Electrophysiological and histological observations were made on the heart of the African lungfish. Impulse origin and propagation were studied using simultaneously recorded epicardial and pericardial electrograms. The primary pacemaker site in the lungfish was found to be at the sinus venosus at its junction with the left cardinal vein. Under a variety of circumstances, pacemaker function shifted to other sites. In response to stress, probably under vagal influence, the regular and rapid sinus venosus rate was generally superseded by an irregular and slower atrial pacemaker. Heart rate and sinoatrial and atrioventricular conduction times varied with changes in temperature [Q10=3.77, 2.55, and 5.46, respectively]. Although alterations in impulse formation and conduction did occur, the site of impulse formation and the patterns of conduction between heart chambers were usually fixed, implying the existence of an organized conduction system. Nonetheless, extensive histological study failed to disclose either organized nodal structures or specialized conduction pathways.
We investigated the function of the conus arteriosus in the bullfrog Rana catesbeiana using a combination of anatomical and physiological techniques. Although there is a normal delay in ventriculoconal conduction and we could induce a spectrum of ventriculoconal conduction disturbances by manipulating the region of the ventriculoconal junction, we found no histological evidence of specialized conducting myocardial tissue in this region. The performance of the conus arteriosus was explored during various disturbances of ventriculoconal conduction and also during hemodynamic disturbances produced by hemorrhage and afterloading. The conus was found to contribute little to forward flow under ordinary circumstances, but its contribution increased greatly during bleeding or partial occlusion of the truncus. In contrast to the conclusion of others, no evidence could be adduced to support the idea that the conus serves as a depulsating chamber. Disparities in previous reports concerning the operation of the conus as a booster pump are attributed to special experimental circumstances.
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