Contraction increases the T 2 of muscle in fresh water but not in marine invertebrates

Previous studies suggest that the activity‐induced increase in 1H‐NMR transverse relaxation time (T2) observed in mammalian skeletal muscles is related to an osmotic effect of intracellular metabolite accumulation. This hypothesis was tested by comparing T2 (measured by 1H‐NMR imaging at 4.7 T) and metabolite changes (measured by 31P‐NMR spectroscopy) after stimulation in the muscles of a freshwater (crayfish, Orconectes virilis) vs two osmoconforming marine invertebrates (lobster, Homarus americanus; scallop, Argopecten concentricus). Intracellular pH significantly decreased after stimulation in the lobster tail muscle, but not in the crayfish tail or scallop phasic adductor muscles. The decrease in phosphoarginine‐to‐ATP ratio after stimulation was similar in the three muscles. Muscle T2 increased from 37 to 43 ms (p