[The mechanism of thermo-induced functional uncoupling of the Ca-pump in the sarcoplasmic reticulum of skeletal muscles].

Short-term heating at 45 degrees C of diluted suspensions of rabbit skeletal muscle sarcoplasmic reticulum (SR) vesicles sharply reduces the Ca/ATP ratio without causing any appreciable changes in the Ca-ATPase activity. The mechanism underlying the uncoupling effect of thermoinactivation was investigated. It was shown that the decrease in the effectiveness of Ca2+ accumulation is correlated with the level of Ca2+ accumulated by SR vesicles. At the same time, thermal inactivation has no effect on passive permeability of SR membranes for Ca2+. Ca2+ (100 microM) added to the inactivation medium partly protects the enzyme against uncoupling. Thermal inactivation of the SR Ca-pump is concomitant with a decreased microviscosity of SR membranes as measured by pyrene excimerization. Addition of 100 microM Ca2+ to the thermoinactivation medium prevents the decrement of microviscosity. It was found that N-3-pyrenylmaleimide, a maleimide derivative of pyrene covalently linked to the SH-groups of Ca-ATPase can also form excimers, which testifies to the formation of Ca-ATPase oligomeric complexes in the native membrane. The probability of oligomerization increases with a thermoinduced decrease in membrane microviscosity, which is correlated with the lowered efficiency of SR Ca-pump operation.