Adenylate‐Cyclase Activity of Rat‐Liver Plasma Membranes

The adenylate‐cyclase activity from a rat‐liver plasma‐membrane preparation purified according to the Neville's procedure was increased 10 to 12‐fold by glucagon, but the addition of epinephrine, at any concentration, failed to activate the enzyme. When the membranes were isolated from adrenalectomized animals, the responses of the enzyme to glucagon and NaF were enhanced and mainly the adenylate‐cyclase activity became sensitive to epinephrine with an optimal effect at 0.05 mM epinephrine. Intraperitoneal administration of cortisol for 2 days to the adrenalectomized animals reversed the effect of the hormonal deprivation. But addition of cortisol in vitro was without effect. The use of liver plasma membranes from adrenalectomized rats is therefore proposed as a tool to study the adenylate‐cyclase activation by both glucagon and epinephrine. The epinephrine‐sensitive adenylate‐cyclase activity was completely inhibited by propranolol but not by phentolamine, thus confirming that the metabolic effect of epinephrine is linked to a β‐like receptor. Epinephrine and glucagon effects upon adenylate‐cyclase activity were both inhibited by concentration of calcium above 0.01 mM but differed with respect to the Mg‐ATP concentration. The two hormonally stimulated adenylate‐cyclase activities were found to be additive. The effect of epinephrine upon adenylate‐cyclase activity was more sensitive to stimulation by low concentration of GTP and other nucleotides than the effect of glucagon. Addition of insulin in vitro at any concentration failed to alter the adenylate‐cyclase activity regardless of the concentration of magnesium, the presence of glucose or GTP.