INTRINSIC CARDIAC RATE REGULATION IN SEA LAMPREY, PETROMYZON MARINUS AND RAINBOW TROUT, SALMO GAIRDNERI

1. 1. Isolated spontaneously beating ventricles of five adult sea lampreys, Petromyzon marinus, and two mature rainbow trout, Salmo gairdneri, have been perfused in vitro with small volumes of Ringer's solution in order to determine whether or not the hearts of these species possessed an intrinsic pressure-sensitive regulatory mechanism in their pacemakers. 2. 2. In the lamprey hearts, 79 per cent, and in the trout hearts, 77 per cent of the experiments yielded positive results. A total of 135 observations were recorded from the seven hearts studied from the two species. 3. 3. The data consistently indicate that the pacemakers of both lamprey and trout hearts exhibit property of intrinsic rate regulation. 4. 4. It is concluded that this inherent pacemaker adaptive mechanism represents an ancient phylogenetic development. © 1969.