IP3-induced tension and IP3-receptor expression in rat soleus muscle during postnatal development

The present study was designed to examine whether changes in Ca2+ release by inositol-1,4,5-trisphosphate (IP3) in 8-, 15-, and 30-day-old rat skeletal muscles could be associated with the expression of IP3 receptors. Experiments were conducted in slow-twitch muscle in which both IP3-induced Ca2+ release and IP3-receptor (IP3R) expression have been shown to be larger than in fast-twitch muscle. In saponin-skinned fibers, IP3 induced transient contractile responses in which the amplitude was dependent on the Ca2+-loading period with the maximal IP3 contracture being at 20 min of loading. The IP3 tension decreased during postnatal development, was partially inhibited by ryanodine (100 muM), and was blocked by heparin (20-400 mug/ml). Amplification of the DNA sequence encoding for IP3R isoforms (using the RT-PCR technique) showed that in slow-twitch muscle, the type 2 isoform is mainly expressed, and its level decreases during postnatal development in parallel with changes in IP3 responses in immature fibers. IP3-induced Ca2+ release would then have greater participation in excitation-contraction coupling in developing fibers than in mature muscle.