In situ study of the sarcoplasmic reticulum function in control and mdx mouse diaphragm muscle

Sarcoplasmic reticulum (SR) calcium handling in diaphragm was compared betw een mdr mice (7-8 weeks old) and age-matched controls. The total SR Ca2+ lo ad was released from the SR by rapidly cooling muscle bundles from 22 to -1 degrees C. The plateau amplitude of the rapid cooling contracture (RCC) wa s considered as an index of the SR Ca2+ content. The steady-state RCC ampli tude was significantly lower by 50% in mdx bundles mainly because of a decr eased capacity of the dystrophic diaphragm to generate maximal tension. The re was no significant difference between either RCC time to peak or the tim e to half-relaxation of the transient, spike-like, contractile response ind uced by muscle rewarming. The recovery process of RCC was studied by using a paired RCC protocol. In both groups, at the shortest interval (10 s) betw een two RCCs, the amplitude of the second RCC was decreased by 25% compared with the first RCC. Increasing the time interval led to progressive monoex ponential recovery of the second RCC with similar time constants in control and mdx diaphragm. These results indicate that the dystrophic process does not significantly alter SR Ca2+ uptake nor Ca2+ redistribution within the muscular cell.