Measurement of sodium ion concentration in the unstirred layer of rat small intestine by polymer Na+-sensitive electrodes.

The concentration of sodium ion at the surface of rat small intestine both in vitro and in vivo was measured with plastic polymer sodium ion sensitive electrodes. In vitro the surface sodium ion concentration [Nas+] was found to be significantly higher than bulk bathing solutions of 25 mM concentration. This value could be increased by the addition of glucose to the medium and was significantly higher in the jejunum than in the ileum. Ouabain, deoxycholic acid and dithiothreitol all reduced the [Nas+] under in vitro conditions. In vivo, very high values for [Nas+] were found in the jejunum (approx. 80 mM) when the bulk concentration was 25 mM, indicating a substantial local accumulation of sodium ion at or near the brush border. This could be reduced by omission of glucose from the buffer and further reduced when magnesium was the substituent cation rather than choline. Consequently, in vivo, an appropriately orientated sodium ion gradient persists in the face of adverse tissue to bulk solution concentration gradients and potentially explains why solute absorption occurs under these circumstances. The further reduction of the correctly aligned sodium ion gradient, as required by the gradient hypothesis, by magnesium-substituted buffers and indicates that there is no real need to postulate additionally a sodium-independent magnesium-sensitive glucose transport system in vivo.