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 µM), and was blocked by heparin (20-400 µg/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.