A REEVALUATION OF THE CRITICAL MEMBRANE-POTENTIAL OF THE EFFECTIVE REFRACTORY PERIOD IN GUINEA-PIG VENTRICULAR FIBERS

The traditional critical membrane potential (CMP), -55--60 mV, which c orresponds to effective refractory period (ERP), was anew investigated in guinea pig ventricular muscle fibres. The electrical and contracti le responses to the stimulus during repolarization of action potential (AP), particularly from +10 to -60 mV, were observed. One third of 35 tested cells displayed testing action potential (TAP) and local respo nse at greater-than-or-equal-to -54 mV when they were stimulated by te sting pulses in 37-degrees-C normal Tyrode's solution. Potential level of TAP which occurred earliest was at -30 mV and that of local respon se which appeared earliest was at 0 mV during repolarization among 95 systematic tests. Most of the TAPs belonged to the slow response poten tial type. The ratio of TAP evoked at greater-than-or-equal-to -54 mV initial membrane potential (IMP) was as high as 86% when the experimen t was carried out in 37-degrees-C 1.5 mmol KCl/L Tyrode's solution. In view of distribution of IMPs of TAPs, the CMP of ERP in guinea pig ve ntricular muscle fibres was more positive than traditional CMP measure d by Hoffman et al. in dog, sheep Purkinje fibres and had a quite chan geable range. The CMP of every cell in ventricular muscle was not all the same, and their CMPs approximated to normal distribution. There wa s no sharp line separating ERP from relative refractory period in myoc ardium. Higher temperature and low [K]0 were the important factors ele vating CMP of ERP.