Phase-Response Analysis of Stretch-Mediated Beat Coordination in the Oyster Heart. II. Mutual Entrainment of Auricle and Ventricle

Beat coordination between auricle and ventricle in the oyster heart was analyzed in situ. When the heart was exposed in the pericardial cavity, auricular and ventricular beats were not synchronized and the heart beat was irregular. In that state, the phase relationship between the auricular and ventricular beats changed successively and the beat frequency of both the auricle and ventricle changed cyclically. The relationship between stretch phase and beat interval change in the auricle and ventricle was identical with the phase response curve (PRC) of auricle and ventricle to brief stretches. By perfusing the pericardial cavity with sea water, the magnitude of reciprocal stretching between auricle and ventricle increased gradually. Then the auricle and ventricle began to beat synchronously with a fixed phase relation and coordinated beating of the heart was achieved. These results strongly support the idea that a coordinated heart beat of the oyster is achieved as a result of mutual entrainment of the auricle and ventricle mediated by reciprocal stretching.