A Chromanol Type of K⁺ Channel Blocker Inhibits Forskolin- but Not Carbachol-Mediated Cl- Secretion in Rat and Rabbit Colon

Abstract

In a previous study [Lohrmann et al: Pflügers Arch Eur J Physiol 1995;429:517-530] we have shown that chromanol K⁺ channel blockers inhibit Cl^– secretion in rabbit colon. Their effect was easily demonstrable after stimulation by hormones acting through increases of cytosolic cAMP. The present study was undertaken to test in more detail the mechanism of action of one of these compounds (293 B). Two types of studies were performed: Ussing chamber experiments in rabbit distal colon and whole cell patch clamp studies in the isolated in vitro perfused rat colonic crypts. Carbachol (CCH, 100 µmol/l) enhanced Cl^– secretion, quantified as equivalent short circuit current (I_sc), in rabbit colon significantly more than did prosta-glandin E₂ (PGE₂). In whole cell patch clamp studies in rat colonic crypt cells from the base, CCH hyperpolarized the membrane voltage (V_m) and enhanced whole cell conductance (G_m). In agreement with previous impalement studies, 10 µmol/l 293 B depolarized V_m in forskolin-treated rat colonic crypt base cells even further and reduced G_m. In Ussing chamber experiments in rabbit colon, 293 B abolished the I_scinduced by PGE2. CCH, in the continued presence of 293 B, still induced a large I_sc. These data indicate that 293 B specifically inhibits the forskolin- but not the CCH-induced Cl^– secretion, and supports our previous conclusion that this class of substances inhibits a cAMP-regulated K⁺ conductance.

ATP increases [Ca2+]i and ion secretion via a basolateral P2Y-receptor in rat distal colonic mucosa

“…From these parameters an increase in I sc (∆I sc ) of -36.4 ± 5.4 µA/cm 2 can be calculated. These results are qualitatively comparable with observations that carbachol acts via an increase of [Ca 2+ ] i [12]. This suggests that ATP induces secondarily active NaCl secretion in rat distal colonic mucosa.…”

Specific blockade of slowly activating IsK channels by chromanols — impact on the role of IsK channels in epithelia

“…293B (at 30 gmol/l; for chemical name see Table 1), the most potent cAMP-regulated K + channel inhibitor in rabbit and rat colon crypts [9,11,12], had no inhibitory effects on Kvl.1 and Kir2.1 ( Fig. 1; n=6 and 6 for Kvl.1 and Kir2.1, respectively).…”

“…Ecke et al [12] demonstrated recently in whole cell patch clamp experiments an inhibition of the basolateral K + conductance in rat colon crypts by novel chromanol derivatives, thereby completely inhibiting C1 secretion caused by several hormones. A number of observations of the present study indicate that I~K channels are the source of this epithelial K + conductance: (a) IsK inhibition mediated by chromanols occurred at similar concentrations and with a similar rank order of potency as observed for the colon cAMP-regulated K + conductance [11]; (b) the chromanols inhibited I~n channels potently without affecting other cloned K + channels; (c) especially intriguing is the stereospecificity of the I~K inhibition by chromanols.…”

“…Recently, Lohrmann et al [11] demonstrated that the inhibition of cAMPactivated K + channels by several chromanol derivatives completely abolishes C1 secretion in colon crypts. In whole-cell patch clamp experiments the inhibitory effects of these chromanols were confirmed on the basolateral, cAMP-activated K + conductance in rat colon crypts [9,12]. The aim of this study was therefore to identify the molecular targets for these *Corresponding author.…”

The Small Conductance K+ Channel, KCNQ1

“…This finding is in contrast to the colon where there is only little if any 293B-sensitive current without previous stimulation with agonists increasing cAMP (28). As the ion transport under control conditions predominately resemble Na ϩ reabsorption via the epithelial sodium channel (ENaC) and only slight anion secretion, it was interesting to address the question of whether KCNQ1 complexes also form part of the K ϩ conductance, which is needed for Na ϩ reabsorption.…”