| Abstract: | The effects of the mixed agonist epinephrine and the beta agonist isoproterenol, each alone and in combination with the alpha adrenergic blocker phentolamine and the beta blocker propranolol on the adenylate cyclase activity of human adipocyte membrane fragments were determined in a calcium free buffer. Neither phentolamine (10 muM) nor propranolol (32 muM) affected basal adenylate cyclase activity. Epinephrine (10 muM) stimulated adenylate cyclase activity and this effect was slightly enhanced by phentolamine. The combination of epinephrine plus propranolol depressed adenylate cyclase below the basal level. Isoproterenol (10 muM) markedly stimulated adenylate cyclase; the addition of phentolamine caused an equivocal further increase while the addition of propranolol depressed adenylate cyclase activity to, but not below, the basal level. These findings are consistent with the hypothesis that human adipocytes have both alpha and beta adrenergic receptors and that these receptors are associated with the cell membrane adenylate cyclase system. |
