AGE-RELATED LOSS OF CALCITRIOL STIMULATION OF PHOSPHOINOSITIDE HYDROLYSIS IN RAT SKELETAL-MUSCLE

We have examined the effects in vitro of calcitriol [1,25(OH)(2)D-3], the hormonal form of vitamin D-3, on the breakdown of membrane phospho inositides in skeletal muscle from young (3 months) and aged (24 month s) rats. Calcitriol (10(-9) M) induced a rapid and transient release o f IP3/inositol phosphates and diacylglycerol (DAG) from muscle slices/ membranes prelabeled with [H-3]myo-inositol and [3H]arachidonate, resp ectively. Inositol phosphate release was maximal at 15 s and then decl ined. The effects of hormone specificity exhibited as the closely rela ted derivatives of vitamin D-3, 250HD(3), 1 alpha OHD3 and 24,25(OH)(2 )D-3 did not alter muscle inositol phosphate levels. The stimulation o f DAG was biphasic, the early phase (15 s) being abolished by neomycin (0.5 mM), an inhibitor of phosphoinositide hydrolysis, similar to IP3 formation and consistent with a role of phospholipase C (PLC) in intr acellular signal generation. Neomycin had no effect on the second DAG peak (2 min) induced by calcitriol, suggesting that the late phase of DAG formation is independent from the hydrolysis of phosphoinositides. Higher basal inositol phosphate and DAG levels were detected in muscl e from aged rats thereby reducing the effects of the hormone on second messenger generation (-80 and -60% for IP3 and DAG, respectively). Ca lcitriol stimulation of PLC was mimicked, in both young and old rats, by GTP gamma S, a non-hydrolyzable analogue of GTP, while GDP beta S, a G protein inhibitor, suppressed the effect of the hormone. The early effects of calcitriol and GTP gamma S were not additive. Bordetella p ertussis toxin abolished by 85% the effects of calcitriol on inositol phosphate release in young rats but was without effect in aged animals . These results demonstrate that calcitriol activates phosphoinositide -PLC in rat skeletal muscle by a mechanism which involves a pertussis- sensitive G protein and that the effects of the hormone are altered wi th ageing. (C) 1998 Elsevier Science Ireland Ltd.