Secretory Modulation of Basolateral Membrane Inwardly Rectified K+ Channel in Guinea Pig Distal Colonic Crypts

Cell-attached recordings revealed K ⁺ channel activity in basolateral membranes of guinea pig distal colonic crypts. Inwardly rectified currents were apparent with a pipette solution containing 140 mM K ⁺ . Single-channel conductance (γ) was 9 pS at the resting membrane potential. Another inward rectifier with γ of 19 pS was observed occasionally. At a holding potential of −80 mV, γ was 21 and 41 pS, respectively. Identity as K ⁺ channels was confirmed after patch excision by changing the bath ion composition. From reversal potentials, relative permeability of Na ⁺ over K ⁺ ( P _Na / P _K ) was 0.02 ± 0.02, with P _Rb / P _K = 1.1 and P _Cl / P _K < 0.03. Spontaneous open probability ( P _o ) of the 9-pS inward rectifier ( ^gp K _ir ) was voltage independent in cell-attached patches. Both a low ( P _o = 0.09 ± 0.01) and a moderate ( P _o = 0.41 ± 0.01) activity mode were observed. Excision moved ^gp K _ir to the medium activity mode; P _o of ^gp K _ir was independent of bath Ca ²⁺ activity and bath acidification. Addition of Cl ⁻ and K ⁺ secretagogues altered P _o of ^gp K _ir . Forskolin or carbachol (10 μM) activated the small-conductance ^gp K _ir in quiescent patches and increased P _o in low-activity patches. K ⁺ secretagogues, either epinephrine (5 μM) or prostaglandin E ₂ (100 nM), decreased P _o of ^gp K _ir in active patches. This ^gp K _ir may be involved in electrogenic secretion of Cl ⁻ and K ⁺ across the colonic epithelium, which requires a large basolateral membrane K ⁺ conductance during maximal Cl ⁻ secretion and, presumably, a lower K ⁺ conductance during primary electrogenic K ⁺ secretion.