Lack of a direct role for cyclic AMP in parathyrin action on phosphate reabsorption by the kidney

Adenylate cyclase blockers dissociate PTH-stimulated bone resorption from cAMP production

It is uncertain whether adenosine 3',5'-cyclic monophosphate (cAMP) or the inositol-calcium pathway mediates the stimulation of bone resorption by parathyroid hormone (PTH). Incubation of bone organ cultures with cAMP analogues and forskolin has not resolved this question because of the cellular inhomogeneity of bone and the consequent presence of adenylate cyclase-linked receptors for both PTH and calcitonin, hormones with opposite effects on bone resorption. We have used two new inhibitors of adenylate cyclase, 9-(tetrahydro-2-furyl)adenine (SQ 22536) and 2',5'-dideoxyadenosine (DDA), to directly reassess the role of cAMP in PTH-stimulated osteolysis. SQ 22536 (0.01-1.0 mM) and DDA (0.01-1.0 mM) completely blocked PTH stimulation of cAMP production measured in the absence of a phosphodiesterase blocker. In the presence of 1 mM 3-isobutyl-1-methylxanthine, half-maximal inhibition of PTH-induced cAMP production occurred with 0.2 mM SQ and 0.1 mM DDA, respectively. These concentrations of SQ and DDA had no effect on PTH-stimulated 45Ca release from calvaria, although both agents inhibited bone resorption when present at concentrations of 1-2 mM. At these levels, SQ and DDA caused equivalent inhibition of 45Ca release stimulated by 1,25-dihydroxyvitamin D3 but did not affect basal 45Ca release or [3H]-phenylalanine incorporation. It is concluded that substantial blockade of PTH-induced cAMP production does not affect this hormone's stimulation of bone resorption, which is therefore likely to be mediated by another intracellular messenger system, possibly calcium. In millimolar concentrations, SQ and DDA appear to be nonspecific blockers of osteoclastic bone resorption.

The effect of gluconeogenesis on phospholipid turnover in isolated chick proximal tubule cells

Previous work from this laboratory has shown that isolated chick renal proximal tubule cells possess an Na+-dependent Pi transport system and that Pi uptake is stimulated under gluconeogenic conditions. It is shown in the present paper that gluconeogenesis is associated with a rapid incorporation of Pi into membrane phospholipids, particularly phosphatidylinositol, and some evidence has been obtained for a change in the relative amounts of phosphatidylinositol polyphosphates under gluconeogenic conditions. There is no increase in the total phospholipid phosphate content however, suggesting that pyruvate-induced incorporation of Pi into phospholipids represents accelerated turnover rather than a net increase in synthesis. It is suggested that the stimulation of Na+-dependent Pi uptake by pyruvate is related to the increased rate of phospholipid turnover. Thus Pi transport may be a further example of a physiological system that is influenced by phosphatidylinositol metabolism. The role of phosphatidylinositol phosphates could be to stimulate transfer of transporter molecules from internal stores to the brush-border membrane of the cell.