ENERGETICS OF CONTRACTILE ACTIVITY IN ISOLATED RADULA PROTRACTOR MUSCLES OF THE WHELK BUSYCON CONTRARIUM: ANAEROBIC END PRODUCT ACCUMULATION AND RELEASE.

Abstract

Anaerobic energy metabolism during contractile activity was investigated in the isolated radula protractor muscle of the whelk Busycon contrarium. Spectrophotometric assay of enzyme activities in crude tissue extracts revealed particularly high pyruvate reductase activities with octopine dehydrogenase displaying the highest activity. During electrically induced isotonic contractions of the radula protractor muscles, the following end products, listed in order of increasing level, accumulated in the tissue: strombine, octopine and alanopine (the "opines"), and D-lactate. Pyruvate levels increased three-fold during muscle contraction, suggesting that pyruvate plays a key role in the regulation of the pyruvate reductases. The muscle released lactate, but none of the opines, into the incubation medium, with rates exceeding 3 µmoles · min-1 · g wet wt-1 · During the later phases of contraction, more D-lactate was released into the medium than accumulated in the muscle. We conclude that transport of D-lactate permits sustained flux through lactate dehydrogenase because of a reduction in product inhibition. Furthermore, we hypothesize that D-lactate transport may be coupled to H+ export or OH- import, which would then serve to regulate the extent of accumulation of glycolytically produced protons.