MODULATION OF 1,25-DIHYDROXYVITAMIN-D3-DEPENDENT CA2+ UPTAKE IN SKELETAL-MUSCLE BY PROTEIN-KINASE-C

In vitro studies have shown that short exposure (1-10 min) of vitamin D-deficient chick soleus muscle to 1,2,5-dihydroxyvitamin D3 [1,25(OH)2D3] causes an acute stimulation of tissue Ca-45 uptake through voltage-gated Ca2+ channels, with parallel increases in cyclic AMP levels, adenylate cyclase activity and membrane protein phosphorylation. We further investigated the involvement of protein kinases in the rapid effects of 1,25(OH)2D3 on skeletal muscle. The hormone was found to stimulate the protein kinase C(PKC) activity of muscle membranes. The PKC activator phorbol 12-myristate 13-acetate (PMA, 100 nM) was found to rapidly stimulate muscle Ca-45 uptake, mimicking 1,25(OH)2D3. Increases of 68% and 46% were observed at 1 and 15 min of exposure to PMA respectively. The effects of PMA were dose-dependent (50-200 nM) and were specific, since the inactive analogue 4-alpha-phorbol was without effect. Analogously to the effects of the sterol, PMA-enhanced Ca-45 uptake was abolished by the Ca(z)-channel antagonists nifedipine (30-mu-M) and verapamil (50-mu-M). Staurosporine (10 nM), a PKC inhibitor, surprisingly potentiated 1,25(OH)2D3-dependent stimulation of Ca-45 uptake. Exposure of skeletal muscle to PMA (100 nM) plus plus 1,25(OH)2D3 (1 nM) produced a less pronounced effect on Ca-45 uptake than either agent alone. PMA also decreased muscle cyclic AMP levels. These results suggest a regulatory link between the two major transmembrane signalling systems in the mechanism of action of 1,25(OH)2D3 in skeletal muscle.