Effect of triamcinolone on parathyroid hormone-stimulated second messenger systems and phosphate transport in opossum kidney cells.

Although PTH is known to stimulate both the adenylate cyclase/protein kinase-A system and the phospholipase-C/protein kinase-C second messenger systems, the relative roles of these second messenger pathways remain unclear. The present studies were designed to examine the effect of triamcinolone on PTH-stimulated second messenger systems and phosphate transport in confluent cultures of opossum kidney cells. Triamcinolone was added to these cultures at a concentration of 10 nM for 24-48 h. Neither cell number nor protein content was changed by this treatment. The addition of triamcinolone did not alter PTH receptor binding or competitive displacement radioligand binding assay curves. PTH-stimulated cAMP generation and activation of protein kinase-A were not altered by triamcinolone. The glucocorticoid, however, increased basal phosphate uptake from 1.0 +/- 0.1 to 1.28 +/- 0.1 pmol/5 min.culture (P < 0.01). Phosphate transport was significantly decreased by PTH (0.01 nM) in the triamcinolone-treated cultures, but not in control cultures. Phosphate uptake in the presence of maximal doses of PTH was similar in both control and triamcinolone-treated cultures. Thus, the PTH-responsive component of phosphate transport was preserved, and the threshold dose for the effect of PTH was reduced after treatment with triamcinolone. Studies were then performed to evaluate the alternate second messenger pathway. In control cultures, PTH rapidly increased the level of diglyceride mass, as measured by diglyceride kinase assay, from 0.18 +/- 0.01 to a peak of 0.26 +/- 0.02 mol/100 mol total phospholipid (P < 0.002), 1 min after addition of the hormone. Triamcinolone pretreatment for 48 h, however, elevated the basal diglyceride levels, but the increase after the addition of PTH was totally abolished. The absence of an increase in diglyceride upon stimulation with PTH correlated with elimination of the PTH-stimulated increase in the activity of particulate protein kinase-C. Thus, in triamcinolone-treated cells, the effect of PTH on phosphate transport was preserved, and the threshold dose of PTH-induced alteration in phosphate transport was reduced in the absence of stimulation of this alternate second messenger pathway. These data show that triamcinolone in opossum kidney cells does not alter PTH activation of the cAMP/protein kinase-A system, but eliminates the increase in diglyceride and the activation of protein kinase-C in response to PTH. These studies emphasize the major role of the protein kinase-A system in the regulation of phosphate transport by PTH.