Regulation of maxi-K+ channels on pancreatic duct cells by cyclic AMP-dependent phosphorylation

Abstract

Using the patch-clamp technique we have identified a Ca2(+)-sensitive, voltage-dependent, maxi-K+ channel on the basolateral surface of rat pancreatic duct cells. The channel had a conductance of approximately 200 pS in excised patches bathed in symmetrical 150 mM K+, and was blocked by 1 mM Ba2+. Channel open-state probability (Po) on unstimulated cells was very low, but was markedly increased by exposing the cells to secretin, dibutyryl cyclic AMP, forskolin or isobutylmethylxanthine. Stimulation also shifted the Po/voltage relationship towards hyperpolarizing potentials, but channel conductance was unchanged. If patches were excised from stimulated cells into the inside-out configuration, Po remained high, and was not markedly reduced by lowering bath (cytoplasmic) Ca2+ concentration from 2 mM to 0.1 microM. However, activated channels were still blocked by 1 mM Ba2+. Channel Po was also increased by exposing the cytoplasmic face of excised patches to the purified catalytic subunit of cyclic AMP-dependent protein kinase. We conclude that cyclic AMP-dependent phosphorylation can activate maxi-K+ channels on pancreatic duct cells via a stable modification of the channel protein itself, or a closely associated regulatory subunit, and that phosphorylation alters the responsiveness of the channels to Ca2+. Physiologically, these K+ channels may contribute to the basolateral K+ conductance of the duct cell and, by providing a pathway for current flow across the basolateral membrane, play an important role in pancreatic bicarbonate secretion.

Concomitant activation of Cl‐ and K+ currents by secretory stimulation in human epithelial cells.

“…Such Ca2+-activated K+ channels sensitive to cAMP-dependent phosphorylation have been found in cultured kidney cells (Guggino, Suarez-Isla, Guggino & Sacktor, 1985) and in pancreatic duct cells (Gray, Greenwell, Garton & Argent, 1990).…”

Purinergic receptors in the endocrine and exocrine pancreas

“…3a) [97,98]. The driving force for anion transport is kept by opened K + channels [91,[99][100][101][102]. Interestingly, in rat pancreatic ducts UTP (which activates rat P2Y 2 and P2Y 4 receptors with about equal efficiency) decreased the whole-cell K + conductance and no effect on the Cl − conductance could be detected [94].…”

Transepithelial Bicarbonate Secretion: Lessons from the Pancreas

“…The molecular identity of the channels was subsequently determined as the MaxiK channels encoded by KCNMA1 (Nehrke et al 2003;Romanenko et al 2006) and the IK1 channels encoded by KCNN4 (Begenisich et al 2004;Hayashi et al 2004), respectively. In the pancreatic duct, MaxiK seems to be the major channel maintaining a negative membrane potential during stimulated bicarbonate secretion (Gray et al 1990). However, MaxiK channels do not appear to contribute to resting membrane potential, possibly because they have a very low open probability during the unstimulated state.…”

“…Deletion of Kcnma1, which encodes MaxiK, impairs salivary K þ secretion, suggesting MaxiK to be the channel responsible for K þ efflux in salivary glands (Nakamoto et al 2008). A recent study showed that MaxiK channels are also expressed at the apical membrane of pancreatic duct cells in guinea pigs (Venglovecz et al 2011), which may contribute to the potentiation of secretin and CCK (or other Ca 2þ agonists such as cholinergic) response in pancreatic secretion (Gray et al 1990). However, the physiological role of the apical K þ channel in pancreatic duct cells is not fully understood at present, because unlike saliva, the major cation in pancreatic juice is Na þ and pancreatic duct cells do not secrete the bulk of K þ .…”