Glucose Metabolism in Isolated Fat Cells: Enhanced Response of Larger Adipocytes from Older Rats to Epinephrine and Adrenocorticotropin[*]

Abstract

The effects of insulin and of two lipolytic hormones (epinephrine and ACTH¹)on the rate and pattern of glucose metabolism were compared during incubation of isolated fat cells, obtained from epididymal fat pads of rats of varying age and degrees of adiposity. Glucose metabolism and the intracellular free fatty acid levels were expressed on a per cell basis and in relation to adipocyte size.

The data for total glucose metabolism show that, in contrast to the declining insulin effect observed with adipocyte enlargement, the stimulation of glucose uptake and metabolism by these lipolytic hormones was significantly greater in the larger fat cells from the older fatter rats than in the smaller ones from the younger leaner rats. Lipolytic hormones suppressed, whereas insulin enhanced, fatty acid synthesis; moreover, the lipolytic hormones stimulated glucose conversion to glyceride glycerol to a much greater extent than did insulin. The effect of epinephrine on the intracellular free fatty acid levels was greater in the small fat cells than in the large ones; this effect of epinephrine was markedly curtailed by the presence of glucose in the incubation medium, making it unlikely that acceleration of glucose metabolism by the lipolytic stimulus was mediated by an elevation of the intracellular free fatty acid level.

The present results show a markedly enhanced capacity of the large adipocytes to accelerate glucose metabolism in response to these lipolytic hormones. Thus, in contrast to prevailing notions of declining hormonal responsiveness with expanding fat cell size in older and more obese animals, this study documents an instance of increased hormonal response in enlarged adipocytes and points to the need for a more comprehensive reevaluation of the various hormonal effects in adipocytes of different size. ACTH, adreno-corticotroic-hormone; FFA, free fatty acids; IFFA, intracellular free fatty acids.

Key words

Fat Cell Size - Fat Cell Number - Pattern of Glucose Metabolism - Fatty Acid Synthesis - Glyceride Synthesis - Intracellular Free Fatty Acids - Lipolysis

1 Supported in part by USPHS Grants AM-13940 and AM-17403, and by a grant-in-Aid of the American Heart Association.

1 Supported in part by USPHS Grants AM-13940 and AM-17403, and by a grant-in-Aid of the American Heart Association.

2 This work was done during the tenure of M. DiGirolamo of an Established Investigatorship of the American Heart Association.