To gain information on the manner in which insulin suppresses lipolysis in man, isolated adipocytes, prepared from subcutaneous adipose tissue, were incubated with insulin (100 μU/ml) alone and in combination with isoproterenol (10−7 M or 10−8 M). Cyclic AMP concentration was measured at 60 min; glycerol release, used as an index of lipolysis, was determined at 45 and 75 min. Insulin consistently reduced both basal and stimulated cyclic AMP and glycerol release: the degree of suppression of each was comparable. In subsequent experiments, the ability of insulin to suppress glycerol release stimulated by isoproterenol, theophylline, and dibutyryl cyclic AMP (dbcAMP), respectively, was compared. Insulin substantially reduced the raised levels of cyclic AMP and glycerol release prompted by isoproterenol and theophylline, but it had little effect on increases caused by dbcAMP. These findings support the view that reduction in cyclic AMP is an important component in the regulation of fat mobilization by insulin.

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