Adaptational Responses of Rainbow Trout to Lowered External Nacl Concentration: Contribution of the Branchial Chloride Cell

ABSTRACT Whole-body ionic fluxes and gill chloride cell (CC) morphology were monitored in rainbow trout (Salmo gairdneri) exposed acutely or chronically to natural fresh water (NFW; [Na+]=0·120 mmol l−1; [Cl−]=0·164 mmol l−1) or artificially prepared fresh water with reduced [NaCl] (AFW; [Na+]=0·017 mmoll−1; [Cl−]=0·014mmoll−1). Net fluxes of Na+ and Cl− became extremely negative (indicating net NaCl loss to the environment) upon immediate exposure to AFW exclusively as a result of reduced NaCl influx ( and ) and were gradually restored to control rates during prolonged (30 days) exposure to AFW. The restoration of in AFW was due both to increased and to reduced Cl− efflux , whereas the primary response contributing to the restoration of was an increase of . The total apical surface area of branchial CCs exposed to the external environment increased markedly after 24 h in AFW and remained elevated for 1 month as a consequence of enlargement of individual CCs and, to a lesser extent, increased CC density. and were correlated significantly with total CC apical surface area. Plasma cortisol levels rose transiently in fish exposed to AFW. Treatment of NFW-adapted fish with cortisol for 10 days (a protocol known to cause CC proliferation) caused pronounced increases in and as measured in both NFW and AFW. These results suggest that an important adaptational response of rainbow trout to low environmental [NaCl] is cortisol-mediated enlargement of branchial epithelial CCs which, in turn, enhances the NaCl-transporting capacity of the gill as a result of the proliferation of Na+ and Cl− transport sites.