Na+ ATPase activities in chela muscle of the euryhaline crab Neohelice granulata: Differential response to environmental salinity

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). I50 for ouabain-insensitive, furosemide-sensitive Na+ ATPase activity was about 1.4 mM. Both ouabain-insensitive, furosemide-insensitive and furosemide-sensitive Na+ ATPase activities were weakly affected by pH and showed Michaelis–Menten kinetics (Km = 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.