Studies on Sinocaval Conduction

Many studies on the cardiac transmembrane potentials and on the sequence of activation of the heart have been reported. However, there have been only a few reports on the transmembrane potentials of the musculature in the great vessels proximal to the heart and on the excitation conduction from the heart to these vessels. Previously, MASHIBA and coworkers reported that, in the in situ rabbit, the sinus node impulse spreads not only to the atrial muscle but also to the musculature in the vena cava. They called this phenomenon "sinocaval conduction". To extend this observation, in the present paper, the author studied on the sinocaval conduction of the rabbit atriocaval preparation with the microelectrode methods. METHODS In the rabbit, there are three venaec avae which empty into the right atrium by separate ostia: right superior vena cava, left superior vena cava and inferior vena eava. In all experiments, the preparation consisting of atria resected together with three venae cavae (atriocaval preparation) was used (Fig. 1). The preparation was perfused with oxygenated Tyrode solution at a constant temperature (35-37°C) and with a constant flow. As for intracellular electrode, the LING-GERARD type of microelectrode was used which was filled with 3 M KCl solution and had 20-40 Megohm electrical resistance. RESULTS 1 . Shape of the transmembrane action potentials of the musculature in the rabbit venae cavae (Fig. 2 and 3). On the basis of the shape of the observed action potentials, 4 functional regions could be described in the vena caval tissues: I) sinus node, II) sinocaval (SC) areas, 111) superior venae cavae and IV) inferior vena cava. The features of the action potentials in the sinus node are the marked diastolic slow depolarization, the slow rate of rise, the low amplitude and the absence of overshoot.