Changes in sarcoplasmic metabolite concentrations and pH associated with the catch contraction and relaxation of the anterior byssus retractor muscle ofMytilus edulis measured by phosphorus-31 nuclear magnetic resonance

The sarcoplasmic concentrations of phosphorus metabolites and pH (pHin) were measured in the anterior byssus retractor muscle (ABRM) ofMytilus edulis by31P nuclear magnetic resonance spectroscopy. During an active contraction induced by 10−3 m acetylcholine, the concentration of arginine phosphate ([Arg-P]in) decreased from the resting value of 7.47±0.26 (mean±se,n=8) to 6.67±0.29 (n=6) μmol g−1, and that of inorganic phosphate (Pi) consistently increased from 0.84±0.06 (n=7) to 1.61±0.12 (n=5) μmol g−1. In the ‘catch’ state following the active contraction, these concentrations were close to their resting levels, indicating that the catch is an inactive state. 5-hydroxytryptamine caused a rapid relaxation of the catch, which was associated with a slight decrease in [Arg-P]in and an increase in pHin by ca 0.2 units. The sarcoplasmic concentration of ATP (mean, 1.6μmol g−1) did not change throughout the contraction-relaxation cycle.