Effects of Guanosine 3',5'-Monophosphate on Glycerol Production and Accumulation of Adenosine 3',5'-Monophosphate by Fat Cells

Abstract

Abstract The effects of guanosine 3',5'-monophosphate (cyclic GMP) on glycerol production by isolated fat cells resembled those of adenosine 3',5'-monophosphate (cyclic AMP) but were of lesser magnitude. In Krebs-Ringer phosphate medium, both nucleotides had little or no effect on basal lipolysis. Inhibition of theophylline-stimulated lipolysis by cyclic GMP was less and more variable than that by cyclic AMP. In phosphate 0.8% NaCl medium, with Na+ as the only cation, both nucleotides stimulated glycerol production. The increase was directly related to the concentration of each. On a molar basis, cyclic GMP was less effective than cyclic AMP in stimulating lipolysis. Lipolytic effects of cyclic GMP were also observed in the presence of cyclic AMP and theophylline at concentrations that did not maximally stimulate lipolysis. During incubation of fat cells with cyclic GMP in either medium, accumulation of cyclic AMP was observed. The magnitude of this increase in cyclic AMP was related to the concentration of cyclic GMP over the range of 0.1 to 5 mm. In the presence of 1 mm cyclic GMP, the rate of accumulation of cyclic AMP was essentially constant for 2 hours. After incubation with cyclic GMP, the amount of cyclic AMP in the medium was apparently as high as in cells plus medium. The accumulation of cyclic AMP in the presence of cyclic GMP was not affected by prostaglandin E1 (5 µg per ml), insulin (12 milliunits per ml) or probenecid (0.1 mm). The latter agent did not alter the distribution of cyclic AMP between cells and medium whether or not cyclic GMP was present. The rate of accumulation of cyclic AMP in the presence of 1 mm cyclic GMP was 10 to 20% of that observed during the first 4 min after epinephrine (1.1 µm) was added to fat cells. During the 4 min after epinephrine was added, the accumulation of cyclic AMP was almost entirely intracellular and was not altered by the presence of cyclic GMP. After incubation for more than 30 min under all conditions, a large fraction of the cyclic AMP in the system was found in the medium. Cyclic GMP was 30 to 100 times more effective on a molar basis than was theophylline in preventing the degradation of cyclic AMP by extracts from fat cells. Cyclic GMP also inhibited degradation of cyclic AMP by homogenates of rat kidney and liver. This effect may account for the accumulation of cyclic AMP that occurred when kidney cortex, liver, and chicken erythrocytes were incubated with cyclic GMP with or without hormone.