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.