CATION EFFLUXES ASSOCIATED WITH THE UPTAKE OF TPP- EVIDENCE FOR A PREDOMINANTLY ELECTRONEUTRAL INFLUX PROCESS(, TPA(+), AND TPMP(+) BY NEUROSPORA )

Previously observed anomalies in the transport of lipid-soluble cation s (LSI's) - presumed voltage-probe ions - by intact fungal cells [1] p rompted a systematic investigation of ion exchanges induced by high (m illimolar) concentrations of the particular species tetraphenylphospho nium ion (TPP+), tetraphenylarsonium ion (TPA(+)), and triphenylmethyl phosphonium ion (TPMP(+)). With low extracellular free Ca2+ (no calciu m added to the medium), influx of the LSI's was biphasic, indicating r apid entry into the cytoplasm followed by sequestration into a subcomp artment. The latter process, especially, was strongly inhibited by ext racellular Ca2+ (1 mM). Contrary to the expectation for electrophoreti cally driven entry of LSI's into fungal cells, no major efflux of prot ons (acidification of the medium) could be measured; in fact, signific ant alkalinization of the medium was observed. The major cellular inor ganic cations, K+ or Na+ (under different conditions), were released d uring LSI uptake, but with kinetic behavior which clearly ruled out di rect coupling to the uptake of TPP+, TPA(+), or TPMP(+). The major mec hanism for entry of these lipid-soluble cations into Neurospora appear s to be electroneutral diffusion in combination with one or more hydro philic anions. Subsequent penetration of the fungal vacuoles would res ult in binding of LSI's to storage polyanions (viz., polyphosphate) an d concomitant displacement of the normal vacuolar cations, such as bas ic amino acids and polyamines, thus leading to alkalinization of the e xtracellular medium. The observed effluxes of cytoplasmic K+ and Na+ s hould result independently from energetic changes (i.e., uncoupling of the mitochondria) and are most easily described by simple, but asynch ronous, changes in the average rate constants for entry and exit of th e alkali-metal cations.