Comparison of the epinephrine-mediated activation of adenylate cyclase in plasma membranes from liver and ascites hepatomas of rats

Abstract

1. 1. The apparent [ 3H]epinephrine binding parameters of plasma membranes from rat liver and ascites hepatomas such as AH-7974, AH-371A and AH-130, as measured by equilibrium dialysis and/or Millipore filtration, were almost similar to each other. The epinephrine binding sites in the plasma membranes were heterogeneous (α-, β-receptors and non-specific sites), but the pattern of these binding sites in the liver membranes appeared almost similar to that in the hepatoma membranes. 2. 2. The β-receptors seemed to be specifically involved in the epinephrine-mediated activation of adenylate cyclase of the liver membranes. In spite of the presence of almost similar β-receptors and adenylate cyclase, the adenylate cyclase of hepatoma membranes was found to be less sensitive to the epinephrine-mediated activation. 3. 3. GTP alone was found to activate adenylate cyclase of liver and hepatoma membranes to some extents when the concentration of ATP was lower (0.3 mM). When GTP was added with epinephrine, a marked, a synergistic activation of adenylate cyclase was observed in liver plasma membranes, but not in hepatoma ones. 4. 4. The synergistic activation of adenylate cyclase by epinephrine plus GTP showed a characteristic kinetic feature, reaching a maximal peak within 1 min or so after mixing. 5. 5. Binding of [ 3H]epinephrine to liver membranes proceeded monophasically in the absence of GTP, while it proceeded biphasically in the presence of GTP showing the retardation of binding at some earlier stages. GTP added at the time of binding equilibrium induced the temporary release of bound epinephrine from the β-receptors. The GTP-induced temporary release of bound epinephrine, occuring within 4–5 min after the addition of GTP, was less marked in the hepatoma membranes as compared with the liver membranes. 6. 6. Possible impairment of the GTP-dependent coupling mechanism in the receptor-adenylate cyclase system of hepatoma plasma membranes was suggested.