MECHANISMS OF ZINC UPTAKE IN GILLS OF FRESH-WATER RAINBOW-TROUT - INTERPLAY WITH CALCIUM-TRANSPORT

The uptake mechanism of Zn2+ through the gill epithelium of freshwater rainbow trout was investigated both in intact animals and in isolated basolateral membranes. Involvement of the apical Ca2+ uptake sites in Zn2+ uptake was examined in vivo by pharmacological manipulation of t he apical Ca2+ permeability. The apical entries of Ca2+ and Zn2+, but not Na+ and Cl-, were inhibited by addition of La to the water. Additi on of 1.0 mu M La reduced the influxes of Ca2+ and Zn2+ to 22 +/- 3 an d 53 +/- 7% (means +/- SE) of the control value, respectively. Injecti on of CaCl2 also reduced the branchial influxes of Ca2+ and Zn2+ This treatment decreased the influx of Ca2+ to 45 +/- 4% of the control lev el and the Zn2+ influx to 68 +/- 5%. These results strongly imply that Zn2+ passes across the apical membrane of the chloride cells of the i lls via the same pathway as Ca2+. The presence of an active basolatera l transporter for Zn2+ was investigated in vitro on isolated basolater al membranes. There was no ATP-depen dent or Na+-gradient driven trans port of Zn2+ at physiological Zn2+ activities. The same system was use d to study potential effects of Zn2+ on the basolateral Ca2+-adenosine triphosphatase. Zn2+ was found to be a potent blocker of this transpor ter, causing a mixed inhibitory effect on the ATP-driven Ca2+ transpor t at a free Zn2+ activity of 100 pM.