Investigations of the Electrical Properties of Cardiac Muscle Fibres with the Aid of IntraceUular Double-Barrelled Electrodes

Current has been passed through the cell membrane of muscle fibres of the isolated rabbit right ventricle with the aid of intraceUular doublebarrelled microelectrodes. Two types of muscle fibres were distinguished which are called P and V fibres. The relation between the intensity of a hyperpolarising current applied during the rising phase and the maximum amplitude of the action potential was different in these fibres. For P fibres the relation was essentially linear over most of the range of currents used. For V fibres the change in maximum action potential amplitude was either negligible or did not appear until a certain value of hyperpolarising current was reached. This behaviour of V fibres can be understood if a drop in polarisation resistance occurs during the rising phase and is of such short duration that the polarisation resistance has returned to its resting value before the crest of the action potential is reached. P fibres have an estimated mean resting polarisation resistance of (106 -413) K ohms, and a rheobase current strength of (0.08 ± 0.02) #a. In V fibres the resting polarisation resistance was (47 -429) K ohms and the rheobase current strength (0.47 40.28) /za. I N T R O D U C T I O N Weidmann has carried out a detailed investigation of the electrical properties of cardiac Purkinje fibres. He found that a profound fall in membrane resistivity occurred during the depolarisation phase of the Purkinje fibre action potential (Weidmann, 1951). The only investigations related to ventricular fibres have been confined mainly to the estimation of the changes in membrane resistance during the course of the repolarisation phase (Coraboeuf, Zacouto, Gargouil, and Laplaud, 1958). The results presented in this paper form part of an investigation and comparison of the electrical behaviour of muscle fibers in the rabbit ventricular