On the problem of season and cold dependence of calcium transport by skeletal muscle sarcoplasmic reticulum.

Calcium transport of skeletal muscle sarcoplasmic reticulum from golden hamsters was studied in January and in June on animals kept at 22 degrees C under natural photoperiod and in January after cold-acclimation at +/- 2 degrees C in the dark for 55 days. Crude homogenates from psoas and soleus muscles and from mixed skeletal muscles were used. No differences were observed in the calcium storing capacity of sarcoplasmic reticulum among the three groups of animals. Kinetic studies on the dependence of the calcium uptake rate on the concentration of free calcium revealed a significant increase of the uptake rates and a decrease of the calcium affinity in the control animals sacrificed in winter as compared to those killed in June. Cold-acclimation in winter leads to a further small reduction of the calcium affinity. This shift of calcium uptake rate and affinity in the sense of that of a fast-twitch muscle may be related to the functional demands of the cold season and cold-acclimation respectively.