P2Y2 and P2Y4 receptors regulate pancreatic Ca2+-activated K+ channels differently

Abstract

Extracellular ATP is an important regulator of transepithelial transport in a number of tissues. In pancreatic ducts, we have shown that ATP modulates epithelial K+ channels via purinergic receptors, most likely the P2Y2 and P2Y4 receptors, but the identity of the involved K+ channels was not clear. In this study, we show by RT-PCR analysis that rat pancreatic ducts express Ca(2+)-activated K+ channels of intermediate conductance (IK) and big conductance (BK), but not small conductance (SK). Possible interactions between P2Y receptors and these Ca(2+)-activated K+ channels were examined in co-expression experiments in Xenopus laevis oocytes. K+ channel activity was measured electrophysiologically in oocytes stimulated with UTP (0.1 mM). UTP stimulation of oocytes expressing P2Y4 receptors and BK channels resulted in a 30% increase in the current through the expressed channels. In contrast, stimulation of P2Y2 receptors led to a 20% inhibition of co-expressed BK channel activity, a response that was sensitive to TEA. Furthermore, co-expression of IK channels with P2Y4 and P2Y2 receptors resulted in a large hyperpolarization and 22-fold and 5-fold activation of currents by UTP, respectively. Taken together, this study shows that there are different interactions between the subtypes of P2Y purinergic receptors and different Ca(2+)-activated K+ channels.

Purinergic receptors in the endocrine and exocrine pancreas

“…Low concentrations of UTP caused Ca 2+ oscillation and synchronous activity of K + currents, presumed to be IK channels, and pH i measurements indicated HCO À 3 secretion [131]. This finding supports conclusions drawn from studies on rat and human P2Y 2 receptors and IK channels [94,105]. In addition to P2Y 2 receptors, there are also P2Y 11 receptors on the basolateral membrane of canine epithelium that activate large Ca 2+ -independent Cl − conductance and cAMP increases (see Fig.…”

“…A similar effect of UTP on secretory processes was also detected in polarised Capan-1 cells, which are epithelial cells derived from pancreatic adenocarcinoma [104]. RT-PCR analysis of the single rat ducts revealed that they express Ca 2+ -activated K + channels of the intermediate conductance (IK also known as SLO, Maxi K and KCNMA1) and big conductance (BK also known as SK4, KCNN4) [105]. Human IK or BK channels were coexpressed with hP2Y 2 or hP2Y 4 receptors in Xenopus oocytes and cell currents were measured.…”

“…In contrast to BK channels, IK channel regulation by P2Y 4 or even P2Y 2 receptors is stimulatory, as would be required for secretory processes [105]. Therefore, localization of these channels and receptors and conditions that will favour secretory events need to be elucidated.…”

“…Functionally, the K + channels are seen as clotrimazole-sensitive Ca 2+ -activated K + channels and their molecular identity corresponds most likely to IK. Interestingly, this cell line lacks Ba 2+ sensitivity indicating an absence of BK channels, which are expressed in native duct cells [105]. In most studies on cultured cells, cells were grown on coverslips and nucleotides were added to the apical side.…”

“…They do possess CFTR channels that have been repeatedly in and out of favour as the ATP release channels [146,147]. Another possibility is that pancreatic ducts distend with oversecretion and release ATP, which then via P2Y 2 receptors and BK channels inhibits further secretion and thus overdistension of ducts [94,105]. In contrast to salivary glands that have myoepithelial cells to hold around ducts, pancreatic ducts do not have such support and may rely more on this self-regulation.…”