Abstract
1. The contractility and transmembrane action potential (AP) of the isolated left superior vena cava (LSVC) or the inferior vena cava (IVC) proximal to the heart of the rabbit were studied in the presence of various drugs: epinephrine (Epin.); norepinephrine (Norep.); isoproterenol (Isop.); propranolol (Prop.); acetylcholine (Ach.); atropine; ouabain, by usinga stra in gauge and microelectrodes. The results were compared with those obtained from the isolated left atrium (LA).
2. Epin. and Norep.(3.3×10-6 g/ml) produced increase in the twitch tension and slight prolongation in the AP-duration of the LSVC. Isop.(1.3×10-6 g/ml), conversely, shortened AP-duration in spite of increase in the twitch tension. The contraction and potential changes produced by Epin. on the LA were virtually similar to those on the LSVC.
3. Prop.(1.5×10-5 g/ml) produced a slurr or step in the rising phase of the AP and gradual decrease in the AP-height together with negative inotropic effects, resulting finally complete conduction block and disappearances of the twitch tension in the LSVC as well as the LA.
4. The resting tension (tone) of the LSVC pretreated by Prop.(1.5-8.3×10-6 g/ml) was markedly elevated by the application of Epin. and Norep.(3.3×10-6 g/ml), but not by Isop.(1.3×10-6 g/ml). All of Epin., Norep. and Isop. did not affect at all the resting tension of the IVC as well as the LA, even though pretreated equally by Prop. These results were discussed in relation to adrenergic alpha- and beta-receptors present in the tissue.
5. The application of Ach.(1.7×10-5g/ml) induced the marked shortening of AP-duration and the drastic decrease in contraction-strength in the LSVC, and the succeeding addition of atropine (8×10-7 g/ml) produced the supernormal contractions with a prolonged AP-duration. In the next application done after 20 minutes washout of the preparation, Ach. displayed directly the positive inotropic action. These diverse actions of Ach. were essentially similar to those observed in the LA.
6. Ouabain (2.5×10-7 g/ml) increased the twitch tension of the almost all strips (90%) of the LA; but only 40% of the LSVC. On the other hand, no significant change was observed in the APs of both the LA and LSVC. This result may imply that the striated muscle component contained in the LSVC is not the same to the atrial muscle in the contractility to ouabain.
7. Finally, physiological significance of spontaneous rhythmic contractions (twitch tension) and sustained contraction (resting tension or tone) of the superior venae cavae near the heart was briefly speculated from the hemodynamic point of view.
