Force frequency relation in the myocardium of rainbow trout

Summary Isolated heart ventricular preparations from rainbow trout were electrically stimulated to contraction. Following a temporary change in stimulation rate from 0.2 Hz to a higher value, the force fell to a minimum after which it increased and levelled off. Upon the return to 0.2 Hz a further transient increase in force appeared. The latter two responses were stimulated by an increased extracellular K⁺, which is known to inactivate the Na⁺ channel. The initial negative inotropic effect, in contrast to the two subsequent positive effects, was associated with a parallel decrease in amplitude of the action potential measured in 15 mM K⁺, used as an index of the Ca²⁺ influx. One micromolar (1 M) ryanodine did not affect either the negative or the positive responses due to an increase in stimulation rate, but depressed the force developed after prolonged periods of rest. Ten micromolar (10 M) adrenaline strongly inhibited the positive effects of an elevation of frequency. An elevation of extracellular Na⁺ from 141 to 166 mM had a similar effect. In conclusion, the positive effects occurring in 15 mM K⁺ do not seem to depend on the initial Na⁺ current. They may nevertheless depend on changes of the cellular Na⁺ balance as suggested by the effects of adrenaline, K⁺ and Na⁺. The functional role of the sarcoplasmic reticulum is unclear.