inositol-1-4-5-trisphosphate and Contracture

Induction of cardiac contractures by 4-AP in Ca2+-free medium implied the involvement of SR and PLC-IP3 cascade. Thus, the role of PLC-IP3 cascade against contractile actions of 4-AP in electrically-driven rat atrial and diaphragmatic strips were studied both in the presence, and absence of Ca2+ using neomycin, a PLC inhibitor, and heparin, an IP3-R antagonist. 4-AP was applied cumulatively in logarithmically increasing concentrations in the range of 1-16µg/ml, and the preparations were treated with neomycin (400µM) or heparin (400µg/ml) for 3min prior to 4-AP injection. Post-rest potentiation in atrial strips was obtained by interruption of stimulation for 30min. 4-AP caused biphasic alteration in twitch amplitudes, as initially increased up to 16mM and then depressed due to contracture development, which were not affected significantly by neomycin and heparin. Both atrial and denervated diaphragmatic strips challenged to 4-AP in the presence and absence of Ca2+ developed dose dependent contractures which were significantly antagonized both by neomycin and heparin (p<0.05). Post-rest first contractions in controls were found to be reduced by 2min exposure to 4mM 4-AP and augmented by 3min exposure to heparin alone. 4-AP responses in the presence of neomycin and heparin were significantly higher than with those only treated with 4-AP alone and lesser than controls. Because of the fact that 4-AP inducing contracture in Ca2+-free medium, Ca2+ causing contracture should be of SR in origin. Depending on these results, it was concluded that activation of PLC-IP3 cascade by 4-AP is involved in the mediation of contracture and contractile actions of this molecule.

Topics: 4-Aminopyridine; Animals; Calcium; Contracture; Diaphragm; Electricity; Female; Heart Atria; Heparin; In Vitro Techniques; Inositol 1,4,5-Trisphosphate; Male; Neomycin; Rats, Wistar; Type C Phospholipases

Previous reports have shown that cooling striated muscles induces contractile responses that are related to Ca2+ release from the sarcoplasmic reticulum. However, the effect of cooling has generally been studied in the presence of pharmacological agents that potentiate rapid cooling-induced contractures. The present study shows that in saponin-skinned rat skeletal muscle preparations, a drop in temperature from 22 degrees C to 2 degrees C per se induces a contracture which relaxes on return to 22 degrees C. In fast-twitch fibres, rapid cooling-induced contractures are fully blocked by ryanodine, an inhibitor of ryanodine receptors. By contrast, in slow-twitch fibres, ryanodine partially inhibits the rapid cooling-induced contractile response, leaving a residual tension that dissipates after application of inositol 1,4,5-trisphosphate (InsP3). At low concentrations, heparin, an inhibitor of InsP3 receptors, decreases rapid cooling-induced contractures in both types of muscle. The present results suggest that in skeletal muscle, rapid cooling-induced contractures are due to both ryanodine-sensitive and InsP3-sensitive Ca2+ release from the sarcoplasmic reticulum.

Topics: Animals; Calcium; Calcium Channels; Cold Temperature; Contracture; Heparin; Inositol 1,4,5-Trisphosphate; Inositol 1,4,5-Trisphosphate Receptors; Male; Muscle Fibers, Fast-Twitch; Muscle Fibers, Skeletal; Muscle Fibers, Slow-Twitch; Muscle, Skeletal; Rats; Rats, Wistar; Receptors, Cytoplasmic and Nuclear; Ryanodine; Ryanodine Receptor Calcium Release Channel; Saponins; Sarcoplasmic Reticulum