Osciliatory after-potentials of cardiac muscle fibers of the rabbit induced by repetitive stimulation in low pH solution

65 OSCILIATORY AFTER-POTENTIALS OF CARDIAC MUSCLE FIBERS OF THE RABBIT INDUCED BY REPETITIVE STIMULATION IN LOW pH SOLUTION. T. Kanoh, Y. Nishizono, K. Nishi. Department of Pharmacology, Kumamoto University Medical School Kumamoto, Japan. Effects of low pH and catecholamines on afterpotentials of cardiac muscle fibers of the rabbit were examined using a conventional microelectrode technique in vitro. In the isolated right ventricular papillary muscle, delayed afterdepolarization (DAD) followed by afterhyperpolarization was induced by repetitive stimulation in Tyrode's solution at pH 7.1. The amplitude of DAD was dependent on both the interval of stimuli and the number of preceeding stimuli. In some cases, the DAD developed to the threshold level and triggered action potentials. Noepinephrine or isoproterenol (1-5 uM) increased the amplitude of DADS and induced sustained rhythmic action potentials. Results suggest the possibility that a slight change in pH of the external medium surrounding the myocardium and an increase in sympathetic activity would trigger arrhythmia in situ.

66 EFFECTS OF Ca-IONOPHORE A23187 ON BEAT RATES AND MEMBRANE POTENTIALS OF CULTURED CHICK EMBRYONIC HEART CELLS. H. Toyota, M. Matsumura. Department of Physiology, Kawasaki Medical School, Kurashiki, Japan Cells of 7-13 days chick embryonic ventricles were cultured for 3-5 days. The beating rhythm was continuously recorded by photoelectric device and the membrane potential was measured by the intracellular microelectrode. Neither norepinephrine nor DBcAMP changed the beating rhythm. Ca-ionophore A23187, at the concentrations of 5~10~~ - 2x10e5M, increased the beat rate by 30200%, depending on the control rate. A23187 also turned the irregular rhythm, which was often observed under poor oxgen supply condition, into regular one. These actions of A23187 were transient but were brought about repeatedly when the drug was further added. The positive chronotropism is explained by the shortening of the action potential duration, and the regulatory action of beating rhythm is explained by the hyperpolarization of the membrane. Supported by the Research Grant for Cardiovascular Disease (57-Cl) from the Ministry of Health and Welfare.