Aging and calcitriol regulation of IP3 production in rat skeletal muscle and intestine.

We previously reported that calcitriol [1,25(OH)2-vitamin D3] in rat skeletal muscle and duodenum stimulates the hydrolysis of polyphosphoinositides by phospholipase C (PLC), generating the second messengers inositol trisphosphate (IP3) and diacylglycerol (DAG), and that this mechanism is altered in old animals. As previously reported in muscle, we show in the present study that GTPgammaS (100 microM, 15 s), the non-hydrolyzable analogue of GTP, increased IP3 release from young rats duodenum to the same extent as 1 nM calcitriol (+ 100%), while GDPbetaS (100 microM) suppressed hormone-dependent IP3 production. Similarly to calcitriol, GTPgammaS response was diminished in old rats. Contrary to muscle, pretreatment with Bordetella pertussis toxin did not modify calcitriol-dependent IP3 in duodenum. The antibody, anti-G alpha q/11 (1:200) and anti-G alpha i (1:200) blocked calcitriol-dependent IP3 release in muscle from young rats, indicating that the hormone activates an isoform of PLC coupled to the alpha subunit of Gq/11 and possibly the betagamma subunits of Gi. The aged muscle was insensitive to anti G alpha i. In rat duodenum the hormone effects were suppressed by anti-Gq/11 both in young and aged animals. In 24-month-old rats, Gq/11 and Gi protein levels were greatly reduced both in muscle and duodenum, suggesting that a deficiency in G protein expression with aging may have important consequences for correct receptor/effector coupling and could explain age-related declines in the function of second messenger systems linked to G-proteins.