Effect of temperature on insulin binding and action in cultured human fibroblasts

Insulin stimulation of glycogen synthase activity and insulin binding were measured in fibroblast monolayers at 24, 32, and 37 degrees C. Insulin stimulation of %I glycogen activity increased with increasing temperature. Maximum response was greater at 37 degrees C than at 32 degrees C, and half maximal stimulation required at 2.0 nM insulin at 37 degrees C vs. 10 nM at 32 degrees C. Insulin stimulation of glycogen synthase was greater and somewhat faster at 37 degrees C than at 32 degrees C. No insulin effect was observed at 24 degrees C. 125I-insulin binding to monolayers became maximal in 15 min at 37 degrees C, 60 min at 32 degrees C, and 120 min at 24 degrees C. However, insulin binding decreased with increasing temperature, and this decline was due to decreased numbers of receptors. Insulin binding and stimulation of glycogen synthase were comparable at 32 degrees C, with half maxima at 10 nM, indicating no evidence of "spare" receptors. The data indicate that temperature effects on insulin binding and action in fibroblasts are not directly related. The results also suggest that a rate limiting step(s) of insulin action is temperature sensitive, and that this step is not insulin binding.