IP(3)-induced tension and IP(3)-receptor expression in rat soleus muscle during postnatal development

The present study was designed to examine whether changes in Ca(2+) release by inositol-1,4,5-trisphosphate (IP(3)) in 8-, 15-, and 30-day-old rat skeletal muscles could be associated with the expression of IP(3) receptors. Experiments were conducted in slow-twitch muscle in which both IP(3)-induced Ca(2+) release and IP(3)-receptor (IP(3)R) expression have been shown to be larger than in fast-twitch muscle. In saponin-skinned fibers, IP(3) induced transient contractile responses in which the amplitude was dependent on the Ca(2+)-loading period with the maximal IP(3) contracture being at 20 min of loading. The IP(3) tension decreased during postnatal development, was partially inhibited by ryanodine (100 microM), and was blocked by heparin (20-400 microg/ml). Amplification of the DNA sequence encoding for IP(3)R 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 IP(3) responses in immature fibers. IP(3)-induced Ca(2+) release would then have greater participation in excitation-contraction coupling in developing fibers than in mature muscle.