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

Previous studies suggest that the activity-induced increase in H-1-NMR tran sverse relaxation time (T-2) observed in mammalian skeletal muscles is rela ted to an osmotic effect of intracellular metabolite accumulation. This hyp othesis was tested by comparing T-2 (measured by H-1-NMR imaging at 4.7 T) and metabolite changes (measured by P-31-NMR spectroscopy) after stimulatio n in the muscles of a freshwater (crayfish, Orconectes virilis) vs two osmo conforming marine invertebrates (lobster, Homarus americanus; scallop, Argo pecten concentricus). Intracellular pH significantly decreased after stimul ation in the lobster tail muscle, but not in the crayfish tail or scallop p hasic adductor muscles. The decrease in phosphoarginine-to-ATP ratio after stimulation was similar in the three muscles. Muscle T-2 increased from 37 to 43 ms (p < 0.02, n = 7) after stimulation in crayfish, but was unchanged in lobster muscle (32 ms, n = 7), and significantly decreased (from 40 to 36 ms, p < 0.02, n = 11) in scallop muscle. The observation that T-2 does n ot increase after stimulation in muscles of marine invertebrates with high natural osmolarity is consistent with the hypothesis that the T-2 increase in mammalian muscle is related to osmotically driven shifts of fluid betwee n subcellular compartments. Copyright (C) 2001 John Wiley & Sons, Ltd.