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Abstract:
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We studied the effect of IGF-I and insulin on intracellular Ca2+ in primary cultured myotubes. IGF-I induced a fast and transient Ca2+ increase, measured as fluo-3 fluorescence. This response was blocked by both genistein and AG538. IGF-I induced a fast in ositol-1,4,5-trisphosphate (IP3) increase, kinetically similar to the Ca2+ rise. The Ca2+ signal was blocked by inhibitors of the IP3 pathway. On the other hand, insulin produced a fast (<1 s) and transient Ca2+ increase. Insulin-induced Ca2+ increase was blocked in Ca2+-free medium and by either nifedipine or ryanodine. In the normal muscle NLT cell line, the Ca2+ signals induced by both hormones resemble those of primary myotubes. GLT cells, lacking the α1-subunit of dihydropyridine receptor (DHPR), responded to IGF-I but not to insulin, while GLT cells transfected with the α1-subunit of DHPR reacted to both hormones. Moreover, dyspedic muscle cells, lacking ryanodine receptors, responded to IGF-I as NLT cells, however they show no insulin-induced calcium increase. Moreover, G-protein inhibitors, pertussis toxin (PTX) and GDPβS, blocked the insulin-induced Ca2+ increase without major modification of the response to IGF-I. The different intracellular Ca2+ patterns produced by IGF-I and insulin may improve our understanding of the early action mechanisms for these hormones in skeletal muscle cells. |