at low temperatures appears to be a key feature that allows the hibernator's cells to function continually despite the prolonged and profound depression of body temperature during hibernation.

The sodium borohydride reduction of 3,5-disubstituted 1,4-dihydro-2,6-dimethyl-4-(pyridinyl)pyridines 2 and 5 in the presence of methyl, phenyl, or tert-butyl chloroformate afforded the respective 4-(dihydropyridinyl)-1,4-dihydropyridines 4 and 6 in good yield. Products 4 comprised a mixture of the 1,2- and 1,6-dihydropyridinyl regioisomers 4a and 4b where 4a was always the predominant regioisomer. Compounds possessing a 4-[dihydro-1-(phenoxycarbonyl)-3-pyridinyl] substituent, such as 26, were also a mixture of two regioisomers 26a and 26b, and each regioisomer existed as a mixture of two rotamers in Me2SO-d6 at 25 degrees C (26a', 26a'', and 26b', 26b'') due to restricted rotation about the nitrogen-to-carbonyl carbamate bond. The calcium antagonist activities for 4 and 6 were determined by using the muscarinic receptor-mediated Ca2+-dependent contraction of guinea pig ileal longitudinal smooth muscle. The relative order of activities for the 4-(dihydropyridinyl) analogues was 4-(dihydro-3-pyridinyl) greater than 4-(dihydro-4-pyridinyl). Increasing the size of the C-3(5) alkyl ester substituents increased activity. Compounds having nonidentical ester substituents were more active than those having identical ester substituents. Replacement of the C-3 and/or C-5 ester substituents by a cyano substituent(s) decreased activity significantly. An approximate 1:1 correlation between the IC50 value for inhibition of [3H]nitrendipine binding and inhibition of the tonic component of the muscarinic-induced contractile response was observed. The test results suggest that a 4-(dihydropyridinyl) substituent is bioisosteric with a 4-(nitrophenyl) substituent on a 1,4-dihydropyridine ring where m- and p-nitrophenyl are bioisosteric with the 4-[1,2(1,6)-dihydro-3-pyridinyl] 4 and 4-(1,2-dihydro-4-pyridinyl) 6 isomers, respectively.

The Hantzsch condensation of alkyl acetoacetates 3 with methyl 3-aminocrotonate (4) and pyridinecarboxaldehydes 5 afforded the unsymmetrical alkyl methyl 1,4-dihydro-2,6-dimethyl-4-(pyridinyl)-3,5-pyridinedicarboxylates 6, whereas condensation of 3 with 5 and ammonium hydroxide gave the symmetrical dialkyl 1,4-dihydro-2,6-dimethyl-4-(pyridinyl)-3,5-pyridinedicarboxylates 7. The calcium channel antagonist activities of disubstituted 1,4-dihydro-3,5-pyridinedicarboxylates 6,7, and 9 were determined with use of the muscarinic-receptor-mediated Ca2+-dependent contraction of guinea pig ileal longitudinal smooth muscle. The relative potency order for isomeric pyridinyl analogues 6 and 7 was 2-pyridinyl greater than 3-pyridinyl greater than 4-pyridinyl. Increasing the size of the alkyl ester substituents enhanced activity. Compounds having nonidentical ester substituents were more potent than those having identical ester substituents. Replacement of the C-3 and/or C-5 ester substituent(s) by a cyano substituent(s) decreased activity significantly. An approximate 1:1 correlation between the IC50 value for inhibition of [3H]nitrendipine binding and inhibition of the tonic component of the muscarinic-induced contractile response was observed. The test results suggest that a 4-(pyridinyl) substituent is bioisosteric with a 4-(nitrophenyl) substituent on a 1,4-dihydropyridine ring system where o-, m-, and p-nitrophenyl are bioisosteric with 2-pyridinyl, 3-pyridinyl, and 4-pyridinyl, respectively.