Effect of aging on the mechanisms of PTH-induced calcium influx in rat intestinal cells

We have investigated the effects of aging on parathyroid hormone (PTH) modu lation of intracellular calcium homeostasis and their relationship to signa l transduction pathways in isolated rat duodenal cells (enterocytes). PTH ( 10(-8)-10(-9) M) increased enterocyte Ca-45(2+) influx and intracellular Ca 2+ concentration ([Ca2+](i)) to a greater extent (twofold and 50%, respecti vely) in aged (24 months) than in young (3 months) animals. The [Ca2+](i) r esponse of old cells to the hormone was slower, lacking the early phase of changes in cytosolic Ca2+. Ca2+ influx induced by PTH was prevented by the protein kinase A antagonist Rp-cAMPS in both young and aged enterocytes, wh ereas neomycin and compound U73122, inhibitors of PLC-calalyzed phosphoinos itide hydrolysis, abolished hormone-dependent Ca2+ influx in young but had no effect on aged cells. Higher basal adenylyl cyclase (AC) activity and cA MP content were detected in old enterocytes. PTH increased the absolute lev els of cAMP in aged cells and AC activity of microsomes isolated therefrom to a greater extent (greater than or equal to twofold) than in young entero cytes/membranes. In young cells, the hormone also induced a rapid and trans ient release of inositoltrisphosphate (IP3) and diacylglycerol (neomycin-se nsitive) at 45 sec, and a delayed phase of DAG at 5 min (neomycin-insensiti ve). The early formation of IP3 and DAG was blunted in aged animals. These results suggest that both the PLC and adenylyl cyclase cascades are involve d in PTH stimulation of Ca2+ influx in duodenal cells. During aging, howeve r, only the cAMP pathway is operative, mediating a potentiation of the effe cts of the hormone. Additional studies are required to establish the relati ve role of PTH-dependent messenger systems in the regulation of intestinal calcium absorption and age-related abnormalities. (C) 2000 Wiley-Liss, Inc.