Simultaneous heat and tension measurements from single muscle cells.

Simultaneous force and heat measurements were made in single cells from skeletal muscle of the frog during isometric twitches and tetani at 10 and 0 degree C. A Hill- Downing type thermopile of low heat capacity was used. In twitches, peak force development was found to be well correlated with heat production at both temperatures, during posttetanic twitch potentiation (at 10 degrees C) and during posttetanic twitch depression (at 0 degree C). In a twitch at 0 degree C, heat production started less than 14 msec after the stimulus had begun, before force development. As in whole muscle, the heat during a tetanus could be separated into two components: an early component produced at an exponentially decreasing rate, labile heat, and a steady rate, stable maintenance heat rate. Increasing temperature from 0 to 10 degrees C doubled the stable maintenance heat rate. At the higher temperature the time constant of labile heat production was halved and the quantity of labile heat decreased. When two tetani were given at 10 degrees C, a 5 min rest interval was required before the second tetanus produced the same force and heat as the first. At 0 degree C this interval was at least 10 min. With shorter intervals, both heat and force were depressed. At 10 degrees C both were depressed equally but at 0 degree C the effect on heat was greater than on force. At both temperatures labile heat was depressed to a greater extent than the stable maintenance heat rate. Results are interpreted in terms of possible calcium-parvalbumin interaction during a tetanus.