Transport by the (Na+, K+) ATPase: Modulation by differentiation inducers and inhibition of protein synthesis in the MDCK kidney epithelial cell line

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Abstract
MDCK kidney epithelial cell cultures exposed to the differentiation inducer hexamethylene bisacetamid (HMBA) for 24 hours exhibited a 50% decrease in transport activity per (Na+, K+)‐ATPase molecule (turnover number) but an unchanged number of pump sites (Kennedy and Lever, 1984). Inhibition of protein synthesis by either 10 μM cycloheximide or 2 μM emetine blocked the inhibitory effects of HMBA on Na+/K+ pump efficiency assessed by measurements of [3H]‐ouabain binding to intact cells, (Na+, K+) ATPase activity of detergent‐activated cell extracts, and ouabain‐sensitive Rb+ uptake. In the absence of inducer treatment, inhibition of protein synthesis increased Na+/K+ pump turnover number by twofold while maintaining Na+/K+ pump activity per cell at a constant level. Intracellular Na+ levels were decreased after cycloheximide treatment; therefore, pump stimulation was not due to substrate effects. Furthermore, cycloheximide effects of Rb+ uptake could be dissociated from effects on tight junctions. These observations suggest that the transport activity of the (Na+, K+) ATPase is tightly regulated by factors dependent on protein synthesis.