An analysis of changes in resting membrane potential induced by high-rate stimulation in frog atria

7 EFFECTS OF CATECHOLAMINE ON K-RELATED CURRENTS OF THE BULLFROG ATRIAL MUSCLE. T. Hyodo, K. Ikeda, M. Goto. Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan. Effects of catecholamines on K-related membrane currents of the bullfrog atria1 muscle were studied using double sucrose-gap technique. In normal Ringer solution, isoproterenol (10m7~ 10-6~) as well as noradrenaline (lo-5 M) augmented the slow inward current (Isi) andthe delayed outward current (Ix) as expected. Unexpected findings were an enhancement of the background current (Ib) for hyperpolarization and the instantaneous outward current (Ikl) for depolarization. These effects disappeared after propranolol (5 X10-TM), but remained after Co (1 mM). In the media with Co, the I-V curves in the presence and absence ofp-agonists crossed at nearly Ek level. When Ikl was inhibited by Ba (0.3 mM), thep-agonists merely produced the enhancement of Isi and I, in the same intermediate voltages of depolarization, and Co eliminated these effects. The results indicate thatp-agonists have a direct effect on Ikl and produce an increase of I, through enhanced Ikl and Isi.

8 AN ANALYSIS OF CHANGES IN RESTING MEMBRANE POTENTIAL INDUCED BY HIGH-RATE STIMULATION IN FROG ATRIA. T. Fujino, M. Goto. Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan. The membrane potential and current change induced by high rate stimulation in bullfrog atria1 fibers were investigated using double sucrose-gap technique under volAfter the onset tage clamped and unclamped conditions. the basal membrane potential depolarized of stimulation, first rapidly (RID) and later slowly (SID), and then gradually repolarized (LR). At the termination of stimulation, the potential further repolarized rapidly (RPDR) and then slowly (SPDR), and this was followed by a longlasting hyperpolarization (PDH). The time course of the membrane potential changes could be defined into six disLR and PDH components were tinct exponential components. depressed in ouabain (lo-GM), K-free and Na-deficient the membrane current changes media. Under voltage clamp, were also found to consist of six factors: activation and deactivation of delayed outward current (Ix), accumulation and depletion of extracellular K ions, and activation and deactivation of electrogenic Na-K pump.