The cAMP-regulated and 293B-inhibited K⁺ conductance of rat colonic crypt base cells

Abstract

We have shown previously that secretagogues acting via the second messenger adenosine 3′,5′-cyclic monophosphate (cAMP) activate, besides their marked effect on the luminal Cl conductance, a K⁺ conductance in the basolateral membrane of colonic crypt cells. This conductance is blocked by the chromanol 293B. This K⁺ conductance is examined here in more detail in cell-attached (c.a.) and cell-excised (ce.) patchclamp studies. Addition of forskolin (5 μmol/1) to the bath led to the activation of very small-conductance (probably < 3 pS) K⁺ channels in c.a. patches (n = 54). These channels were reversibly inhibited by the addition of 0.1 mmol/1 of 293B to the bath (n = 21). Noise analysis revealed that these channels had fast kinetics and produced a Lorentzian noise component with a corner frequency (f_c) of 308 ±10 Hz (n = 30). The current/voltage curves of this noise indicated that the underlying ion channels were K⁺ selective. 293B reduced the power density of the noise (S₀) to 46 ± 8.7% of its control value and shifted f_c, from 291 ± 26 to 468 ± 54 Hz (n = 8). In c.e. patches from cells previously stimulated by forskolin, the same type of current persisted in 3 out of 18 experiments when the bath solution was a cytosolic-type solution without adenosine 5′-triphosphate (ATP) (CYT). In 15 experiments the addition of ATP (1 mmol/1) to CYT solution was necessary to induce or augment channel activity. In six experiments excision was performed into CYT + ATP solution and channel activity persisted. 293B exerted a reversible inhibitory effect. The channel activity was reduced by 5 mmol/1 Ba²⁺ and was completely absent when K⁺ in the bath was replaced by Na⁺. These data suggest that forskolin activates a K⁺ channel of very small conductance which can be inhibited directly and reversibly by 293B.