Some Factors Modifying the Actions of Hormones on Glucose Uptake by Adipose Tissuein Vitro

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Abstract
The uptake of glucose by rat epididymal adipose tissue incubated in vitro in a Krebs-Ringer bicarbonate medium is influenced by the presence or absence of bovine serum albumin in the medium. Without albumin glucose uptake is significantly higher than when albumin is present at concentration of 2–4%. Adrenaline, noradrenaline and ACTH, all of which stimulate lipolysis in adipose tissue in vitro, inhibit glucose uptake when there is no albumin in the medium and stimulate it when albumin is present. Insulin promotes glucose uptake irrespective of the presence of albumin, but this response to insulin is inhibited by the lipolytic hormones when there is no albumin in the medium. Both the inhibitory and stimulatory actions of ACTH in the different media are abolished by chemical inactivation of ACTH. Furthermore, the unnatural L(+) isomers of adrenaline and noradrenaline are less active than the natural D(−) isomers. Evidence is presented to support the hypothesis that while some increase in intracellular concentration of free fatty acids in adipose tissue may stimulate glucose utilization, these same fatty acids, or degradation products thereof, exert an inhibitory action on glucose uptake if they accumulate in excess. The latter situation holds when lipolysis in adipose tissue is stimulated by lipolytic hormones thereby preventing the escape of the fatty acids into the medium. The significance of these findings is discussed in regard to the physiological effects of hormones on adipose tissue in vitro.