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

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 mmoll⁻¹; [Cr]=0.164 mmoll⁻¹) or artificially prepared fresh water with reduced [NaCl] (AFW; [Na+]=0.017 mmoll⁻¹; [CT]=0.014 mmoll⁻¹).

Net fluxes of Na⁺ (J_net^Na) and Cl⁻ (J_net^Cl) became extremely negative (indicating net NaCl loss to the environment) upon immediate exposure to AFW exclusively as a result of reduced NaCl influx (J_in^Na and J_in^Na). J_net^Na and J_net^Cl were gradually restored to control rates during prolonged (30 days) exposure to AFW.

The restoration of J_net^Cl in AFW was due both to increased J_in^Cl and to reduced Cl⁻ efflux (J_out^Cl) whereas the primary response contributing to the restoration of J_net^Na a t was an increase of J^Na_in.

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. J_in^Na and J_in^Na 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 J_in^Cl and J_in^Na, 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.