SUMMARY1. The whole-cell configuration of the patch-clamp technique has been used to investigate the conductance properties of villus enterocytes isolated from guinea-pig small intestinal epithelium.2. With near physiological ionic gradients inward and outward rectification was observed in the hyperpolarizing and depolarizing voltage domains respectively.3. Replacement of intra-and extracellular K+ with N-methyl-D-glucamine (NMG) eliminated inward rectification but did not alter outward currents. In symmetrical low Cl-solutions outward currents were reduced but inward rectification was not affected. Under these conditions increases in extracellular K+ shifted both the current-voltage relation and the extrapolated reversal potential as expected for a K+-selective current.4. The inwardly rectifying nature of the K+ current observed here remained unaltered after chelation of internal Mg2+ with ATP or EDTA.5. Extracellular application of 5 mm-Ba2+ or 50 ,g ml-' of the venom of the scorpion Leiurus quinquestriatus abolished the inward K+ current, while 5 mmextracellular tetraethylammonium (TEA) had little effect.6. The current remaining in the presence of symmetrical Cl-solutions and in the complete absence of K+ rectified outwardly and reversed at 0 mV. The anionic nature of this current was confirmed by replacing Cl-with different anions. SCN-and Brcarried more current than Cl-, while F-and gluconate were less permeant.7. Anionic currents of villus guinea-pig enterocytes were not stimulated by cyclic AMP and were strongly and reversibly inhibited by the Cl-channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 10-5 M).8. The inwardly rectifying K+ current described here shares some, but not all, characteristics with others previously described. It is postulated that this conductance might function to couple K+ permeability and the Na+-K+ pump rate in enterocytes. Absorption of chloride may be mediated by the Cl-channels.

The possible regulation of guinea pig enterocyte ion channels by GTP-binding proteins has been investigated by using the whole-cell recording mode of the patch-clamp technique and non-hydrolysable analogues of GTP. The main K+ currents in these cells are mediated by inwardly-rectifying K+ channels. Intracellular dialysis with hydrolysis-resistant GTP analogues leads to the deactivation of the inward K+ currents. Non-hydrolysable analogues of ATP or GDP were without effect. The effect occurs after a lag phase of 2 to 7 min, suggesting a multistep mechanism. Cl currents were not affected by any of the nucleotide analogues. It is suggested that inwardly-rectifying K+ currents are deactivated by a G-protein-dependent process.