clc-2c is regulated by salinity, prolactin and extracellular osmolality in tilapia gill

Teleosts inhabiting fresh water (FW) depend upon ion-absorptive ionocytes to counteract diffusive ion losses to the external environment. A Clc Cl−channel family member, Clc-2c, was identified as a conduit for basolateral Cl−transport by Na+/Cl−cotransporter 2 (Ncc2)-expressing ionocytes in stenohaline zebrafish (Danio rerio). It is unresolved whether Clc-2c/clc-2cis expressed in euryhaline species and how extrinsic and/or intrinsic factors modulate branchialclc-2cmRNA. Here, we investigated whether environmental salinity, prolactin (Prl) and osmotic conditions modulateclc-2cexpression in euryhaline Mozambique tilapia (Oreochromis mossambicus). Branchialclc-2candncc2mRNAs were enhanced in tilapia transferred from seawater (SW) to FW, whereas both mRNAs were attenuated upon transfer from FW to SW. Next, we injected hypophysectomized tilapia with ovine prolactin (oPrl) and observed a marked increase inclc-2cfrom saline-injected controls. To determine whether Prl regulatesclc-2cin a gill-autonomous fashion, we incubated gill filaments in the presence of homologous tilapia Prls (tPrl177and tPrl188). By 24 h, tPrl188stimulatedclc-2cexpression ~5-fold from controls. Finally, filaments incubated in media ranging from 280 to 450 mosmol/kg for 3 and 6 h revealed that extracellular osmolality exerts a local effect onclc-2cexpression;clc-2cwas diminished by hyperosmotic conditions (450 mosmol/kg) compared with isosmotic controls (330 mosmol/kg). Our collective results suggest that hormonal and osmotic control of branchialclc-2ccontributes to the FW adaptability of Mozambique tilapia. Moreover, we identify for the first time a regulatory link between Prl and a Clc Cl−channel in a vertebrate.

Switching of Na+, K+-ATPase isoforms by salinity and prolactin in the gill of a cichlid fish

We identified and investigated the changes in expression of two gill Na+, K+-ATPase α-subunit isoforms (α-1a and α-1b) in relationship with salinity acclimation in a cichlid fish, Mozambique tilapia. Transfer of freshwater (FW)-acclimated fish to seawater (SW) resulted in a marked reduction in α-1a expression within 24 h and a significant increase in α-1b expression with maximum levels attained 7 days after the transfer. In contrast, transfer of SW-acclimated fish to FW induced a marked increase in α-1a expression within 2 days, while α-1b expression decreased significantly after 14 days. Hypophysectomy resulted in a virtual shutdown of α-1a mRNA expression in both FW- and SW-acclimated fish, whereas no significant effect was observed in α-1b expression. Replacement therapy by ovine prolactin (oPrl) fully restored α-1a expression in FW-acclimated fish, while cortisol had a modest, but significant, stimulatory effect on α-1a expression. In hypophysectomized fish in SW, replacement therapy with oPrl alone or in combination with cortisol resulted in a marked increase in α-1a mRNA to levels far exceeding those observed in sham-operated fish. Expression of α-1b mRNA was unaffected by hormone treatment either in FW-acclimated fish or in SW-acclimated fish. The mRNA expression of fxyd-11, a regulatory Na+, K+-ATPase subunit, was transiently enhanced during both FW and SW acclimation. In hypophysectomized fish in FW, oPrl and cortisol stimulated fxyd-11 expression in a synergistic manner. The clear Prl dependence of gill α-1a expression may partially explain the importance of this hormone to hyperosmoregulation in this species.