guanosine-triphosphate and Drug-Hypersensitivity

Fetal and neonatal tissues are resistant to catecholamine-induced desensitization of essential physiological responses. We examined the mechanisms underlying the ontogeny of desensitization in neonatal rat heart for the beta-adrenergic receptor/adenylyl cyclase signaling cascade. Animals of different ages received isoproterenol daily or 4 days and cardiac membrane preparations were evaluated on the 5th day (6, 15, 25 days old and adults). Measurements were made of basal activity, activity stimulated by two agonists (isoproterenol or glucagon) that operate at different receptors but that share Gs as the transduction intermediate, or by forskolin-Mn' to assess total catalytic capacity of the cyclase subunit; we also assessed inhibition of activity by carbachol which acts via muscarinic cholinergic receptors and G. Adult rats exhibited robust desensitization of the adenylyl cyclase response but the effect was heterologous in that equivalent loss of activity was seen for basal, isoproterenol- and glucagon-stimulated activity forskolin-Mn(2+)-stimulated activity was also decreased. Two factors contributed to desensitization; generalized reduction in membrane protein concentrations caused by cell enlargement (reduced surface-to-volume ratio), and specific interference with the G-protein component that couples receptors to the cyclase. Thus, after adjustment for changes in membrane protein, the desensitization of the forskolin-Mn2, response was no longer evident, but the effects on the other measures were still present. In addition, isoproterenol treatment produced crosstalk with the carbachol/Gi signaling pathway, with significant reductions in the ability of carbachol to inhibit adenylyl cyclase activity. Heterologous desensitization by isoproterenol was also present in 15 and 25 day old rats, but involved only selective components of the effects seen in adults. At 25 days, uncoupling of signals operating through Gs and Gi was obtained without a reduction in forskolin-Mn(2+)-stimulated activity. At 15 days, only the effect on Gs coupling was seen. At 6 days, agonist-induced desensitization was not detectable and instead, heterologous sensitization was found. In these youngest animals, isoproterenol treatment produced a parallel increase in basal, isoproterenol-, glucagon- and forskolin-Mn(2+)-stimulated activity, unaccompanied by changes in membrane protein concentrations, indicating an increase in adenylyl cyclase catalytic activity. These results indicate th

Topics: Adenylyl Cyclases; Adrenergic beta-Agonists; Age Factors; Animals; Animals, Newborn; Body Weight; Carbachol; Cell Membrane; Colforsin; Drug Hypersensitivity; Female; Glucagon; GTP-Binding Protein alpha Subunits, Gi-Go; GTP-Binding Protein alpha Subunits, Gs; GTP-Binding Proteins; Guanosine Triphosphate; Heart; Isoproterenol; Male; Manganese; Membrane Proteins; Muscarinic Agonists; Myocardium; Pregnancy; Rats; Rats, Sprague-Dawley; Receptors, Adrenergic, beta; Time Factors