Phosphate uptake in Chara: membrane transport via Na/Pi cotransport

Phosphate uptake in the freshwater charophyte plant Chara corallina was fou nd to be strongly dependent on the presence of Na in the external medium. B ased on the reciprocal stimulations of (32)Pi uptake by Na and Na-22 uptake by Pi, the logical mechanism for Pi uptake appears to be a nNa/Pi symport with a half-maximal stimulation (K-m) for Na of approximately 300 mu M and a K-m for Pi of approximately 10 mu M. Comparison of the stimulations of (3 2)Pi and Na-22 influxes at pH 6 gives a stoichiometry of Na : Pi of 5.68. T he reduction in Pi influx with increasing pH is consistent with the transpo rted species being the monovalent H2PO4-. In voltage-clamp experiments, cur rents elicited by Pi in the presence of Na were equivalent to an influx of positive charge which exceeded the measured influxes of P-32 by a factor of 6.26. Intracellular perfusion was used to examine the dependence of Pi inf lux on ATP and Na. In perfused cells, Pi influx was low when ATP was absent from the internal medium or Na was absent from the external medium. Additi on of ATP alone had little effect whereas addition of Na alone increased th e (32)Pi influx slightly. Addition of both ATP and Na together restored Pi influx to rates comparable to those of intact cells. It is suggested that t he ATP is required for membrane hyperpolarization which in turn drives the highly electrogenic flux of Pi with up to 6 Na. However, consideration of t he electrochemical potential differences for Na and Pi at pH less than 6 sh ows that nNa/Pi would not be feasible. It is suggested that at low pH, H+ m ay substitute for Na.