The autonomic nervous system plays a major role in regulating cardiac activity. Thus changes in post synaptic receptors, which initiate the actions of the autonomic transmitters acetylcholine and catecholamines, may contribute to possible alterations in the ability of the autonomic nervous system to regulate cardiac activity. The long term objective of this proposal is to study the regulation of the high and low agonist affinity states of cardiac beta-adrenoreceptors and muscarinic receptors and their ability to stimulate and inhibit adenylate cyclase activity. Both intact animals (rats) and isolated atria will be used. Parameters to be determined include the total receptor concentrations (as measured with labeled antagonists), the fraction of receptors that form the high and low agonist affinity states and the affinity constants for agonists. These parameters will be determined by computer analysis of agonist competition curves (for both receptors) and by direct agonist binding to high affinity sites for the muscarinic receptor. In addition, the ability of muscarinic agonists to attenuate and beta-agonists to stimulate adenylate cyclase activity will be measured. The conditions under which these parameters will be measured include development, thyroid status and during treatment with isoproterenol, 6-hydroxydopamine, nadolol, methacholine, atropine and glucocorticoid. Irreversible beta and muscarinic antagonists will be used to define the relationship between receptor number and biochemical response (coupling efficiency) in isolated atria for cAMP accululation (beta), attenuation of cAMP production and cGMP accumulation (muscarinic). The effect of microtubule inhibitors on atrial beta and muscarinic biochemical responses will also be determined and if the altered response is associated with a change in receptor-response coupling efficiency. These studies are designed to determine further, changes that may occur under various conditions, to different agonist binding states of cardiac autonomic receptors and their ability to modulate the activity of adenylate cyclase. In addition, the use of irreversible receptor antagonists will allow a further investigation on how cellular structures such as microtubules, modulate the ability of receptors to produce a biochemical response.
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