Cadmium transport and toxicity in isolated perfused segments of the renal proximal tubule.

We measured the lumen-to-bath transport and assessed the toxicity of inorganic cadmium (Cd2+) in isolated, perfused segments of the rabbit renal proximal tubule. To determine the dose range for acute toxicity the segments (S1, S2, and S3) were perfused with cadmium chloride (CdCl2) and the vital dye, FD & C green. We observed the tubular epithelial cells under the light microscope for signs of cellular injury and necrosis. Cellular swelling, blebbing of the luminal membrane, and cellular vacuolization were indicators of cellular injury, and the uptake of dye was indicative of cellular necrosis. Visible cellular damage occurs within 45 min after exposure of renal proximal tubular cells to cadmium concentrations greater than 500 microM. To determine rates of transport and cellular uptake of cadmium, the segments were perfused with a mixture of 109CdCl2 and the volume marker, L-[3H]glucose. We added nonradioactive CdCl2 to vary the total cadmium concentration from 1.5 to 2000 microM. After perfusion, we treated the tubules with 3% trichloroacetic acid or with a buffer solution of reduced osmolality in an attempt to determine the fate of the cadmium reabsorbed from the lumen. The tubular transport of cadmium was measured as the rate of disappearance of cadmium from the lumen (JD, pmol min-1 mm-1) and as the rate of appearance of cadmium in the bath (JA, pmol min-1 mm-1). In transport experiments, increasing the concentration of cadmium in the lumen caused an increase in the leak of the volume marker from the lumen into the bath. Cadmium disappeared from the lumen much more rapidly than it appeared in the bath for all three tubular segments. We conclude that (i) ionic cadmium, at concentrations greater than 500 microM, is acutely toxic to cells of isolated, perfused renal proximal tubules, and this toxicity is greater in the S1 than in the S2 or S3 segments; (ii) it is avidly taken up at the luminal membrane in all three segments; uptake is greater in the S1 than in the S2 or S3 segments; (iii) less than 10% of the cadmium that disappears from the lumen is transported across the basolateral membrane into the bath; and (iv) appearance flux into the bath does not show saturation in any of the segments over the concentration range studied; disappearance flux from the lumen shows saturation in the S2 and S3 segments, but not in the S1 segment.