An examination of the ability ofinositol 1,4,5-trisphosphate to induce calcium release and tension development in skinned skeletal muscle fibres of frog and crustacea

Abstract We have examined the ability of inositol 1,4,5-trisphosphate (InsP 3 ) to cause contractions of mechanically skinned muscle fibres of frog and barnacle. InsP 3 (10–500 μM) did not cause any tension development in 25 frog skinned fibres and 26 barnacle myofibrillar bundles, although contractions could be readily evoked by caffeine and by replacement of an impenneant anion by Cl − , treatments known to release calcium from the sarcoplasmic reticulum (SR). Four barnacle bundles did give responses to InsP 3 . InsP 3 did not modify responses to caffeine or calcium-induced calcium release. Free Mg 2+ was lowered to 40 μM and 15 mM D-2,3-diphosphoglycerate was added in order to inhibit the possible breakdown of InsP 3 by inositol trisphosphatase. Neither measure revealed a response to InsP 3 . Arsenazo III absorbance measurements failed to detect any binding of Mg 2+ (0–0.5 mM) by 0.35 mM InsP 3 in our solutions. Inhibitors of SR calcium uptake (cadmium, quercetin, furosemide), omission of EGTA from the solution and varying the temperature from 4° to 22°C also failed to reveal a response of frog skinned fibres to InsP 3 . The nucleotide GTP, which has been reported to enhance InsP 3 -induced calcium release from rat liver microsomes, had no effect at 50 μM on the response of frog fibres to InsP 3 . It is concluded that under conditions in which other calcium release mechanisms operate well, InsP 3 is relatively ineffective at releasing calcium from the SR in amounts sufficient to induce contraction. Although we have been unable to find evidence to support the proposed role of InsP 3 as an essential link in excitation-contraction coupling of skeletal muscle, we cannot entirely reject its role if essential cofactors are lost in the skinned preparations.