Meeting Abstract

76.1  Friday, Jan. 7  Salinity dependent gill Na,K-ATPase isoform shifts in a cichlid fish and prolactin dependence of the freshwater enzyme TIPSMARK, CK*; BREVES, JP; HIRANO, T; GRAU, EG; University of Arkansas, Fayetteville; University of Hawaii, Manoa; University of Hawaii, Manoa; University of Hawaii, Manoa tipsmark@uark.edu

In euryhaline fishes, shifts between a freshwater (FW) and a seawater (SW) environment involves a complete switch in the compensatory gill epithelial transport from NaCl uptake to secretion. In salmonid fish, this transition involves a reciprocal shift in expression of two gill Na,K-ATPase isoforms, alpha-1a (in FW) and alpha-1b (in SW). The present study on Mozambique tilapia, investigated whether such an isoform shift may be involved in the euryhalinity of this cichlid species and how gill Na,K-ATPase is regulated by the osmoregulatory hormones cortisol, prolactin and growth hormone. We identified two expressed Na,K-ATPase isoforms in the tilapia gill and showed how switching from the alpha-1a to the alpha-1b isoform is induced by transfer from FW to SW. Hypophysectomized tilapia experienced a virtual shutdown of gill alpha-1a mRNA expression when compared to sham operated fish, demonstrating the dependence of a pituitary factor. Replacement therapy by prolactin injection salvaged this drop off in expression. Cortisol also had a stimulatory effect on gill alpha-1a expression but was not able to fully counteract the effect of hypophysectomy. Gill alpha-1b mRNA levels were unaffected by hypophysectomy and hormone treatment. Our findings suggest that a alpha-1a and alpha-1b shift may indeed be a general pre-requisite for teleost euryhalinity and possibly a thermodynamical necessity for coping with the diverse conditions of FW and SW media. The prolactin dependence of gill alpha-1a expression may partially explain the hormones significance to hyperosmoregulation in this species.