Membrane Action of Ethmozin on Normoxic and Hypoxic Canine Purkinje Fibers

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Abstract
Transmembrane potentials were recorded from canine Purkinje fiber preparations before and after perfusion with ethmozin (0.05, 0.2, and 1.0 microgram/ml). Under normal oxygenation (95% O2 + 5% CO2; n = 9), ethmozin at 0.05 microgram/ml significantly (p less than 0.05) reduced maximal rate of depolarization (MRD), from 750 +/- 61 to 709 +/- 58 V/s, without affecting other parameters. MRD was further decreased to 647 +/- 53 and 540 +/- 78 V/s at 0.2 and 1.0 microgram/ml of this agent. Reduction of maximal diastolic potential, from -86.1 +/- 6.0 to -84.6 +/- 5.9 mV, occurred only at 1.0 microgram/ml, suggesting that the decrease in MRD is not voltage dependent. Ethmozin significantly shortened the action potential duration at 50 (APD50) and 90% (APD90) repolarization, from 231 +/- 16 and 306 +/- 18 to 199 +/- 26 and 279 +/- 14 ms, respectively. At 1.0 microgram/ml, these values were further reduced. The effective refractory period also showed a concentration-dependent shortening, but the ratio of the refractory period to APD90 was increased. The membrane responsiveness curve was shifted by ethmozin to more negative potentials. The h infinity (inactivation of the fast Na+ current) curve was shifted by 3.8 mV to more negative values by 1.0 microgram/ml ethmozin. "Slow response" produced by high K+ + isoproterenol (0.025 microgram/ml) was not affected by ethmozin (1.0 and 5.0 microgram/ml). Ethmozin exaggerated the electrophysiologic effects of hypoxia (95% N2 + 5% CO2; n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

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