Interactions of acid-base balance and osmoregulation across a salinity gradient in early life stage sheepshead minnow (<i>Cyprinodon variegatus</i>)

Estuaries are dynamic systems with daily and seasonal fluctuations in salinity, pH, CO 2 and oxygen that can impose significant physiological stress on resident fishes. High CO 2 causes a respiratory acid-base disturbance that must be corrected through proton excretion, which in larval fish occurs directly across the skin through specialized ionocytes. For fish in hyperosmotic environments, this is thought to occur primarily through sodium-proton exchangers (NHE) owing to a favorable sodium gradient imbued by high environmental sodium concentrations. However, this gradient becomes less favorable in freshwater where we hypothesize V-type H+ ATPase (VHA) contributes to acid-base balance primarily. Yet, NHE is also known to function in an osmoregulatory capacity in freshwater, which raises questions of how acid excretion is partitioned between NHE and VHA in estuarine species that can quickly transition between hypo- and hyperosmotic environments. Here, we acclimated embryos to 0, 25, and 60 ppt for 7 days (i.e., fertilization through hatch) to explore the relative importance of NHE and VHA using a combination of qPCR, immunofluorescence microscopy and scanning ion microelectrode measurements of H + flux. Larval survivorship was not affected by salinity treatments and little evidence of phenotypic plasticity was observed via qPCR; NHE3 was significantly upregulated at 0 ppt, while NHE2c was downregulated. VHA was ubiquitously localized to seawater ionocytes while restricted to an ionocyte subpopulation in freshwater. Freshwater larvae had higher net H + flux than seawater fish and showed a greater increase following hypercapnia exposure, indicating higher acid excretion capacity. Freshwater fish also showed significant inhibition of H + flux by NHE-specific inhibitors while seawater fish was not significantly affected; VHA specific inhibitors did not decrease net proton flux in either seawater or freshwater fish. This work was funded, in part, through the Matagorda Mitigation Trust. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.