Comparative characterization of Na + transport in Cyprinodon variegatus variegatus and Cyprinodon variegatus hubbsi: A model species complex for studying teleost invasion of freshwater

Kevin V. Brix, Martin Grosell

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


The euryhaline fish Cyprinodon variegatus variegatus is capable of tolerating ambient salinities ranging from 0.3 to 160PSU, but is incapable of long-term survival in freshwater (<2mmoll -1 Na +). A population isolated in several freshwater (0.4-1mmoll -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 (Km7000-38,000μmoll -1) when acclimated to 2 or 7mmoll -1 Na +, but C. v. hubbsi switches to a high affinity system (Km100-140μmoll -1) in low-Na + freshwater (≤1mmoll -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 0.1mmoll -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 0.1mmoll -1Na + 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 7mmoll -1Na + 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 mitochondriarich 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.

Original languageEnglish (US)
Pages (from-to)1199-1209
Number of pages11
JournalJournal of Experimental Biology
Issue number7
StatePublished - Apr 2012


  • Mitochrondria-rich cells
  • Na uptake
  • NHE
  • NKCC
  • Osmoregulation

ASJC Scopus subject areas

  • Animal Science and Zoology
  • Ecology, Evolution, Behavior and Systematics
  • Molecular Biology
  • Physiology
  • Insect Science
  • Aquatic Science


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