Labile heat and changes in rate of relaxation of frog muscles.

Observations were made of the labile heat and the progressive slowing of relaxation as a function of tetanus duration (Abbott effect) during isometric tetani of frog extensor longus digiti IV (e.l.d. iv) and sartorius muscle. Both the labile heat and slowing of relaxation are less marked in e.l.d. iv than in sartorius muscle. Both effects are depressed to the same extent in the second of two closely spaced tetani in sartorius muscle. The repriming of both effects follows the same time course in sartorius muscle. The hypothesis is discussed that both effects result from binding of calcium to parvalbumin during a tetanus, and that their repriming is due to the removal of calcium from parvalbumin by the sarcoplasmic reticulum.