Zinc Influx Across the Isolated, Perfused Head Preparation of the Rainbow Trout (Salmo gairdneri) in Hard and Soft Water

Abstract

At a waterborne [Zn] of 1.9 mg∙L⁻¹in hard water (~1 mmol Ca∙L⁻¹), Zn influx across an isolated, saline-perfused head preparation of rainbow trout (Salmo gairdneri) was about 1.5 nmol∙kg⁻¹∙h⁻¹through the lamellar pathway and about 1 nmol∙kg⁻¹∙h⁻¹through the filamental route. Flux rates came rapidly to steady state in both pathways. Trout preexposed to artificial soft water (~0.05 mmol Ca∙L⁻¹) for 5 d showed differential stimulation of flux rates to about 42 and 5 nmol Zn∙kg⁻¹∙h⁻¹through the lamellar and filamental pathways, respectively. Under these conditions, steady-state fluxes across the lamellae did not occur until 15–20 min after the start of perfusion. Preparations from hardwater-acclimated trout tested in soft water gave typical hardwater fluxes showing that these changes in influx were not simply due to acute exposure of the gill surface to low waterborne [Ca]. Influxes in softwater trout, studied over [Zn] from 0.4 to 7.5 mg Zn∙L⁻¹, revealed a saturable, first-order uptake with apparent J_maxand K_mof 150 nequiv∙kg⁻¹∙h⁻¹and 1.5 mg Zn∙L⁻¹(23 μmol∙L⁻¹), respectively. Because the apparent K_mis in the toxic range, Zn is clearly not the primary substrate. Scanning electron micrography revealed hypertrophy and increased apical exposure of chloride cells; this stimulation, coupled with the increase in Zn influx, suggests that chloride cells may be the site of entry of Zn across the gill.