ZINC TRANSPORT AND METALLOTHIONEIN INDUCTION IN PRIMARY CULTURES OF RABBIT KIDNEY PROXIMAL CELLS

Primary cultures of isolated rabbit renal proximal cells were grown on collagen-coated permeable supports. The confluent epithelia were pola rized, making possible the measurement of uptakes and effluxes across the apical and the basolateral membranes. Uptakes of Zn-65 were assess ed under initial rate conditions, after 0.5 min incubation. The kineti c parameters of apical uptake were a J(max) of 25.1 +/- 5.3 pmol min(- 1) (mu g DNA)(-1), a K-m of 43.3 +/- 7.3 mu M and an unsaturable const ant of 0.105 +/- 0.029 (n = 7) at 37 degrees C. Cadmium competitively inhibited the zinc uptake, with a K-i value of 24.5 +/- 7.3 mu M. Baso lateral uptake was characterized by a high capacity (J(max) = 227.9 +/ - 46.6 pmol min(-1) (mu g DNA)(-1)) and an affinity similar to that of the apical uptake (K-m = 35.4 +/- 14.2 mu M). Cadmium had no effect o n the basolateral zinc uptake. Effluxes across the basolateral face of the epithelium always exceeded those across the apical face. Excess z inc in the culture medium induced the synthesis of metallothionein in the epithelia, as judged by the rate of [S-35]cysteine incorporation i nto a fraction of cytosolic proteins. Metallothionein induction did no t appear to modify the kinetic parameters of the apical zinc uptake. T hese data suggest that separate saturable transport systems are respon sible for the apical and basolateral zinc uptakes in proximal renal ce lls. Induction of metallothionein had no apparent effect on apical zin c uptake in this system.