Abstract:
Medium' alkaliniiation occurred -lipon the addition of L-Glu to mechanically
isolated Asparagus sprenger-i mesophyll cells suspended in 1 mM CaS04.
Alkalinization resulted from the coupled entry of H+ and L-Glu anion into the
cells. This H+ IL-Glu symport did not stimulate K+ efflux. K+ efflux has been
observed during H~ lamino acid symport in other systems. The stimulation of K+
efflux by proton coupled symport is regarded as an indicator of a plasma
membrane depolarizing electrogenic symport process. H+ IL-Glu symport in
Asparagus sprengerimesophyl1 cells was investigated to determine whether or
not the process was electrogenic.
The rate of uptake of 0.25 11M 3H-MTPP+ ( Methyltriphenylphosphonium,
methyl-3H ) is a probe for monitoring changes in the membrane potential. 3HMTPP+
uptake was reduced by K+ or CCCP, agents known to depolarize the
membrane potential. Uptake of 3H-MTPP+ was also inhibited by L-Glu but not
by D-Glu. Conversely, 10 mM external MTPP+ inhibited the uptake of 14C-U-LGlu.
Simultaneous measurements of the rates of 14C-U-L-Glu uptake and L-Glu
dependent H+ influx showed that the molar stoichiometry of H+ IL-Glu symport
was 2 to 1.
K+ or Na+ stimulated H+ efflux was completely inhibited by DCCD, DES,
oligomycin and antimycin reagents which inhibit ATP driven H+ efflux. The H+
efflux \Vas also stimulate.d by the weak acids, butyric acid and acetic acid, which
are known fo-aCidify the cytoplasm. This weak acid stimulated H+ efflux was
also completely inhibited by oligomycin. It was calculated that net L-Glu
dependent H+ influx increased by 100% in the presence of oligomycin and that
despite net medium alkalinization H+ IL-Glu symport stimulates ATP dependent
H+ efflux.
11
The data presented in this study indicate that H+ IL-Glu symport is
electrogenic. The data also show that ATP dependent Ht efflux rather than K+
efflux is the- process compensating for thi~ electrogenic H+ IL-Glu symport.
