Fig. 1. Experimental steps and time course in series 1 and 2 for ventricular muscle isolated from turtle and trout. Preparations were either left to rest or electrically paced to isometric twitch force development at the rate indicated. A representative recording of twitch force development of a turtle preparation is included. In series 1, preparations were sampled for biochemical determinations at the times indicated. In series 2, twitch force, oxygen consumption and hypoxic lactate production were recorded during 30 min of oxygenation and then during 30 min of severe hypoxia.