Ionic currents and ion fluxes in Neurospora crassa hyphae

Voltage dependence of ionic currents and ion fluxes in a walled, turgor-regulating cell were measured in Neurospora crassa. The hyphal morphology of the model organism Neurospora simplifies cable analysis of ionic currents to determine current density for quantitative comparisons with ion fluxes. The ion fluxes were measured directly and non-invasively with self-referencing ion-selective microelectrodes. Four ions (H+, Ca2+, K+, and Cl-) were examined. H+ net uptake and Ca2+ net release were small (10.2 nmol m(-2) s(-1) and 1.1 nmol m(-2) s(-1), respectively) and voltage independent. K+ and Cl- fluxes were larger and voltage dependent. Maximal K+ net release (similar to 1440 nmol m(-2) s(-1)) was observed at positive voltages (+15 mV), while maximal Cl- net release (similar to 905 nmol m(-2) s(-1)) was observed at negative voltage (-210 mV). A possible function of the net outward K+ and Cl- fluxes is regulation of the plasma membrane potential. Total ion fluxes were 37-58% of the total ionic current density (about +/- 244 mA m(-2), equivalent to +/- 2500 nmol m(-2) s(-1), at 0 mV and -200 mV) so other ions must contribute significantly to the ionic currents.