The rat small intestine has the capacity to transfer a number of substances, including hexoses, inorganic salts and fluid. Riklis & Quastel (1958) showed that movement of hexose was dependent on the presence of sodium, and Barry, showed that a part of the fluid transfer was dependent on the presence of glucose, and part was independent of it. It is generally assumed that fluid movement in living tissues is related to movement of inorganic salts, and there must therefore be a complex inter-relation between transfer of sodium, glucose and fluid. Since electrical changes in other tissues are related to ion pumps, an investigation was undertaken into the relation between the electrical potential across the wall of the intestine and the transfer of fluid, various ions, hexoses and their derivatives. The present paper deals with transfer of fluid and sugars in relation to electrical changes, and preliminary communications of the results have been given by Barry, Dikstein, Matthews & Smyth (1961), and Barry, Matthews, Smyth & Wright (1962). METHODSWhite male rats of the Sheffield strain were used. Before experiments the rats were maintained on an unrestricted diet of rat cubes No. 86 (Oxoid, London). The work included three different kinds of experiments: (1) the effect of hexoses on the electrical potential difference across the gut wall, and this was studied both in vitro and in vivo; (2) determination of the rate of fluid transfer maintained by different sugars or their derivatives; and (3) the extent of metabolism of these sugars by the intestine: these last two were studied by in vitro methods. In the in vitro experiments the intestine was suspended in bicarbonate saline (Krebs & Henseleit, 1932), which was in equilibrium with a gas mixture of 5 % CO2 and 95 % 02-When anoxic conditions were required a gas mixture of 5 % CO2 and 95 % N2 was used.Measurement of electrical potentials across intestine Electrical recording. The potentials were led off from the solutions on each side of the gut wall to calomel half-cells by means of polythene tubes filled with M-KCI-3 %-agar, and these tubes are referred to subsequently as salt bridges. The pairs of calomel half-cells were selected to balance each other to within 1 mV. The half-cells were connected with a Hewlett-Packard vacuum tube voltmeter, with a minimum input impedance of 10 MO.
1. Stripped sacs of rat jejunum in which the outer muscle layers had been removed were found to maintain substantial transport and electrical activities. 2. Mucosal and serosal membrane potentials of epithelial cells of normal and stripped everted sacs of rat jejunum were recorded in vitro together with the transmural potential difference. 3. The cell interior was negative relative to both serosal and mucosal fluids, the transmural potential being the sum of the two membrane potentials. 4. Changes in the transmural potentials in the presence of actively transferred hexoses and amino acids were entirely due to variations in the serosal potential, the mucosal potential being unchanged. 5. Serosal and transmural potential increases on the addition of galactose were consistent with Michaelis—Menten kinetics, giving apparent Km values of 14·9 and 14·1 m M respectively. 6. Phlorrhizin, ouabain, 2,4‐dinitrophenol and sodium fluoroacetate inhibited serosal potential changes in the presence of galactose. 7. Osmotic potentials resulting from transmural osmotic gradients originated from the serosal layers of the tissue. 8. The results are consistent with the concept of a serosally located, electrogenic sodium pump which is stimulated by actively transferred hexoses and amino acids. The sodium‐dependent entry mechanism at the mucosal membrane is non‐electrogenic.
SUMMARY1. The nature of Na transfer across rat jejunum was studied by measuring the Na and fluid transfers and the electrical potential in the presence of galactose, mannose and glucose.2. Galactose, actively transferred but not metabolized, caused an increase in the potential but no change in Na transfer or tissue resistance.3. Mannose, not actively transferred but metabolized, caused a large increase in Na and fluid transfer without affecting the potential or tissue resistance. Similar results were obtained with glucose in the presence of phlorrhizin.4. Galactose in the presence of mannose increased fluid and Na transfer to the level found with glucose, which is both actively transferred and metabolized.5. These results are consistent with the view that there are two Na pumps associated with hexoses in rat jejunum, both moving Na towards the serosal side. One is an electrogenic pump, stimulated by hexose transfer, and which results in a rise in potential, and the other is a neutral pump which is stimulated by hexose metabolism and which does not affect the potential.
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