Circulation of potassium across the plasma membrane of Blastocladiella emersonii: K+ channel.

A previous paper reported that the water mold Blastocladiella emersonii generates a transcellular electrical current, such that positive charges enter the rhizoid and leave from the thallus (Stump et al., Proc. Natl. Acad. Sci. U.S.A. 77: 6673-6677, 1980). To begin to understand the genesis of this current we investigated ionic relationships in this organism by use of intracellular microelectrodes. In cells suspended in buffered CaCl2, the membrane potential could be accounted for as a K+ diffusion potential; no evidence for an electrogenic pump was obtained. Potassium ions diffuse outward by a pathway that also carries Rb+ and Ba2+, but excludes both smaller and larger ions (Li+, Na+, Cs+, Mg2+, Ca2+, and choline). Chloride and other anions make little contribution to the potential, but the presence of Ca2+ in the external medium is required for successful potential measurements. In growing cells, the internal K+ concentration is generally somewhat higher than would be expected if the K+ distribution were determined entirely by the membrane potential. Under certain conditions, net uptake of K+ against the electrochemical potential gradient was observed. We suggest that K+ is actively accumulated by a primary transport system that may exchange K+ for H+, and that K+ leaks passively outward through the K+ channel. The K+ circulation across the membrane amounts to about 2% of the K+ pool per min, or 4.5 microA/cm2 of surface area. We propose that this K+ circulation is one arm of the transcellular current, carrying positive charge out of the thallus.