Characterization of adenylate cyclase-coupled alpha 2-adrenergic receptors in rat renal cortex using [3H]yohimbine

Alpha 2 adrenergic receptors in rat renal cortex were measured with the antagonist ligand [3H]yohimbine. Renal cortical membranes contained 120 +/- 11.7 (SE) fmoles of binding sites per milligram of protein (n = 9). Their affinity for [3H]yohimbine was 10.4 +/- 0.5 (SE) nM (n = 9) from equilibrium studies and 7.2 +/- 3.3 (n = 4) from kinetic measurements. Alpha-adrenergic agonists and antagonists bound [3H]yohimbine sites with affinities consistent with alpha 2-receptor binding. The slope factors for the binding of all antagonists tested were close to 1, whereas agonists bound with slope factors of less than 1, consistent with the presence of receptors of more than one affinity. The alpha 2-receptors measured with [3H]yohimbine appeared to be coupled to the inhibition of adenylate cyclase. The affinities of alpha-adrenergic catecholamines measured in cyclase inhibition studies were similar to their affinities in binding studies when these were performed under conditions optimal for adenylate cyclase inhibition. Both sodium ion and GTP are required for maximal inhibition of renal cortical adenylate cyclase [Woodcock, E.A., C. I. Johnston, and C.A. Olsson. J. Cyclic Nucleotide Res. 6:261-271, 1980)]. Each of these factors produced a 10- to 20-fold decrease in the binding affinity of epinephrine. Together, a 100- to 200-fold decrease was produced. The concentrations of both sodium ion and GTP required to decrease the affinity of epinephrine were similar to the concentrations required for adenylate cyclase inhibition. This suggested that similar mechanisms were involved in decreasing the affinity of agonists for the alpha 2-receptor and promoting agonist-induced inhibition of adenylate cyclase.