Rapid Effects of 1,25-Dihydroxyvitamin D3and Extracellular Ca2+on Phospholipid Metabolism in Dispersed Porcine Parathyroid Cells

We examined the rapid effects (less than 120 sec) of 1,25(OH)2D3 on membrane phospholipid hydrolysis in porcine parathyroid cells and compared these effects to those produced by extracellular Ca2+. Cells were labeled with 3Hmyo-inositol or 14Carachidonic acid for 3 h, then exposed to various 1,25(OH)2D3 concentrations in 0.5 mM Ca2+ for different time periods or to 2 mM Ca2+. Parathyroid cells showed a biphasic increase in diacylglycerol (DAG), monoacylglycerol (MG), phosphatidic acid (PA), and inositol trisphosphate (IP3) in response to 1,25(OH)2D3 (10(-12)-10(-8) M) or to 2 mM Ca2+. This effect was rapid (within 5 sec) and dose-dependent, with a maximal stimulation with 10 pM of 1,25(OH)2D3. At this concentration, the first peak of DAG, MG, and IP3 was at 5 sec and reached 176 +/- 9%, 134 +/- 4%, and 154 +/- 13%, respectively vs. basal levels. For PA, the first maximum increase was at 20 sec (130 +/- 6%). At 30 sec MG, PA, and IP3 returned to basal levels, whereas the decrease in DAG was under the basal level (-25 +/- 5%). The second peak reached a maximum at 60 sec for the four products (145 +/- 8%, 119 +/- 5%, 125 +/- 6%, and 175 +/- 20%, respectively) then decreased to basal level at 120 sec. High extracellular Ca2+ (2 mM) and fluoride (5 mM) also produced similar increase in phosphatidylinositol metabolites, except that DAG levels returned to basal level at 30 sec. In conclusion, the present data shows the existence of rapid effects of 1,25(OH)2D3 in porcine parathyroid cells. The short time sequence suggests that they are mediated by a direct interaction with the membrane, possibly through a receptor-mediated process linked to phospholipase C by a G-protein.