1966 Volume 16 Issue 4 Pages 462-480
1. In the rabbit, there exist three venae cavae which empty into the right atrium (RA) by separate ostia: the left superior vena cave (LSVC), the right superior vena cava (RSVC), and the inferior vena cava (IVC). Electrical and mechanical responses of the venae cavae proximal to the heart were recorded and compared with those of the atria in vitro, by the aid of microelectrodes and a strain gauge transducer.
2. Spontaneous or electrically driven twitch tension, elicited by an single action potential, could be recorded from each of the superior venae cavae (SVC);however, neither action potential nor contraction was obtained from the IVC.This fact was discussed in relation to the histological findings.
3. The orifice of the SVC contracted 20-30 msec earlier than the RA. This supports a hypothesis that rhythmical contractions of the SVC play a role of a venous valve in preventing blood regurgitation from the RA.
4. Under certain conditions, the LSVC exhibited a tetanus like contraction followed by the contractile summation, and a ‘seeming dissociation’ of E-C coupling. These results may suggest that the SVC have a more fragile intercellular connection than the atria, from the functional point of view.
5. Abrupt changes of stimulus frequency, from a higher (4 cps) to a lower (1 cps) value or vice versa, brought about normal or reverse staircase phenomena in the LSVC as well as in the left atrium (LA). In the LSVC, however, the contractile inhibitory process (fatigue) during normal staircase effect occurred more easily than in the LA.
6. Frequency-tension relationships in the LSVC and the LA were similar to each other on the whole. But the frequency causing the ‘infinitesimal phase ’of contractile force was significantly different from each other, e. g., the LSVC, 20±2.7 cps ; the LA, 43±3.8 cps.
7. Phenomena of rest contraction were recognized in the LSVC as well as in the LA. The rest contraction reached a maximum after 60-200 sec of quiescene in each of them. After the overly long rest-intervals (1000 sec or more), however, the rest potentiation of the LSVC had disappeared more rapidly than that of the LA. That is, the LSVC falls into hypo-dynamic state more rapidly than the LA.
The possible explanations for above-mentioned (5, 6 and 7) differences between the LSVC and the LA were discussed with reference to the calciumsusceptibility of their fiber membranes.
8. In the electrically induced arrhythmias, an arbitrary contraction ‘z ’ had, at first sight, no particular relation to the preceding cycle length ‘y ’ nor the cycle length ‘x’ prior to ‘y’. However, highly positive correlation existed between ‘z’ and ‘y/x’ in each of the LSVC and LA. In these arrhythmias, moreover, the longer the 85%-duration of an action potential and the shorter the 35%-duration, the larger was the contraction ; 60%-duration was nearlyconstant regardless of the marked changes of twitch tension. The wide fluctuation of the strength of twitch tension was discussed with relation to calcium and/or potassium-concentration at the immediate extracellular spaces.
9. In general, the vascular muscle of the SVC proximal to the heart of a rabbit shows a similar nature to the atrial muscle in both electrical activity and contractility. Nevertheless, some difference in their nature was recognized.These differences may depend upon the complexity of the structure of the wall of the proximal SVC which is composed of striated muscle and smooth muscle, or the mixture of the two.
From the above-mentioned results, it seems reasonable to assume that the muscle of the superior venae cavae proximal to the heart has a position between the cardiac striated muscle and the vascular smooth muscle from the functional point of view. Further studies into this problem may be necessary.
