Effects of ACh and IJPs on the AV Valve and the Ventricle ofDolabella auricularia

Abstract

Some differences exist between myocardial tissue of the auriculo-ventricular valve and the ventricle of Dolabella, which have been revealed by means of electron microscopic and electrophysiological investigations. The muscle cells in the ventricle are more neatly arranged than those in the auriculoventricular valve, in which the alignments of the muscle cells run in many directions, indicating an interwoven structure. Myocardial cells of the ventricle have more condensed myofibrils and more apparent sarcomeres than those of the auriculo-ventricular valve. When both the sites are dissected out together, it is noticeable that the auriculo-ventricular valve almost always fires an action potential before an action potential appears in the ventricle. Thus the pacemaker of the auriculo-ventricular valve dominates that of the ventricle. Acetylcholine usually causes depolarization in the ventricle and hyperpolarization in the auriculo-ventricular valve and has similar actions on the auriculo-ventricular valve as do inhibitory junctional potentials. Membrane conductance of the myocardium in both sites is increased by acetylcholine. This possibly resultsfrom increase of sodium and potassium permeability in both sites. Thus, the difference in effects of acetylcholine between the two sites, depolarization in the ventricle and hyperpolarization in the auriculo-ventricular valve, might result from the difference in the initial membrane potential and in the ratio of increasedconductance for sodium and potassium between the sites. Results show that there is some functional differentiation between the sites. It indicates that even though the heart has a diffuse myogenic nature, a conduction system has been developed in Dolabella auricularia.