In Vitro Effects of Insulin and Glucose on Leucine-2-¹⁴C Incorporation into Protein of Diaphragms of Hypophysectomized Rats

Abstract

INSULIN, added in vitro, has been found to stimulate the incorporation of labelled amino-acids into the protein of isolated diaphragms of normal rats^1–4. Glucose, when added to the incubation medium, depressed the stimulatory action of insulin on the uptake of alanine labelled with carbon-14 ^1,3 and α-aminoisobutyric acid-¹⁴C⁴ by normal diaphragm. Even when insulin was not added to the incubation medium the uptake of these amino-acids was markedly depressed by supplementing the medium with glucose. In one report, however, the incorporation of glycine labelled with carbon-14 into protein of diaphragms of normal fasting rats was found to be increased by the addition of glucose alone to the medium². A later study³ failed to confirm this observation and indicated further that neither glucose nor pyruvate seriously altered the uptake of a number of amino-acids labelled with carbon-14 by normal diaphragm. From the above studies it was apparent that insulin had a stimulatory effect on the uptake of amino-acids by isolated muscle of normal rats, and that in certain instances this effect was altered by the concomitant utilization of carbohydrate. Thus, it was of interest to study the effects of insulin and of glucose, alone and in combination, on the in vitro incorporation of labelled leucine by the diaphragm of hypophysectomized rats, which show a decreased rate of incorporation of amino-acid⁵ but which utilize glucose at a greater than normal rate⁶.