Chloride conductance and Pi transport are separate functions induced by the expression of NaPi-1 in Xenopus oocytes.
Expression of the protein NaPi-1 in Xenopus oocytes has previously been shown to induce an outwardly rectifying Cl- conductance (GCl), organic anion transport and Na+-dependent Pi-uptake. In the present study we investigated the relation between the NaPi-1 induced GCl and Pi-induced currents and transport. NaPi-1 expression induced Pi-transport, which was not different at 1-20 ng/oocyte NaPi-1 cRNA injection and was already maximal at 1-2 days after cRNA injection. In contrast, GCl was augmented at increased amounts of cRNA injection (1-20 ng/oocyte) and over a five day expression period. Subsequently all experiments were performed on oocytes injected with 20 ng/oocytes cRNA. Pi-induced currents (Ip) could be observed in NaPi-1 expressing oocytes at high concentrations of Pi (>/= 1 mm Pi). The amplitudes of Ip correlated well with GCl. Ip was blocked by the Cl- channel blocker NPPB, partially Na+-dependent and completely abolished in Cl- free solution. In contrast, Pi-transport in NaPi-1 expressing oocytes was not NPPB sensitive, stronger depending on extracellular Na+ and weakly affected by Cl- substitution. Endogenous Pi-uptake in water-injected oocytes amounted in all experiments to 30-50% of the Na+-dependent Pi-transport observed in NaPi-1 expressing oocytes. The properties of the endogenous Pi-uptake system (Km for Pi > 1 mM; partial Na+- and Cl--dependence; lack of NPPB block) were similar to the NaPi-1 induced Pi-uptake, but no Ip could be recorded at Pi-concentrations </=3 mM. In summary, the present data suggest that Ip does not reflect charge transfer related to Pi-uptake, but a Pi-mediated modulation of GCl.
Bröer
S
S
Institute of Physiological Chemistry, Eberhard-Karls-Universität Tübingen, D-72076 Tübingen, Germany.
Schuster
A
A
Wagner
C A
CA
Bröer
A
A
Forster
I
I
Biber
J
J
Murer
H
H
Werner
A
A
Lang
F
F
Busch
A E
AE
D000818:Animals; D001692:Biological Transport / Q000235:genetics; D002352:Carrier Proteins / Q000096:biosynthesis* / Q000235:genetics / Q000502:physiology*; D018118:Chloride Channels / Q000378:metabolism*; D002712:Chlorides / Q000502:physiology; D005110:Extracellular Space / Q000737:chemistry; D015870:Gene Expression / Q000187:drug effects; D015640:Ion Channel Gating / Q000502:physiology*; D008845:Microinjections; D009579:Nitrobenzoates / Q000494:pharmacology; D009865:Oocytes / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D018075:RNA, Complementary / Q000494:pharmacology; D012964:Sodium / Q000502:physiology; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters; D014981:Xenopus
D002352:Carrier Proteins; D018118:Chloride Channels; D002712:Chlorides; D009579:Nitrobenzoates; D010710:Phosphates; D018075:RNA, Complementary; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters; C058176:5-nitro-2-(3-phenylpropylamino)benzoic acid; D012964:Sodium