Changes in the Action Potential and Contraction of Isolated Frog Muscle after Repetitive Stimulation

Abstract

AbstractThe effect of repetitive stimulation of twitch fibres was studied in isolated whole muscle from Rana pipiens. Intracellular electrodes were used for recording membrane potentials, muscle tension was recorded isometrically and the decline of the active state of the contractile elements was determined by the quick release method. Repetitive stimulation caused an increase in the duration of the action potential and in the amplitude of the early negative after‐potential. To judge from experiments on fibres with disrupted transverse tubules, the increase in the duration of the action potential is due to changes in the surface membrane, while the increase in the amplitude of the after‐potential is related to the transverse tubules. The changes in the action potential and the after‐potential were independent of whether or not the muscle contracted and thus cannot be secondary to the contraction. The shape of the action potential and the after‐potential returned to normal within 5 min after stimulation at 10/sec for 60 sec while after 30 min there was still an increase in the peak twitch tension and a slower‐than‐normal decay of the active state in response to test shocks. This indicates that the post‐stimulatory potentiation is due to a prolonged active state which, in turn, is due mainly to causes other than changes in the action potential and after‐potential.