Effects of extracellular pH and D‐lactate efflux on regulation of intracellular pH during isotonic contractions in a molluscan muscle: A 31p‐nuclear magnetic resonance study

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Abstract
Isotonic contractions were electrically induced in isolated radula protractor muscles of the whelk Busycon canaliculatum, mounted and superfused under anoxic conditions in a special 5 mm phosphorus nuclear magnetic resonance (31P‐NMR) probe. 31P‐NMR spectra showed that there was a decrease in intracellular pH (pHi) from 7.29 to 7.01 during the time course of 30 min of contractile activity. Subsequent biochemical analysis of the tissue and superfusion medium showed that D‐lactate and, to a much lesser extent, alanopine and octopine were the major glycolytic end products. Approximately 75% of the total D‐lactate produced was released into the medium. When the superfusate pH was changed from 7.8 to 7.2, a much more pronounced intracellular acidification took place, as the pHi fell to 6.80. However, there was no significant effect on the extent of release of D‐lactate from the muscle. Elevation of the superfusate pH to 8.2 had no significant effect on final pHi or D‐lactate efflux. D‐lactate efflux was reduced when α‐cyanohydroxycinnamic acid or L‐lactate was added to the superfusate at pH 7.8. These treatments had no significant effect on the final pHi. Movement of proton equivalents and D‐lactate efflux are not tightly linked in this muscle system.