The goal of this project is to ascertain the development of mechanisms, in embryonic and neonatal animals, for the regulation of cardiac function by the parasympathetic and sympathetic divisions of the autonomic nervous system. This goal is incorporated into tests of hypotheses for beta-adrenoceptor stimulation and muscarinic receptor (mAChR) inhibition of biochemical (adenylate cyclase activity), electrical (membrane potentials, ionic currents), and mechanical (inotropic effects) properties of cardiac tissues. Agonist occupancy of receptors and the regulation of occupancy by guanine nucleotides will be determined by displacement of antagonist (125-I-CYP for beta-adrenoceptors and H-3-QNB for mAChR). The coupling of autonomic transmitter receptors to cellular effectors depends, in most instances, on guanine nucleotide binding proteins that mediate inhibition (Ni) and stimulation (Ns). The properties of Ns will be determined with cholera toxin which promotes the ADP-ribosylation of Ns. These experiments will test mechanism(s) for subsensitivity (positive inotropic effect, adenylate cyclase stimulation) to beta-adrenoceptor agonist seen late in embryonic life. Whether atrium and ventricle contain different or differing amounts of Ni will be tested in experiments with pertussis toxin which catalyzes the ADP-ribosylation of Ni and thereby uncouples mAChR from adenylate cyclase and gK1. Does the ventricle lack a subunit of Ni and/or will voltage clamp experiments show ventricular gK1 to be different from that in the atriuim so as to render mAChR unable to increase current through resting K+ channels of ventricle? The positive inoptropic effect of muscarinic agonists appears independent of Ni and somehow to involve stimulation of phosphatidylinositol hydrolysis. This hypothesis will be tested in both embryonic and hatched chick cardiac tissues. A novel mechanism for beta-adrenoceptor agonist, to reduce tau-rec of i-Ca, will be tested in voltage clamp experiments with isolated ventricular myocytes. Intracellular injection of cyclic AMP will be done to test a cyclic AMP hypothesis in which agonist occupancy of mAChR inhibits the effects of accumulated cyclic AMP without changing cyclic AMP content. Altogether, the results can increase our understanding not only of how autonomic transmitters act and interact on heart cells but also of how imbalances between muscarinic and beta-adrenoceptor agonists can disturb membrane electrical activity (arrhythmias) and contractility (mechanical dysfunction).
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