Involvement of calmodulin in Ca(2+)-activated K+ efflux in human colonic cell line, HT29-19A

Regulation of Ca(2+)-activated K(+) channels by multifunctional Ca(2+)/calmodulin-dependent protein kinase

Activation of mesangial cells by ANG II provokes release of intracellular Ca(2+) stores and subsequent Ca(2+) influx through voltage-gated channels, events that are reflected by a large transient increase in intracellular concentration [Ca(2+)](i) followed by a modest sustained elevation in [Ca(2+)](i). These ANG II-induced alterations in [Ca(2+)](i) elicit activation of large Ca(2+)-activated K(+) channels (BK(Ca)) in a negative-feedback manner. The mechanism of this BK(Ca) feedback response may involve the direct effect of intracellular Ca(2+) on the channel and/or channel activation by regulatory enzymes. The present study utilized patch-clamp and fura 2 fluorescence techniques to assess the involvement of multifunctional calcium calmodulin kinase II (CAMKII) in the BK(Ca) feedback response. In cell-attached patches, KN62 (specific inhibitor of CAMKII) either abolished or reduced to near zero the ANG II-induced BK(Ca) feedback response. This phenomenon did not reflect direct effects of KN62 on the BK(Ca) channel, because this agent alone did not significantly alter BK(Ca) channel activity in inside-out patches. KN62 also failed to alter either the transient peak or sustained plateau phases of the [Ca(2+)](i) response to ANG II. In inside-out patches (1 microM Ca(2+) in bath), calmodulin plus ATP activated BK(Ca) channels in the presence but not the absence of CAMKII. These observations are consistent with the postulate that CAMKII is involved in the BK(Ca) feedback response of mesangial cells, acting to potentiate the influence of increased [Ca(2+)](i) on the BK(Ca) channel or a closely associated regulator of the channel. An additional effect of CAMKII to activate a voltage-gated Ca(2+) channel cannot be ruled out by these experiments.

Involvement of calmodulin and protein kinase C in the regulation of K+ transport by carbachol across the rat distal colon

The cholinergic agonist carbachol stimulates the apical H+-K+-ATPase and apical as well as basolateral K+ channels in the rat distal colon. The effect of carbachol was tested in the presence of different inhibitors of the Ca2+ signaling pathway in order to characterize the intracellular mechanisms involved. Both carbachol-stimulated Rb+-efflux as well as carbachol-stimulated mucosal Rb+-uptake were dependent on the presence of serosal Ca2+. The Ca2+-calmodulin antagonist calmidazolium (10(-7) mol l(-1)) inhibited the stimulation of mucosal and serosal Rb+ efflux by carbachol. A similar effect had KN-62 (10(-5) mol l(-1)), an inhibitor of the Ca2+-calmodulin-dependent kinase II, suggesting the regulation of basolateral and apical K+ channels by this kinase. Staurosporine (10(-6) mol l(-1)), which potently inhibits protein kinase C, did not alter the effect of carbachol on Rb+ efflux, although the stimulation of apical Rb+ efflux by carbachol seemed to be less prolonged, indicating that protein kinase C is not involved in the regulation of K+ permeability. In contrast, mucosal Rb+ uptake, which is determined by the ouabain- and vanadate-sensitive K+ transport via the apical H+-K+-ATPase, was decreased to nearly one third of control values in the presence of calmidazolium. Both calmidazolium and staurosporine, but not KN-62, prevented the stimulatory action of carbachol on the H+-K+-ATPase, suggesting a synergistic control of this ion pump by both Ca2+-calmodulin and protein kinase C.

Characteristics of two basolateral potassium channel populations in human colonic crypts

The basolateral membrane of human colonic crypt cells contains Ca2+ and cAMP activated, Ba2+ blockable, low conductance (23 pS) K+ channels, which probably play an important part in intestinal Cl- secretion. This study has defined more clearly the basolateral K+ conductive properties of human colonic crypts using patch clamp recording techniques. High conductance (138 pS) K+ channels were seen in 25% of patches (one or two channels per patch), and significantly inhibited by the addition of 5 mM Ba2+, 1 mM quinidine or 20 mM tetraethylammonium chloride (TEA) to the cytosolic side of excised inside-out patches, whereas 1 mM diphenylamine-2-carboxylic acid (DPC) had no effect. In contrast, clusters of the 23 pS K+ channel (two to six channels per patch) were present in > 75% of patches, and channel activity was inhibited by quinidine and DPC, but not by TEA. Activity of the 138 pS K+ channel in inside-out patches was abolished almost completely by removal of bath Ca2+, but in contrast with its effect on the 23 pS K+ channel, addition of 0.1 mM carbachol had no effect on the 138 pS K+ channel in cell attached patches. It is concluded that human colonic crypt cells possess two discrete basolateral K+ channel populations, which can be distinguished by their responses to K+ channel blockers, and their different sensitivities to changes in intracellular Ca2+ concentration.

Evidence against direct activation of chloride secretion by carbachol in the rat distal colon

Carbachol (5 x 10(-5) mol.1-1) induced a biphasic increase in short-circuit current (Isc) consisting of an initial peak phase followed by a long-lasting plateau. Complete dependence on the presence of Cl- ions and sensitivity to bumetanide confirmed that carbachol induces Cl- secretion. The plateau phase was blocked by indomethacin, and both the plateau and the peak phase were suppressed in the combined presence of indomethacin and tetrodotoxin. Inhibition of the carbachol response could be overcome by agonists of the cAMP pathway like prostaglandin E2, forskolin or 8-(4-chlorophenylthio)-adenosine-3',5'-cyclic monophosphate. The increase in Isc was inhibited by a blocker of cAMP-activated Cl- channels, glibenclamide, but was resistant to an inhibitor of Ca(2+)-activated Cl- channels, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). The K+ channel blockers Ba2+ and charybdotoxin inhibited the first and suppressed the second phase of the carbachol response, whereas a less specific K+ channel blocker, quinine, suppressed both phases. These results suggest that the dominant effect of carbachol in the intact colonic mucosa is an opening of Ca(2+)-dependent, charybdotoxin- and Ba(2+)-sensitive K+ channels, which leads to hyperpolarization of the epithelial cells. This stimulates Cl- secretion only if there are spontaneously open apical Cl- channels which are basically stimulated by a continuous release of neurotransmitters and prostaglandins. Only during the first phase of the carbachol effect is there indirect evidence for activation of a Cl- conductance synergistically with the cAMP pathway as shown by the increase in tissue conductance resistant to K+ channel blockers.