Evidence that parathyroid hormone-mediated calcium transport in rat brain synaptosomes is independent of cyclic adenosine monophosphate.

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Abstract

In vivo PTH administration to rats resulted in increased brain synaptosomal Ca++ transport, while parathyroidectomy (PTX) resulted in decreased transport. To determine the mechanism of action of PTH on Ca++ transport in rat brain synaptosomes, we performed transport studies by the Na-Ca exchanger and also measured cAMP generation in synaptosomes from PTX rats. Ca++ transport was studied after in vivo additions of either bovine (b)PTH, cAMP, or forskolin, and adenylate cyclase activity was assessed after additions of either bPTH, forskolin, sodium fluoride (NaF), or isoproterenol. In the presence of 1-34 bPTH [10(-7) M], Ca++ uptake was significantly increased by 55% (P less than 0.001) above control, while 3-34 bPTH [10(-7) M] had no effect on uptake. Both 8br,cAMP [10(-6) M] and dibut,cAMP [10(-6) M] also significantly increased (P less than 0.001) Ca++ uptake above control by 63 and 44%, respectively. Similarly, forskolin [10(-5) M], the adenylate cyclase activator, increased Ca++ uptake by 41%. We next evaluated Ca++ efflux, and found that 1-34 bPTH [10(-7) M], 1-84 bPTH [10(-7) M], and forskolin [10(-5) M] also increased Ca++ efflux by 50, 73, and 120%, respectively, above control. Since Ca++ transport was increased by either PTH, cAMP, or forskolin, we decided to determine if PTH action on Ca++ transport in synaptosomes was dependent on cAMP. This was investigated by measuring cAMP production during the conversion of 32P-ATP to 32P-cAMP in the presence of an ATP regenerating system (30 micrograms creatine phosphokinase, 10 mM creatine phosphate), and the cyclic nucleotide phosphodiesterase inhibitor (1 mM IBMX). Whereas forskolin [10(-4) M] and NaF [100 mM] significantly increased (P less than 0.001) adenylate cyclase activity in synaptosomes by eight- and fourfold, respectively, neither 1-34 bPTH nor 1-84 bPTH increased synaptosomal cyclase activity. However, in canine renal cortical plasma membranes (CRCPM), we observed significant increases in cAMP production with either forskolin, NaF, or PTH. Finally, to determine if synaptosomes contain an intact adenylate cyclase system, we measured cAMP production in the presence of the beta adrenergic agent, isoproterenol. Isoproterenol significantly increased adenylate cyclase activity in both synaptosomes (90%) and CRCPM (50%). These data suggest that although there is an intact adenylate cyclase system in rat brain synaptosomes, PTH-stimulated calcium transport in synaptosomes appears to be independent of this system.

Authors

C L Fraser, P Sarnacki, A Budayr