ATP-dependent activation of K(Ca) and ROMK-type K(ATP) channels in human submandibular gland ductal cells

Abstract

[Ca2+](i) and membrane current were measured in human submandibular gland ductal (HSG) cells to determine the regulation of salivary cell function by ATP. 1-10 μM ATP activated internal Ca2+ release, outward Ca2+-dependent K+ channel (K(Ca)), and inward store-operated Ca2+ current (I(SOC)). The subsequent addition of 100 μM ATP activated an inwardly rectifying K+ current, without increasing [Ca2+](i). The K+ current was also stimulated by ATP in cells treated with thapsigargin in a Ca2+-free medium and was blocked by glibenclamide and tolbutamide, but not by charybdotoxin. This suggests the involvement of a Ca2+-independent, sulfonylurea-sensitive K+ channel (K(ATP)). UTP mimicked the low [ATP] effects, while benzoyl-ATP activated internal Ca2+ release, a Ca2+ influx pathway, and K(Ca). Thus, ATP acts via P(2U) (P2Y2) and P(2Z) (P2X7) receptors to increase [Ca2+](i) and activate K(Ca), but not K(ATP). Importantly, (i) ROMK1 and the cystic fibrosis transmembrane regulator protein (but not SUR1, SUR2A, or SUR2B) and (ii) cAMP-stimulated Cl- and K+ currents were detected in HSG cells. These data demonstrate for the first time that a ROMK-type K(ATP) channel is present in salivary gland duct cells that is regulated by extracellular ATP and possibly by the cystic fibrosis transmembrane regulator. This reveals a potentially novel mechanism for K+ secretion in these cells.