Impaired formation of β-adrenergic receptor-nucleotide regulatory protein complexes in pseudohypoparathyroidism

Decreased activity of the guanine nucleotide regulatory protein (N) of the adenylate cyclase system is present in cell membranes of some patients with pseudohypoparathyroidism (PHP-Ia) whereas others have normal activity of N (PHP-Ib). Low N activity in PHP-Ia results in a decrease in hormone (H)-stimulatable adenylate cyclase in various tissues, which might be due to decreased ability to form an agonist-specific high affinity complex composed of H, receptor (R), and N. To test this hypothesis, we compared β-adrenergic agonist-specific binding properties in erythrocyte membranes from five patients with PHP-Ia (N = 45% of control), five patients with PHP-Ib (N = 97%), and five control subjects. Competition curves that were generated by increasing concentrations of the β-agonist isoproterenol competing with [¹²⁵I]pindolol were shallow (slope factors <1) and were computer fit to a two-state model with corresponding high and low affinity for the agonist. The agonist competition curves from the PHP-Ia patients were shifted significantly (P <0.02) to the right as a result of a significant (P <0.01) decrease in the percent of β-adrenergic receptors in the high affinity state from 64 ± 22% in PHP-Ib and 56 ± 5% in controls to 10 ± 8% in PHP-Ia. The agonist competition curves were computer fit to a 'ternary complex' model for the two-step reaction: H + R + N ≠ HR + N ⇆ HRN. The modeling was consistent with a 60% decrease in the functional concentration of N, and was in good agreement with the biochemically determined decrease in erythrocyte N protein activity. These in vitro findings in erythrocytes taken together with the recent observations that in vivo isoproterenol-stimulated adenylate cyclase activity is decreased in patients with PHP (Carlson, H.E., and A.S. Brickman, 1983, J. Clin. Endocrinol. Metab. 56:1323-1326) are consistent with the notion that N is a bifunctional protein interacting with both R and the adenylate cyclase. It may be that in patients with PHP-Ia a single molecular and genetic defect accounts for both decreased HRN formation and decreased adenylate cyclase activity, wereas in PHP-Ib the biochemical lesion(s) appear not to affect HRN complex formation.