Energy cost of tonic contraction in a lamellibranch catch muscle

Abstract

1. The oxygen consumption of isolated anterior byssus retractor muscle (ABRM) of Mytilus edulis was measured during tonic contraction induced by acetylcholine (ACh).2. The respiration was measured with an oxygen electrode during 95 min, divided into one period of 5 min and six successive periods of 15 min.3. Tonic contraction induced a prolonged increase of the basal respiration that slowly diminished with a time course roughly similar to that of the tonic tension.4. For each period of measurement, the excess respiration over the resting level could be analysed into a constant amount and an amount that depended on the maintained tonic tension. The analysis was performed by fitting regression equations of the type Y = Q+bP, where Y is the excess respiration in n-moles O(2)/g.min, and P, the isometric tension (kg/cm(2)); term b of the equation expresses the amount of oxygen consumption directly proportional to the tonic tension.5. During the first 20 min of contraction, terms b of the equations are not significant, and most of the excess respiration (terms Q) is independent of the tension. The oxygen consumed during this time is supposed to reflect the recovery metabolism for the energy cost of the development of the tension.6. From the 20th to the 80th min of contraction, terms Q are reduced and terms b are significant and constant. The excess respiration during this period is equal to 16.9 (+/-0.5) n-moles O(2)/g.min + P x 6.8 (+/-0.5) n-moles O(2)/kg.cm.min (+/-S.E. of the means, n = 24).7. During a tonic contraction suppression of tension by a release reduced the oxygen consumption which increased again when tension was restored by stretching the muscle back to its original length. This confirmed the role of tension in determining the intensity of respiration during the catch.8. The oxygen consumption related to this tension restored by stretching the muscle, varied from 8.0 to 12.3 n-moles O(2)/kg.cm.min. These figures are of the same order of magnitude as the coefficient b obtained in the case of tonic contraction without modification of tension by length changes.9. These results are taken as a demonstration that the maintenance of tonic tension is an ;active' phenomenon with a metabolic counterpart.