Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic.
The cloned rabbit intestinal Na+/glucose cotransporter was expressed in Xenopus oocytes, and transmembrane currents associated with this transporter were monitored using a two-electrode voltage clamp. Addition of D-glucose to a Na(+)-containing solution bathing these oocytes generated a current which was blocked by phlorizin. Water-injected control oocytes did not exhibit any currents under these conditions. The magnitude and shape of the currents were dependent on the extracellular glucose and Na+ concentrations and the membrane potential. At Vhold = -50 mV, the Km values for glucose and Na+ were 14 +/- 2 (N = 4) microM and 17 +/- 1 (N = 3) mM, respectively. These Km values and imax exhibited voltage dependence: increasing the membrane potential from -30 to -150 mV increased KGlcm and imax threefold and decreased KNam eightfold. The reversal potential (VR) of the phlorizin-sensitive, glucose-dependent current varied with log Nao+ (slope 46 +/- 6 [N = 9] mV). In the absence of sugar, a Na(+)-dependent, phlorizin-sensitive (Ki = 3 +/- 0.5 microM) current was detected only in RNA-injected oocytes. The amplitude of this current at -50 mV was 6 +/- 1% (N = 13) of the maximum current measured in the presence of D-glucose. The VR of this sugar-independent current varied with log Nao+ (slope 63 +/- 1 [N = 4] mV), indicating that the cotransporter may carry Na+ in the absence of sugar. We conclude that the Na+/glucose cotransporter is electrogenic and that investigations of currents associated with its operation can yield valuable insights into the mechanisms of solute translocation.
Department of Pharmacology, Jerry Lewis Neuromuscular Research Center, Los Angeles, California.
J A
JA
Umbach
M J
MJ
Coady
E M
EM
Wright
D000818:Animals; D004553:Electric Conductivity; D005260:Female; D066298:In Vitro Techniques; D007422:Intestines / Q000502:physiology*; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D008433:Mathematics; D008564:Membrane Potentials / Q000187:drug effects; D008757:Methylglucosides / Q000494:pharmacology; D008845:Microinjections; D008962:Models, Theoretical; D009004:Monosaccharide Transport Proteins / Q000235:genetics / Q000502:physiology*; D009865:Oocytes / Q000502:physiology*; D012333:RNA, Messenger / Q000008:administration & dosage / Q000235:genetics; D011817:Rabbits; D012044:Regression Analysis; D012964:Sodium / Q000494:pharmacology; D014982:Xenopus laevis
D007473:Ion Channels; D008757:Methylglucosides; D009004:Monosaccharide Transport Proteins; D012333:RNA, Messenger; C027020:methylglucoside; D012964:Sodium