The apparent discrepancy of ouabain inhibition of cation transport and of lymphocyte proliferation is explained by time‐dependency of ouabain binding

Mitogenesis of human blood lymphocytes in culture is inhibited by concentrations of ouabain that are approximately one order of magnitude lower than those that block Na and K transport. For example, the 50% inhibition (ID 50 ) of Na‐K transport, 280 nM, is seven‐fold greater than the ID 50 for RNA synthesis, DNA synthesis, or blastogenesis, ˜40 nM. Yet, inhibition of transport and consequent reduction in cell K is considered responsible for the effects of ouabain on mitogenesis. Since synthetic processes are assessed at least 24 hours after lymphocyte stimulation, this discrepancy could be explained by either 1) a progressive increase in K leak, or 2) a progressive inhibition of Na‐K transport by ouabain during 24 hours of PHA treatment. We found that the lymphocyte membrane leak rate of K increased immediately after PHA treatment but did not increase further from 4 to 24 hours. In contrast, the ouabain sensitivity of 42 K uptake was markedly increased with time: ID 50 for 42 K uptake of 35 nM at 24 hours as compared to 280 nM at 30 minutes. Measurement of ouabain binding revealed a seven‐fold increase in the lymphocyte‐associated ouabain after 24 hours compared to binding at 1 hour. These data indicate that the dose response of ouabain inhibition of active K transport and lymphocyte proliferation are closely correlated if one considers the slow membrane binding of ouabain at low concentrations.