A basolateral K+ conductance modulated by carbachol dominates the resting membrane potential of small intestinal crypts. Single-channel recordings were obtained from the crypt basolateral membrane to identify the underlying ion channels. In cell-attached patches with 145mM KCl in the pipette an inwardly-rectifying K+ channel activity with a slope conductance of 32-34 pS shows time-dependent inactivation at depolarized potentials and is carbachol insensitive. Upon patch excision the inward rectifier shows no voltage-dependent inactivation and is Ca2+-dependent. A second single-channel activity is activated by carbachol at both spontaneous and hyperpolarized holding potentials. In excised patches this channel has a unitary conductance of 18-20 pS and is both Ca2+- and voltage-dependent. Ion substitution experiments reveal that this channel is cation non-selective with a PNa/Pk of 1.7. Pipette solutions containing 145mM NaGluconate were used to drive EK to more negative values and ENa and ECl to more positive values. In cell-attached patches both membrane depolarization and carbachol activate a K+ channel with a conductance of 2-4pS when fitted to the GHK equation. Although this K+ channel activity disappears upon patch excision, replacement of 'cytosolic' Na+ with K+ evokes a reversible increase in outward current across the membrane patch. This membrane-patch current exhibits time-dependent activation at depolarized potentials, is insensitive to changes in cytosolic Ca2+ and most strikingly its PK/PNa ratio decreases from 18 to 3 as K+ is increased from 10 to 145mM by equimolar substitution of K+ for Na+. The properties of this membrane patch current suggest that a high density of proteins of low unitary conductance are present in the crypt basolateral membrane.

Funded by the CFRT, U.K..