Calcium concentration in the myoplasm of skinned ferret ventricular muscle following changes in muscle length.

Ferret ventricular muscles were skinned by prolonged application of Triton X-100. Aequorin was allowed to diffuse into the myoplasmic space and the resulting light emission was used to monitor the myoplasmic Ca2+. The muscle was then activated with a lightly buffered Ca2+ solution and the changes in myoplasmic Ca2+ and tension in response to length changes were investigated. 2. A sudden reduction in muscle length led to a rapid increase in myoplasmic Ca2+ to a new level which was maintained as long as muscle length was reduced and which was reversed when the muscle was stretched back to the control length. The rate of increase of Ca2+ when the muscle length was reduced was greater than the rate of decrease in Ca2+ when the muscle was stretched. 3. Increasing the concentration of EGTA in the activating solution, so as to increase its Ca2+-buffering capacity, eliminated the changes in myoplasmic Ca2+ in response to a length change but had little effect on developed tension. 4. On stretching the muscle there was a slow component of recovery of tension with a time course broadly similar to the rate of decrease of myoplasmic Ca2+. The time course of tension redevelopment and of the accompanying reduction in myoplasmic Ca2+ both decreased to a similar extent when the Ca2+ used to activate the muscle was increased. 5. Step reductions of length of increasing amplitude caused increases in myoplasmic Ca2+ which were larger in proportion to the size of the step. 6. Step reductions of length of equal size but from different starting lengths caused changes in myoplasmic Ca2+ the amplitude of which correlated with the change in tension rather than the change in length. 7. The increase in myoplasmic Ca2+ when muscle length is reduced suggests that Ca2+ is released from a site in the muscle, probably troponin C. The time course and magnitude of the changes in myoplasmic Ca2+ correlate more closely with the changes in developed tension than muscle length.