Rapid actions of calcitriol and its side chain analogues CB1093 and GS1500 on intracellular calcium levels in skeletal muscle cells: a comparative study

The ability of synthetic analogues of the secosteroid hormone 1α,25‐dihydroxy‐vitamin‐D 3 [calcitriol, CT; 1,25(OH) 2 D 3 ] to exert non‐genomic (rapid) effects on target cells has been scarcely studied. To evaluate the pharmacological potential of the CT side‐chain analogues CB1093 and GS1500, we compared their fast effects on intracellular calcium concentration ([Ca 2+ ] i ) in chick skeletal muscle cells with those elicited by the natural hormone. Both analogues, similarly to CT, specifically induced rapid (30–60 s) and sustained rises in [Ca 2+ ] i levels. CB1093 and GS1500 were more potent than the natural hormone at concentrations as low as 10 −13 M (4.5 fold stimulation) and 10 −12 M (2.5 fold), respectively, whereas higher concentrations (10 −9 –10 −8   M ) of CT were more effective than the analogues in elevating [Ca 2+ ] i . Cyclic AMP was markedly increased by both analogues pointing for a role of this messenger in the fast actions of the synthetic compounds. In Ca 2+ free medium CT and analogues elicited a transient elevation in [Ca 2+ ] i . The PLC inhibitors U73122 (2 μ M ) and neomycin (0.5 m M ), as well as depletion of intracellular stores with thapsigargin (1 μ M ), completely prevented CB1093/GS1500‐dependent changes in [Ca 2+ ] i suggesting that, similarly to CT, these analogues mobilized Ca 2+ from an IP 3 /thapsigargin‐sensitive store. The voltage‐dependent calcium channel (VDCC) blocker nifedipine (2 μ M ) reduced by 50–60% the influx phase of the [Ca 2+ ] i response to CB1093 and GS1500, indicating that VDCC contributed partially to Ca 2+ entry. The Ca 2+ readdition protocol suggested that analogue‐dependent activation of a SOC entry pathway accounted, to the same extent as for CT, for the remaining non‐VDCC mediated Ca 2+ influx.British Journal of Pharmacology (1999) 126 , 1815–1823; doi: 10.1038/sj.bjp.0702451