Annual variations in the binding of insulin to hepatic membranes of the frog Rana esculenta.

Amphibia undergo regular annual cycles of metabolic activity that are influenced by both exogenous factors and hormones. Insulin binding to crude frog hepatic membranes was studied throughout the year. The general character of insulin binding was similar to that in other vertebrates; the maximum specific binding was achieved after 4 hr at 4 degrees, the optimum pH was 7.8, half-maximal displacement of bound insulin was from 9 x 10(-10) to 1 x 10(-9) M, and insulin analogs competed for the insulin receptor in line with their relative biological potencies. A biphasic Scatchard plot and negative cooperativity of the receptor were also observed in frog liver membranes. Affinity constants from Scatchard plots revealed high and low affinity binding sites which were unchanged during the year. The seasonal cycle, however, markedly affected the binding capacity for both sites. Maximum binding occurred in May-June and the minimum in November-December for both classes of receptors. Binding capacities ranged from 1.71 to 11.33 fmol/mg protein for the high affinity sites and from 432 to 3171 fmol/mg protein for the low affinity sites. It is concluded that annual cycles of insulin binding reflect modulation of receptor number rather than receptor affinity.