SMOOTH-MUSCLE CONTRACTILITY AND CALCIUM-CHANNEL DENSITY IN HIBERNATING AND NONHIBERNATING ANIMALS

Hibernating animals consistently survive prolonged periods of cold with body temperatures near the freezing point. Previous studies have suggested that regulation of calcium influx may be a fundamental cellular mechanism for cold tolerance in hibernating species. The present study was undertaken to compare (i) the calcium dependence of contractility and (ii) [3H]nitrendipine binding in homogenates of ileal longitudinal smooth muscle from the nonhibernating guinea pig (Cavia porcellus) and a hibernator, the ground squirrel (Spermophilus richardsonii). The contractility studies indicate that both the activation threshold for calcium and the concentration - response curve were shifted to the right in ground squirrel when compared with quinea pig. The binding site density in ground squirrel muscle was about an order of magnitude less than in guinea pig (B(max) = 10 ± 2 (n = 12) and 86 ± 6 fmol/mg protein (n = 5), respectively). These results indicate that ground squirrel tissues are less sensitive to external calcium and clearly have fewer calcium channels than the smooth muscle of the non-hibernator. The results continue to support the hypothesis that cold tolerance in hibernating species involves calcium homeostatic control mechanisms.