Long term facilitation of tension in crustacean muscle and its modulation by temperature, activity and circulating amines

Prolonged stimulation of the motor axon of the opener and stretcher muscles of the crayfish claw leads to long-term facilitation (LTF) of transmitter release at the neuromuscular junction. This facilitation is correlated with enhancement of tension development. Factors shown to enhance LTF of transmitter release, such as increased frequency of excitation, lower temperature, and exposure to ouabain also enhance tension development (Figs. 1, 2 and 4). Prolonged stimulation delivered in a bursting pattern enhances the development of tension more than an equivalent amount of stimulation delivered in a regular pattern (Fig. 3).Two circulating neurohormones, serotonin and octopamine, were examined for their effect on the development of tension during short and long periods of muscle activation. Serotonin and LTF of transmitter release appear to have an additive effect on the development of tension. The threshold for a detectable serotonin effect is 10−10 M. The effect of octopamine on the development of tension appears to be enhanced by longer periods of maintained muscle activation. LTF of transmitter release resulting from 5 min of continuous activation at 15 Hz is accompanied by a drop in the threshold of an observable octopamine effect on tension from 10−9Mto 10−10 M. It is proposed that octopamine's trophic effects on metabolism in muscle act to sustain muscular performance during maintained activity.