Calcium homeostasis in the blood and extracellular fluids is maintained to a large extent by parathyroid hormone (PTH), which, by actions on bone and in the kidney, tends to increase the level of extracellular Ca2+. The secretion of PTH, in turn, is depressed by increases in the concentration of extracellular Ca2+. It is unknown, however, how this cell detects a change in the concentration of extracellular Ca2+ and how this recognition event is subsequently transformed into an intracellular signal(s) that regulates PTH secretion. Using dissociated bovine parathyroid cells, the applicant has provided evidence for the presence of a """"""""Ca2+ receptor"""""""" on the surface of the parathyroid cell that enables this cell to detect small changes in the concentration of extracellular Ca2+. Activation of this putative receptor by extracellular Ca2+ causes the rapid mobilization of cellular Ca2+ and increases the plasma membrane permeability to Ca2+, resulting in transient and sustained increases in (Ca2+)i. Thus, in contrast to other secretory cells, increases in (Ca2+)i in the parathyroid cell are associated with an inhibition of secretion. It is uncertain, however, if the putative """"""""Ca2+ in the regulation of PTH secretion is equally unclear.
The aims of this proposal test the validity of the """"""""Ca2+ receptor"""""""" coupled to the mobilization of cellular Ca2+ is also linked to the regulation of secretion. The role of cytosolic Ca2+ receptor"""""""" hypothesis and examined the relationship between cytosolic Ca2+ and secretion. Various agents that impair transmembrane signalling mechanisms common to Ca2+-mobilizing receptors will be examined for effects on (Ca2+)i and PTH secretion. It will also be determined if extracellular divalent cations affect phosphoinositide metabolism and cytosolic pH. Preliminary results show that some agents affecting membrane signal transduction mechanisms have differential effects on (Ca2+)i and secretion, suggesting additional or alternative intracellular signals important for the regulation of PTH secretion. The role of cytosolic Ca2+ in secretion will be assessed by directly manipulating (Ca2+)i with chelators and ionophores. The discovery of a purinergic receptor on parathyroid cells coupled to the mobilization of cellular Ca2+ has provided some initial results suggesting that changes in (Ca2+)i may be necessary but not sufficient to alter PTH secretion. This, together with other findings, prompts study of the possible role of diacylglycerol in secretion. The effects of extracellular divalent cations on diacylglycerol formation under various experimental conditions will therefore be examined. Finally, in an attempt to obtain specific high-affinity ligands that might allow the isolation and characterization of some membrane component (a """"""""Ca2+ receptor""""""""), are initial studies showing similarities between the effects of certain organic polycations and extracellular Ca2+ on (Ca2+)i and PTH secretion. The proposed studies are relevant to understanding the mechanism of secretion in various cells, especially those others that alter secretion in response to changes in the concentration of extracellular Ca2+, like the C-cell and renin-secreting cell.
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