POTASSIUM FLUXES IN CHLAMYDOMONAS-REINHARDTII .1. KINETICS AND ELECTRICAL POTENTIALS

Potassium influx and cellular [Kt] were measured in the unicellular gr een alga Chlamydomonas reinhardtii after pretreatment in either 10 or 0 mM external K+ ([K](0)). K+ (K-42(+) or Rb-86(+)) influx was mediate d by a saturable, high-affinity transport system (HATS) at low [K+](0) and a linear, low-affinity transport system at high [K+](0). The HATS was typically more sensitive to metabolic inhibition (and darkness) t han the low-affinity transport system. Membrane electrical potentials were determined by measuring the equilibrium distribution of tetraphen ylphosphonium. These values, together with estimates of cytoplasmic [K +] (B. Malhotra and A.D.M. Class [1995] Plant Physiol 108: 1537-1545), demonstrated that at 0.1 mM [K+](0) K+ uptake must be active. At high er [K+](0) (>0.3 mM) K+ influx appeared to be passive and possibly cha nnel mediated. When cells were deprived of K+ for 24 h, the V-max for the HATS increased from 50 x 10(-6) to 85 x 10(-6) nmol h(-1) cell(-1) and the K-m value decreased from 0.25 to 0.162 mM. Meanwhile, cellula r [K+] declined from 24 x 10(-6) to 9 x 10(-6) nmol cell(-1). During t his period influx increased exponentially, reaching its peak value aft er 18 h of K+ deprivation. This increase of K+ influx was not expresse d when cells were exposed to inhibitors of protein synthesis. The use of K-42(+) and Rb-86(+) in parallel experiments demonstrated that Chla mydomonas discriminated in favor of K+ over Rb+, and this effect incre ased with the duration of K+ deprivation.