Insulin binding sites in solubilized membranes from rat testis. Effect of age

The effect of aging on the binding of insulin to testicular solubilized microsomal membranes was studied in rats. The insulin receptor was solubilized in triton x-100 and its properties compared with those of solubilized membranes of rat liver. At the three life stages studied (21, 38-40 and 75-80 days) binding specificity in testicular solubilized membranes showed the following order of competition relative to porcine insulin (100): proinsulin (7), guinea-pig insulin and IGF-I (less than 1). Scatchard analysis of the binding data always gave curvilinear plots with a high affinity of about 0.9 10(9) M-1. With aging, high affinity binding expressed as fmol per mg protein did not vary in the testis whereas it decreased in the liver. Where expressed as fmol bound per testis, the binding of insulin increased 14-fold between the age of 21 days and adult age. It is concluded that the characteristics of solubilized membranes receptors for insulin are roughly similar in the testis and liver but the effect of age on the number of high affinity binding sites is quite different in the two tissues.