Abstract
The role played by the sarcoplasmic reticulum in force development and in cellular Ca super(2+) balance and its dependence on extracellular K super(+) were examined in heart ventricular tissue of rainbow trout (Oncorhynchus mykiss) and plaice (Pleuronectes flesus ). Compared with the steady-state twitch at a stimulation rate of 0.2 Hz, a 30-s rest led to a similar increase in twitch force in trout heart, regardless of whether (K super(+)) was 2.5 or 5 mM. At 5 mM (but not at 2.5 mM) post-rest potentiation increased with increasing rest periods (from 30 to 900 s).