The parathyroid (PT) glands play a critical role in mineral ion homeostasis by sensing even minute changes in the extracellular calcium (Ca++) concentration and responding with oppositely directed changes in parathyroid hormone (PTH) secretion. Changes in extracellular Ca++ modulate several mediators in PT cells, including cAMP, the cytosolic Ca++ concentration (Ca[i]), and phosphoinositide (PI) turnover. None of the high Ca++-evoked changes in these mediators, however, totally explain the concomitant reduction in PTH secretion. In addition, the high Ca++-stimulated increases in Ca[i] and PI turnover in PT cells are accompanied by a decrease rather than the expected increase in protein kinase C (PKC) activity. The latter implicates as yet unexplored regulatory factors in the control of PKC and PTH secretion in PT cells. Recent studies in other systems have indicated a potentially important role for agonist-induced activation of phospholipases other than PI-specific phospholipase C (PLC) in generating biologically important mediators. These include PLC(s) acting on phospholipids other than the phosphinositides (non-PI PLC), which form diradylglycerols, phospholipase D (PLD), which produces phosphatidic acid (PA), phospholipase A2 (PLA2), which forms lysophospholipids and free fatty acids, and sphingomyelinase, which produces ceramide and, in turn (though the action of ceramidase), sphingosine. Several of these products are relevant to Ca++-regulated PT function, since some diradylglycerols are ineffective in activating PKC or even inhibit the enzyme, PA and lysoPA both mimic the effects of high Ca++ in raising Ca[i] and inhibiting PTH secretion, and lysophosphatidylcholine and sphingosine both inhibit PKC. This proposal will test the hypothesis that stimulation of these four phospholipases in PT cells by high extracellular Ca++ leads to the formation of mediators which inhibit PKC and PTH release, rather than producing the usual stimulation of these parameters as in other cells. For each of these phospholipases, we will ask the following questions: (a) Does high Ca++ activate the phospholipase in intact PT cells? (b) Is the activation of the enzyme direct (i.e., """"""""Ca++-receptor""""""""-mediated) or indirect [e. g. . due to changes in some other mediator(s) ] ? And (c) what are the biologic effects of mediators generated by the phospholipase on other second messenger systems, including PKC, and PTH release? These studies may provide important insights into the regulation of both normal and abnormal parathyroid function and also into the mechanisms through which other cell types sense changes in extracellular Ca++.
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