Effects of ouabain and low temperature on the sodium efflux pump in excised corn roots

Abstract

Ouabain (0.05 millimolar) and low temperature (4 C) both caused the tissue Na(+) content of excised 5-day-old corn roots to increase, indicating that there is an inhibition of the Na(+) efflux pump. Na(+) efflux was measured utilizing three different methods. Each method gave similar results in terms of rate and ouabain sensitivity. With one of these methods, the compartmental efflux method, it was demonstrated that rates for Na(+) efflux increase as the external Na(+) concentration is increased; e.g. the efflux rates are 0.529, 1.78, and 3.64 microequivalents per gram fresh weight per hour for external NaCl concentrations of 1, 10, and 30 millimolar, respectively. The data indicate that the Na(+) efflux pump is located in the plasmalemma of root cells.Na(+) efflux was stimulated for 30 to 60 minutes after the introduction of ouabain. This was followed in 60 to 90 minutes by an inhibition of Na(+) efflux. The Na(+) efflux rate returned to the original level on the removal of ouabain.The transport of Na(+) to the xylem vessels was stimulated by ouabain which most likely is a consequence of the ouabain-induced increase in cytoplasmic Na(+) content.Ouabain (0.05 millimolar) had little or no effect on K(+) and Cl(-) contents, and this implies the lack of an effect of ouabain on K(+) and Cl(-) fluxes. Ouabain at a concentration of 0.01 millimolar had no effect on Na(+) flux or on tissue ion content. With 0.5 millimolar ouabain the tissue contents of K(+), Na(+), and Cl(-) were greatly reduced.Evidence is presented indicating that ouabain has no effect on Na(+) efflux in pea roots.