Title: Na+ ATPase activities in chela muscle of the euryhaline crab Neohelice granulata: Differential response to environmental salinity
Abstract: The occurrence and characteristics of ouabain-insensitive Na + ATPase activity and the response to environmental salinity of the coexistent Na + -K + ATPase and ouabain-insensitive Na + ATPase activities were studied in chela muscle of the euryhaline crab Neohelice (Chasmagnathus) granulata from Mar Chiquita coastal lagoon (Buenos Aires Province, Argentina).Chela muscle exhibited two ouabain-insensitive Na + ATPase activities (a furosemide-insensitive and a furosemide-sensitive activity).I 50 for ouabain-insensitive, furosemide-sensitive Na + ATPase activity was about 1.4 mM.Both ouabain-insensitive, furosemideinsensitive and furosemide-sensitive Na + ATPase activities were weakly affected by pH and showed Michaelis-Menten kinetics (K m = 0.021 and 0.224 mM, respectively).These characteristics appeared to be quite different from those previously described for Na + -K + ATPase activity in chela muscle of this crab.Na + -K + ATPase and ouabain-insensitive, furosemide-sensitive Na + ATPase activities appeared to be sensitive to environmental salinity.In crabs acclimated to low salinity (10‰), a salinity at which N. granulata exhibits a strong hyperregulatory capacity, Na + -K + ATPase activity was higher (117 ± 26 nmol Pi min -1 mg prot -1 ) than in 35‰ salinity (23 ± 6 nmol Pi min -1 mg prot -1 ) (a salinity at which this crab is osmoionoconforming).On the contrary, ouabain-insensitive, furosemide-sensitive Na + ATPase activity was higher in 35‰ salinity (108±15 nmol Pi min -1 mg prot -1 ) than in crabs acclimated to 10‰ salinity (36±11 nmol Pi min -1 mg prot -1 ).Ouabain-insensitive, furosemide-insensitive Na + ATPase activity was not affected by acclimation of crabs to low salinity.The response to low salinity suggests that Na + -K + ATPase could be a component of muscle regulatory mechanisms at the biochemical level secondary to hyperregulation whereas ouabain-insensitive, furosemide-sensitive activity appeared to be predominant upon osmoconforming conditions.The possible differential functional roles of Na + -K + ATPase and ouabain-insensitive Na + ATPase activities in muscle are discussed.