The effects of quinine on the calcium and magnesium content of the sarcoplasmic reticulum and the temperature-dependence of quinine contractures.

A significant decrease in the Ca2+ and increase in the Mg2+ content of the terminal cisternae (TC) of the sarcoplasmic reticulum (SR) during quinine contraction was demonstrated by electron probe analysis of rapidly frozen frog muscles. The extent of Ca2+ release (71% of total) from the TC and the absence of an increase in total cell Ca2+ support the conclusion that quinine contractures are caused by passive efflux of Ca2+ from the SR when the latter is uncompensated due to inhibition of the SR Ca2+ pump by quinine. A rapid warming contraction (RWC) was observed, in the presence of quinine, when the temperature of intact and skinned muscles was increased from about 5 degrees C to 18-23 degrees C. The duration of the latency of quinine contracture, in intact muscle bundles, was approximately 31 s at 3 degrees C and 2 s at 23 degrees C. The results suggest a significant temperature sensitivity of the passive Ca2+ channels of the SR membrane, although an effect of temperature on the lipid partition coefficient of quinine into the SR has not been ruled out.