Abstract
Studies were made of effects of changing ionic composition of the medium, on the uptake of phenol red by goldfish kidney tissues. Most striking immediate effects were observed when medium Na+ concentration was varied. At a fixed Na+ concentration, the uptake curve for phenol red was composed of two components; one linear with the dye concentration and the other obeying Michaelis-Menten kinetics. The latter was absolutely dependent on the presence of Na+ in the medium, and a graded reduction of Na+ concentration resulted in a graded increase in the apparent Km for phenol red without changing the Vmax. The apparent Km was a function of the reciprocal of the square of Na+ concentration, suggesting the presence of carrier activation by 2 Na+ Li+ had the similar but very small effect, but choline+ and K+ had no activating effect in the absence of Na+. In the presence of Na+, other monovalent cations showed an augmenting effect on the Na+-dependent uptake of the dye. The uptake in K+-free medium did not differ from that in the basic medium until 60 min, thereafter a strong depression was observed. Results of experiments on inhibition by PAH suggest that Na+-dependent carrier process for phenol red may exist at the peritubular border of the tubular cells.
