Potassium fluxes in Neocosmospora vasinfecta

The unidirectional K+ fluxes across the mycelial surface of Neocosmospora vasinfecta were determined using 42K. Influx was mediated by at least two kinetically distinct systems, one having an apparent Km of 6-5 mu-equiv. K+/l and the other of about 1-0 m-equiv. K+/l. The VMAX for both systems was in the range 18 to 22 mu-equiv. K+/100 mg mycelial dry matter/h (1-0 to 1-2 m-equiv. K+/l cell-water/min). Influx was strongly inhibited by 2,4-dinitrophenol, sodium azide, sodium arsenate and anaerobiosis. K+ efflux was dependent on the external K+ concentration and ranged from 3 to 10% of mycelial K+/h. The maximum efflux rate was always considerably less than the initial influx rate for the K+ concentrations examined. During incubation in dilute KCl solutions, K+ influx decreased to a value approaching the K+ efflux rate. It is considered that equilibrium with external K+ is attained primarily by the regulation of K+ influx, and that this may be the principal mechanism controlling cytoplasmic K+ levels. Adsorption of K+ was also observed throughout the K+ concentration range examined and can be attributed to two distinct K+-binding entities at the mycelial surface, half-saturating at approximately O-I mM-and 4-4 mM-KCl respectively.