Insulin regulates protein metabolism in mouse blastocysts

Mouse blastocysts, in vitro, endocytosed 100 micrograms/ml 125I-labelled bovine serum albumin (BSA) at a rate equivalent to 192 +/- 27 microliters/hr/mg embryonic protein over the first 20 min. Insulin stimulated this initial uptake by 30% (P < 0.05). After this time, accumulation of 125I-labelled BSA began to plateau as the endocytosed 125I-labelled BSA was catabolized and 125I was released from the cells. Insulin caused an approximately 72% (P < 0.05) increase in the amount of uncatabolized 125I-labelled BSA remaining in insulin-treated blastocysts after 2 hr as compared to control blastocysts. Insulin partially inhibited catabolism of endocytosed 125I-labelled BSA during the first 2 hr following transfer to nonradioactive medium. After this time, degradation ceased in both control and insulin-treated blastocysts, leaving a small, uncatabolized protein pool remaining in the embryos; however, as a result of insulin's inhibitory effects on the initial catabolic rate, the uncatabolized protein pool was 30% (P < 0.05) larger in insulin-treated blastocysts after the 4 hr chase. Insulin inhibited endogenous protein degradation in blastocysts by 37% (P < 0.05). Combined with previous studies showing a 90% increase in endogenous protein synthesis in blastocysts following short-term stimulation with insulin (Harvey and Kaye, 1988), these results suggest that insulin acts to increase the endogenous protein reserves in the embryo. Dose-response studies indicated an EC50 of 0.5 pM for insulin's stimulation of 125I-labelled BSA accumulation, consistent with action via its own receptor.(ABSTRACT TRUNCATED AT 250 WORDS)