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

Abstract

We have investigated the effects of aging on parathyroid hormone (PTH) modulation of intracellular calcium homeostasis and their relationship to signal transduction pathways in isolated rat duodenal cells (enterocytes). PTH (10(-8)-10(-9) M) increased enterocyte (45)Ca(2+) influx and intracellular Ca(2+) concentration ([Ca(2+)](i)) to a greater extent (twofold and 50%, respectively) in aged (24 months) than in young (3 months) animals. The [Ca(2+)](i) response of old cells to the hormone was slower, lacking the early phase of changes in cytosolic Ca(2+). Ca(2+) influx induced by PTH was prevented by the protein kinase A antagonist Rp-cAMPS in both young and aged enterocytes, whereas neomycin and compound U73122, inhibitors of PLC-catalyzed phosphoinositide hydrolysis, abolished hormone-dependent Ca(2+) influx in young but had no effect on aged cells. Higher basal adenylyl cyclase (AC) activity and cAMP content were detected in old enterocytes. PTH increased the absolute levels of cAMP in aged cells and AC activity of microsomes isolated therefrom to a greater extent (>/= twofold) than in young enterocytes/membranes. In young cells, the hormone also induced a rapid and transient release of inositoltrisphosphate (IP(3)) and diacylglycerol (neomycin-sensitive) at 45 sec, and a delayed phase of DAG at 5 min (neomycin-insensitive). The early formation of IP(3) and DAG was blunted in aged animals. These results suggest that both the PLC and adenylyl cyclase cascades are involved in PTH stimulation of Ca(2+) influx in duodenal cells. During aging, however, only the cAMP pathway is operative, mediating a potentiation of the effects of the hormone. Additional studies are required to establish the relative role of PTH-dependent messenger systems in the regulation of intestinal calcium absorption and age-related abnormalities.

Age-dependent alterations in Ca2+homeostasis: role of TRPV5 and TRPV6

“…Although serum PTH levels remained constant or increased, renal Ca 2ϩ reabsorption and intestinal Ca 2ϩ absorption declined with age. This would be compatible with a mechanism of desensitization as has been described in previous reports, which demonstrated the blunting of PTH-mediated signal transduction pathways as well as loss of renal and intestinal PTH receptors (15,16,18,30). Moreover, the elevated PTH levels in elder TRPV5 Ϫ/Ϫ mice have also been found in humans and may play an important role in age-related bone loss or senile osteoporosis (31,36,42).…”

Parathyroid hormone (PTH) rapidly enhances CFTR-mediated HCO3− secretion in intestinal epithelium-like Caco-2 monolayer: a novel ion regulatory action of PTH

“…Although PTH is known to exert its actions mainly through the cAMP-dependent PKA pathway, other signaling pathways, namely, Ca 2ϩ -dependent PKC and PI3K, have been reported to mediate PTH actions in the renal proximal tubular epithelium (1,16,38) and intestinal epithelium (23), respectively. In the present study, Caco-2 monolayers were exposed on both sides to inhibitors of cAMP/ PKA, Ca 2ϩ /PKC or PI3K pathways prior to PTH 1-34 stimulation.…”

Implication of Gβγ proteins and c-SRC tyrosine kinase in parathyroid hormone-induced signal transduction in rat enterocytes

“…In rat intestinal cells (enterocytes), PTH initiates its effects by interacting with the heterotrimeric G protein-coupled receptor, PTHR1 (Gentili et al 2003b). The transduction of PTH signal through the plasma membrane of rat enterocytes involves both a Gs-mediated stimulation of adenylyl cyclase with cAMP production and protein kinase (PK) A activation (Picotto et al 1997), and a Gq-mediated activation of phospholipase (PL) C , leading to generation of inositol 1,4,5 trisphosphate (IP 3 ) and diacylglycerol, followed by activation of PKC (Massheimer et al 2000). PTH also increases intracellular Ca 2+ levels in rat enterocytes by promoting an initial acute IP 3 -mediated mobilization of Ca 2+ from a thapsigargin-sensitive store, and a sustained phase due to Ca 2+ influx through voltage-dependent Ca 2+ -channels (Gentili et al 2003a).…”