guanosine-triphosphate and hydroxybenzylpindolol

To investigate whether, in developing fetal rabbit myocardium, the affinity of beta-adrenoreceptors (beta-AR) for beta-AR agonists is regulated by guanosine triphosphate (GTP), we performed in vitro competition experiments utilizing 125I-hydroxybenzylpindolol (I-HYP), a beta-AR antagonist, as the radioligand, and l-isoproterenol (l-I) and l-propranolol (l-P), as beta-AR agonist and antagonist, respectively. At gestational ages from 21 to 31 days (term 31 days), in the presence of GTP, there was a 4-fold increase in the KI for the inhibition of specific I-HYP binding by l-I but not by l-P. Moreover, in the presence of GTP, there was a significant shift of the Hill coefficient to values closer to 1 only in the case of l-I. We conclude that: (1) beta-AR can form a 'high affinity' complex with l-I in the absence of GTP, detectable from days 21 to 31 of gestation; (2) the conversion of the former to a 'low affinity' state appears to be a GTP-dependent process.

Topics: Animals; Binding, Competitive; Fetus; Gestational Age; Guanosine Triphosphate; In Vitro Techniques; Isoproterenol; Membranes; Myocardium; Pindolol; Propranolol; Rabbits; Receptors, Adrenergic, beta

The ability of 10 muM epinephrine or isoproterenol to stimulate cyclic AMP accumulation was decreased in hepatocytes isolated from hyperthyroid (triiodothyronine treated) as compared to euthyroid rats. In the presence of methylisobutylxanthine, epinephrine or isoproterenol-stimulated cyclic AMP accumulation was approximately 65% lower in hyperthyroid as compared with euthyroid rat hepatocytes. The ability of glucagon to stimulate a cyclic AMP response was also decreased in the hyperthyroid state, when assayed in either the absence or presence of a methyl xanthine. The character of the catecholamine-stimulated cyclic AMP response was beta adrenergic in both the hyperand euthyroid states. No evidence for an alpha(2) adrenergic mediated component of catecholamine action on cyclic AMP levels was noted. Cyclic AMP phosphodiesterase activity of hepatocyte homogenates was not altered in the hyperthyroid state. Hormone-stimulated, guanine nucleotide- and fluoride-activatable adenylate cyclase activity was reduced in subcellular fractions obtained from hyperthyroid as compared with euthyroid rat hepatocytes. Beta adrenergic receptor binding was reduced approximately 35% and glucagon receptor binding reduced approximately 50% in the hyperthyroid as compared with euthyroid rat hepatocyte membrane fractions. The status of the regulatory components of adenylate cyclase were examined by in vitro treatment of subcellular fractions with cholera toxin. The ability of cholera toxin to modulate adenylate cyclase was not altered by hyperthyroidism. Cholera toxin catalyzed AD[(32)P]ribosylation of hyperthyroid and euthyroid rat hepatocyte proteins separated electrophoretically displayed nearly identical autoradiograms. Studies of the reconstitution of adenylate cyclase activity of S49 mouse lymphoma cyc(-) mutant membranes by detergent extracts from rat hepatocyte membranes, indicated that hyperthyroidism was associated with a reduced capacity of regulatory components to confer fluoride, but not guanine nucleotide activatability to catalytic cyclase. Thyroid hormones regulate the hormone-sensitive adenylate cyclase system of rat hepatocytes at several distinct loci of the system.

Topics: Adenylyl Cyclases; Animals; Benzyl Alcohols; Cholera Toxin; Cyclic AMP; Epinephrine; Female; Glucagon; Guanosine Triphosphate; Hormones; Hyperthyroidism; Isoproterenol; Liver; Pindolol; Prazosin; Propranolol; Rats; Rats, Inbred Strains; Receptors, Cell Surface; Triiodothyronine; Triiodothyronine, Reverse

Incubation of 1321N1 astrocytoma cells for 15 min with 1 microM (-)-isoproterenol resulted in a 50-65% loss of isoproterenol-stimulated adenylate cyclase activity. No decrease occurred in basal adenylate cyclase activity or in the density of beta-adrenergic receptors as assessed by (125I)-hydroxybenzylpindolol binding. Concentration-effect studies indicated that the apparent affinity of isoproterenol for inhibition of (125I)-hydroxbenzylpindolol binding was decreased by approximately 10-fold in membranes prepared from cells that had been exposed to 1 umM isoproterenol for 15 min. In the presence of GTP there was a shift to the right of the concentration-effect curve for isoproterenol in control membranes. GTP had little effect on the apparent affinity of isoproterenol in desensitized membranes. In desensitized cells that were subsequently washed free of catecholamine, the decrement in isoproterenol-stimulated adenylate cyclase activity and the decrease in the capacity of isoproterenol to inhibit 125IHYP binding returned to control levels within 15 min. These data are consistent with the hypothesis that an early event in the process of desensitization in 1321N1 cells involves a reversible uncoupling of beta-adrenergic receptors and adenylate cyclase.

Topics: Adenylyl Cyclases; Astrocytoma; Benzyl Alcohols; Binding Sites; Cell Line; Dose-Response Relationship, Drug; Guanosine Triphosphate; Isoproterenol; Membranes; Pindolol; Protein Binding; Receptors, Adrenergic; Receptors, Adrenergic, beta