Properties of Na-K pump in primary cultures of kidney cells

Abstract

Activities related to Na‐K transport were measured in cell cultures of ground squirrel kidney cortex in order to compare these cells with those of intact kidney and of continuous cell lines. A microsomal preparation containing plasma membrane Na,K‐ATPase from fresh kidney showed twice the activity of a similar preparation from 72‐hour cultured cells. Na,K‐ATPase of homogenates of 72‐hour cells showed one‐third to one‐fourth the specific activity of that from 6‐hour cultured cells. The associated K‐dependent phosphatase activity also declined as a function of time in culture. The ouabain‐sensitive influx of K into 6‐hour cultured cells was twice as great as the K influx into 72‐hour cells. The number of sites binding ³H‐ouabain in intact cultured cells declined 81% on a cell protein basis between 6 and 72 hours in culture. This decline in ouabain binding sites was relatively greater than that of K influx, so that the K turnover number increased over this same time period. The decline in ouabain‐sensitive K influx during culture was complementary to an increase in furosemide‐sensitive K influx. Measurements of unidirectional and net K fluxes showed that there were three components of K influx into 3‐day cultured cells: ouabain‐sensitive Na:K exchange, furosemide‐sensitive K:K exchange, and K diffusion. In the 6‐hour cultures, however, there was no furosemide‐sensitive K:K exchange. Thus, after three days in culture ground squirrel kidney cells lose a feature characteristic of the original parent cells (high Na,K‐ATPase activity), and gain a feature common to many undifferentiated cultured cells (furosemidesensitive K:K exchange).