The series of cellular events which are initiated by agonist occupation of Alpha?1? adrenergic receptors are only partially understood. While it is recognized that an increase in cytosolic Ca?2+? mediates such Alpha?1? receptor responses as contraction of arteries and stimulation of hepatic glycogenolysis, the reactions leading to Ca?2+? mobilization are unclear. Since activation of Alpha?1? receptors is associated with a Ca?2+? independent increase in the generation of the membrane phospolipids phosphatidic acid (PA) and phosphatidyl inositol (PI), it has been suggested that these changes lead to observed increases in Ca?2+? influx and intracellular Ca?2+? release. To test the validity of this suggestion we will quantitate the relationship between Alpha?1? receptor occupany and coupling events in the rabbit aorta. In addition to measurements with the full agonists norepinephrine and phenylephrine we will compare the effects of a series of imidazoline derivatives which are graded partial agonists so as to draw inferences on the basis of their submaximal effectiveness. Occupancy will initially be determined for both full and partial agonists using contractile response as an endpoint. Radioligand binding studies on isolated membranes using the receptor subtype-specific radioligands ?3?H-prazosin and ?3?H-yohimbine will also be used to characterize the interaction of agonists with Alpha?1? and Alpha?2? receptors respectively. Subsequent studies will measure the simulation of ?45?Ca release by increasing concentrations of agonists. ?32?P-labelling of PA and PI will also be assayed at the same agonist concentrations. The results of contraction, receptor binding, Ca?2+? flux and phospholipid turnover measurements will be compared so as to address the following questions: 1) Is the relationship between receptor occupany and response linear when computed using each of the above receptor-initiated parameters? 2) Are non-linear relationships similar for different steps in the coupling process? 3) Is the pattern of PA (or PI) formation compatible with its role as a receptor-operated Ca?2+? gating mechanism? 4) Do phospholipid and Ca?2+? flux events indicate the presence of a receptor reserve? 5) Is the pattern of Ca?2+? handling by arterial smooth muscle cells similar at different levels of receptor-initiated activation.
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