[Ionic nature of repeated responses of myocardial fibers]

Changing ionic composition of the environment and applying exposures which modify the conductivity of electrogenic membrane, the role of entering and leaving currents is studied in the generation of two zones of repeated responses (RR) of cat papillary muscle fibres. The first RR zone activated at a weak depolarization of the membrane (no higher than 30 mV) seems to be concerned with the functioning of the fast Na system. It was not blocked with inderal (2 divided by 5 mg/l), anor with Mn2+ ions (3mM). The second RR zone appearing at a stronger membrane depolarization (higher than 30 mv) is essentially dependent on the activity of the slow Na-Ca-system. It is activated with adrenaline (0.2 mg/l), ions of Sr2+ (5mM), Ba2+ (0.1 mM), Ca2+ (5 divided by 10 mM); blocked with Mn2+ (3 mM), Co2+ (5mM) ions, verapamile (2 divided by 4 mg/l), inderal (2 divided by 4 mg/l). The second RR zone was essentially affected with potassium ions: the 3-fold increase of Ko+ suppressed the RR of the second zone, a 3-fold decrease of Ko+ strengthened them.