The effect of pH on the rate of relaxation of isolated barnacle myofibrillar bundles

CO₂-induced acidosis in barnacle muscle fibres prolongs the relaxation phase of the electrically stimulated contraction (Ashley, C.C., Franciolini, F., Lea, T.J. and Lignon, J. (1979) J. Physiol. 296, 71P). In order to test if this effect is due to a direct action of H⁺ on the relaxation kinetics of the myofilaments, isolated myofibrillar bundles were contracted and relaxed in Ca²⁺ buffer solutions at pH 6.0 and 7.1, in the presence of 20 mM caffeine to inactivate the sarcoplasmic reticulum. At pH 7.1, the relaxation half-time was reduced from 1.5 to 0.3 s as the EGTA concentration in the relaxing solution was progressively increased from 0.3 to 50 mM. The resulting curve was shifted in the direction of increasing EGTA concentration by lowering the pH to 6.0. This effect could be explained by the reduction in affinity of Ca²⁺ for EGTA at pH 6.0, since relaxation half-times for a given relaxing pCa (calculated from the contaminating Ca²⁺ concentrations in the relaxing solutions) were shorter (by about 40%) at pH 6.0 compared with 7.1. However, similar experiments using the new Ca²⁺-chelating agent 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), which is much less pH sensitive than EGTA, indicated that there was no significant difference between relaxation half-times at pH 6.0 and 7.1 for a given relaxing pCa. It is concluded that because no prolongation of relaxation of the myofibrils was observed on lowering the pH from 7.1 to 6.0, the effect of CO₂ on the relaxation of intact muscle fibres is probably due to a modification of sarcoplasmic reticulum activity.