STUDIES ON TRANSMEMBRANE ACTION POTENTIALS AND MECHANICAL RESPONSES OF THE VENAE CAVAE AND ATRIA OF THE RABBIT

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Abstract
Electrical and mechanical responses of rabbit venae cavae proximal to the heart were recorded and compared with those of the atria in vitro, with micro-electrodes and a strain gauge transducer. Spontaneous or electrically driven twitch tension, elicited by a single action potential, could be recorded from each of the superior venae cavae (SVC); however, neither action potential nor contraction was obtained from the inferior vena cava (IVC) which was discussed in relation to the histological findings. The SVC orifice contracted 2030 msec. earlier than the right atrium (RA). Rhythmical contractions of the SVC may play the role of a venous valve in preventing blood regurgitation from the RA. Under certain conditions, the left superior vena cava (LSVC) exhibited a tetanus like contraction followed by contractile summation, and a seeming dissociation of excitation contraction coupling. The SVC may have a more fragile intercellular connection than the atria. Abrupt changes of stimulus frequency brought about normal or reverse staircase phenomena in the LSVC and left atrium (LA). In the LSVC, however, the contractile inhibitory process (fatigue) during normal staircase effect occurred more easily than in the LA. Phenomena of rest contraction were recognized in the LSVC and in the LA. The rest contraction reached a maximum after 60200 sec. of quiescence in each. After long rest-intervals (1000 sec. or more), however, the rest potentation of the LSVC disappeared more rapidly than that of the LA. These differences between the LSVC and the LA were discussed with reference to the Ca susceptibility of their fiber membranes. In electrically induced arrhythmias, an arbitrary contraction ''z'' had, at 1st sight, no particular relation to the preceding cycle length ''y'' nor the cycle length ''x'' prior to ''y''. However, highly positive correlation existed between V and ''y/x'' in both the LSVC and LA. The vascular muscle of the SVC proximal to the heart is similar to the atrial muscle in both electrical activity and contractility. The differences noted may depend upon the complexity of the wall structure of the proximal SVC which is composed of striated muscle and smooth muscle, or a mixture of the 2. The muscle of the superior venae cavae proximal to the heart may have a functional position between cardiac striated muscle and vascular smooth muscle.

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