CALCIUM REGULATION OF RELAXATION IN SKELETAL-MUSCLE

Experiments were designed to test the hypothesis that the time course of skeletal muscle relaxation is determined by the disappearance of the Ca.troponin complex (Ca.TN) which in turn is controlled by the rate of Ca2+ sequestration by the sarcoplasmic reticulum (SR) and the rate of Ca2+ dissociation from TN. The rate of Ca2+ sequestration was altered by: 1) by-passing SR Ca2+ uptake through rapid Ca2+ chelation by photolysis of diazo-2, a caged Ca2+ chelator; and 2) by inhibiting the SR Ca2+ ATPase activity with thapsigargin. The influence of the intrinsic rate of Ca2+ dissociation from TN on relaxation was examined by reducing the rate of Ca2+ dissociation from TN with calmidazolium. Experiments were done at 10-degrees-C with fibers and fiber bundles that were isolated from frog tibialis anterior muscles. Experimental results are consistent with the proposed hypothesis with the proviso that the intrinsic rate of Ca2+ dissociation from TN is a modulated variable during relaxation.