Abstract
Although catecholamines stimulate lipolysis in human fat cells, activation by epinephrine of adenylate cyclase in human fat cell membranes is not readily observed. The possible role of guanine nucleotides in this reaction has now been examined with human material. Fat cell ghosts were prepared from subcutaneous fat obtained from patients undergoing elective surgery. Adenylate cyclase was assayed with [alpha-32P]ATP as substrate. Fluoride ion stimulated the enzyme 8.3-fold relative to basal levels, but epinephrine activation of cyclase was not statistically significant. GTP did not allow expression of an epinephrine effect. However, the addition of the GTP analogue, 5'-guanylyl-imidodiphosphate [GMP-P(NH)P], along with epinephrine produced 5.7-fold activation of the enzyme (P less than 0.001). GMP-P(NH)P alone was without stimulatory effect. Comparable augmentation by GMP-P (NH) P of adenylate cyclase activity was seen with isoproterenol, norepinephrine, and epinephrine. Propranolol blocked catecholamine-GMP-P (NH) P stimulation of the enzyme, suggesting that the nucleotide-dependent activation of catecholamine-sensitive adenylate cyclase is mediated by beta-receptors. GMP-P(NH)P may prove useful in allowing in vitro demonstration of additional hormone-sensitive adenylate cyclase systems.