title: Comparative characterization of Na+ transport in Cyprinodon variegatus variegatus and Cyprinodon variegatus hubbsi: a model species complex for studying teleost invasion of freshwater.
abstract: The euryhaline fish Cyprinodon variegatus variegatus is capable of tolerating ambient salinities ranging from 0.3 to 160 PSU, but is incapable of long-term survival in freshwater (<2 mmol l(-1) Na(+)). A population isolated in several freshwater (0.4-1 mmol l(-1) Na(+)) lakes in central Florida is now designated as a subspecies (Cyprinodon variegatus hubbsi). We conducted a comparative study of Na(+) transport kinetics in these two populations when acclimated to different ambient Na(+) concentrations. Results reveal that the two subspecies have qualitatively similar low affinity Na(+) uptake kinetics (K(m)=7000-38,000 μmol l(-1)) when acclimated to 2 or 7 mmol l(-1) Na(+), but C. v. hubbsi switches to a high affinity system (K(m)=100-140 μmol l(-1)) in low-Na(+) freshwater (≤1 mmol l(-1) Na(+)). Inhibitor experiments indicate that Na(+) uptake in both subspecies is EIPA-sensitive, but sensitivity decreases with increasing external Na(+). EIPA induced a 95% inhibition of Na(+) influx in C. v. hubbsi acclimated to 0.1 mmol l(-1) Na(+), suggesting that this subspecies is utilizing a Na(+)/H(+) exchanger to take up Na(+) in low-Na(+) environments despite theoretical thermodynamic constraints. Na(+) uptake in C. v. hubbsi acclimated to 0.1 mmol l(-1) Na(+) is phenamil-sensitive but not bafilomycin-sensitive, leading to uncertainty about whether this subspecies also utilizes Na(+) channels for Na(+) uptake. Experiments with both subspecies acclimated to 7 mmol l(-1) Na(+) also indicate that a Cl(-)-dependent Na(+) uptake pathway is present. This pathway is not metolazone-sensitive (NCC inhibitor) in either species but is bumetanide-sensitive in C. v. variegatus but not C. v. hubbsi. This suggests that an apical NKCC is increasingly involved with Na(+) uptake for this subspecies as external Na(+) increases. Finally, characterization of mitochondria-rich cell (MRC) size and density in fish acclimated to different ambient Na(+) concentrations revealed significant increases in the number and size of emergent MRCs with decreasing ambient Na(+). A linear relationship between the fractional area of emergent MRCs and Na(+) uptake rate was observed for both subspecies. However, C. v. variegatus have lower Na(+) uptake rates at a given MRC fractional area compared with C. v. hubbsi, indicating that the enhanced Na(+) uptake by C. v. hubbsi at low ambient Na(+) concentrations is not strictly a result of increased MRC fractional area, and other variables, such as differential expression of proteins involved in Na(+) uptake, must provide C. v. hubbsi with the ability to osmoregulate in dilute freshwater.
authors.affiliation: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA. kbrix@rsmas.miami.edu
authors.forename: Kevin V
authors.initials: KV
authors.lastname: Brix
authors.forename: Martin
authors.initials: M
authors.lastname: Grosell
mesh_terms: D000064:Acclimatization / Q000187:drug effects / Q000502:physiology; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D048429:Cell Size / Q000187:drug effects; D002712:Chlorides / Q000494:pharmacology; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D005618:Fresh Water; D005880:Gills / Q000033:anatomy & histology / Q000187:drug effects / Q000648:ultrastructure; D058865:Introduced Species; D017136:Ion Transport / Q000187:drug effects; D007695:Killifishes / Q000378:metabolism*; D007700:Kinetics; D018942:Macrolides / Q000494:pharmacology; D008788:Metolazone / Q000494:pharmacology; D008928:Mitochondria / Q000378:metabolism; D023421:Models, Animal; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D013045:Species Specificity; D013997:Time Factors
chemical_list: D002712:Chlorides; D018942:Macrolides; C057620:bafilomycin A; C047088:phenylamil; D000584:Amiloride; D012964:Sodium; D008788:Metolazone; C039614:ethylisopropylamiloride