Prolactin and Extracellular Osmolality Regulate Branchial clc2c Expression in Tilapia

Abstract

Varying and contending models have been proposed to explain how teleost ionocytes facilitate ion uptake from freshwater environments. A recently characterized member of the clc Cl− channel family (clc2c) has been proposed as a conduit for basolateral Cl− transport by Na+/Cl− cotransporter (ncc)‐expressing ionocytes of stenohaline zebrafish. It is unknown to what extent clc2c is expressed in gill of euryhaline species, such as Mozambique tilapia, and whether expression is modulated in response to extrinsic (salinity) and intrinsic (hormones) factors. Here, we investigated prolactin (Prl) and local osmotic control of clc2c mRNA levels. First, branchial clc2c expression was markedly enhanced in freshwater‐acclimated tilapia compared with seawater‐acclimated tilapia. To investigate endocrine control of this salinity‐dependent expression, we injected hypophysectomized tilapia with ovine Prl. Ovine Prl induced a >8000‐fold increase in clc2c expression from saline‐injected controls. To then test whether Prl regulates clc2c expression in a gill‐autonomous fashion, we incubated gill filaments for 24 hours in the presence of homologous Prls, Prl177 and Prl188. By 24 hours, Prl188 stimulated clc2c expression ~5‐fold from controls. We then incubated filaments in media ranging from 280 to 450 mOsm/kg for 3 hours to investigate the possibility that extracellular osmolality exerts a local effect on clc2c expression. clc2c showed higher expression with decreasing media osmolalities. Collectively, our results suggest that both hormonal and extracellular osmotic conditions direct clc2c expression in branchial ionocytes. We propose that multifactorial modulation of clc2c expression contributes to the excellent adaptability of Mozambique tilapia to variations in environmental salinity.Support or Funding InformationSupported by Start‐Up Funds (Skidmore College) to J.P.B.