Previously we have reported that in the rabbit the sinus node impulse spreads not only to the atrial muscle but also to the musculature in the venae cavae proximal to the heart10, 19). On the basis of this result, Mashiba and coworkers have introduced into electrophysiology the concept of sinocaval conduction. Recently ARITA and coworkers reported on the mechanical activity of the vena caval muscle of the rabbito. Further studies on the sinocaval conduction of the rabbit have revealed that the transmission of the sinus node impulse to the superior venae cavae is delayed during passage through three different sites: one is the sinus node and the others are the junctions of right and left superior venae cavae with the right atrium. These junctional areas have been found to take over the function of impulse initiation under certain conditions. We have called them the sinocaval (SC) areas. The present paper mainly deals with the functional properties of the SC areas. METHODSThe experimental methods employed here were the same as described prey iouslylo. RESULTS

In recent years, it has become evident that the impulse originating in the sinus node spreads also to the right and left superior venae cavae proximal to the heart (sino-caval conduction), as well as to the atria ITO et al., 1963;ITO et al., 1964;YANAGA et al., 1966).In a previous paper (ARITA et al., 1966), spontaneous rhythmic contractions and action potentials were demonstrated in vitro rabbit, and the electrophysiological properties of the vena caval muscle have been studied comparing with those of the atrial muscle.Since some differences recognized between the venae cavae and atria in the electrophysiological properties (frequency-tension relationship, rest potentiation, cellular interconnection, etc.) seemed to include significant problems from the physiological and hemodynamic points of view, further investigations were undertaken.The present paper deals with the effects of epinephrine, norepinephrine, isoproterenol, propranolol, acetylcholine, atropine, ouabain, and their several combinations on the contractility and action potentials of the isolated left superior vena cava or inferior vena cava by using a strain gauge transducer and microelectrode technique. In several cases, the effects of these drugs on the isolated left atrium were also examined for comparison. METHODSThe proximal portion of the isolated left superior vena cava (LSVC) and the inferior vena cava (IVC), or the left atrium (LA) was prepared in essentially the

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Since the pulsation of the vena cava was first noticed in the dog by Wallaenus in 1660 (FRANKLIN, 1937), the existence of rhythmical contractions of the venae cavae proximal to the heart in various animals has been reported

Aconitine induced the automatic activities in the isolated strip of the left superior vena cava in rabbit. In the left superior vena cava near the coronary sinus, action potentials with diastolic slow depolarization were recorded after the application of aconitine. When aconitine was applied to the left superior vena cava near the coronary sinus in situ, cardiac arrhythmias were recorded. From the above-mentioned facts, it was suggested that aconitine induced the ectopic pacemaker in the left superior vena cava proximal to the heart and that the impulse from the ectopic focus spread not only to the periphery but also to the atrium. In the patient with persistent left superior vena cava the ectopic pacemaker formation in the vena caval muscle may have the significance for the genesis of cardiac arrhythmias, especially supraventricular extrasystoles. CONITINE is a characteristic substance which induces cardiac arrhythmias. When locally applied to the cardiac muscle, it induces ectopic pacemakers at the area of application. However, the ectopic pacemaker formation in the left superior vena cava proximal to the heart has never been reported. Recently, it was found that the impulse which originated in the sinus node spread not only to atrial muscle but also to vascular muscle in vena cava (sino-caval conduction),1)-3) that the configuration of transmembrane potential of left superior vena caval muscle had close similarity to that of cardiac muscle,4) and that the contractile tension could be recorded.5) In connection with these facts, the ectopic pacemaker formation by aconitine in the left superior vena cava proximal to the heart was studied in rabbit. METHODSThe rabbits weighing 1.5 to 2Kg. were used. After intraperitoneal injection of pentobarbital sodium, the animals were thoracotomized. Experiments were carried out in vitro and in situ. In the in vitro experiment, after three venae cavae were cut at 2-3cm. from their cardiac orifices, the heart was excised. Then a large