title: Calcium-activated potassium channels and fluid secretion by exocrine glands.
abstract: Fluid secretion by exocrine glands is regulated by neurotransmitters and hormones. The secretagogues act on the acinar cells by switching on two types of conductance pathways: K+-selective channels in the basolateral membrane and Cl(-)-selective channels localized to the luminal membrane. The K+ channels have been quantitatively characterized in patch-clamp single-channel and whole-cell current-recording studies. Opening of the K+ channels is determined by the membrane potential (depolarization enhances the probability of channel opening), and the intracellular free Ca2+ concentration ([Ca2+]i) (a rise in [Ca2+]i increases the open-state probability). The Cl- channels are also controlled by internal Ca2+ in such a way that an elevation of [Ca2+]i favors opening. Secretagogues evoking an increase in [Ca2+]i activate both sets of channels causing a substantial loss of cellular KCl. KCl is taken up via a Na+-K+-2Cl- cotransport mechanism in the basolateral membrane and the Na+ uptake activates the Na+-K+ pump. In the steady-state stimulated situation the three basolateral transport proteins, the K+ channels, the Na+-K+ pump, and the Na+-K+-2Cl- cotransporter operate together as an electrogenic Cl- pump. Cl- exits into the lumen via the Ca2+-activated Cl- channels and Na+ follows through the paracellular shunt pathway. When stimulation of the acinar cells ceases the K+ and Cl- conductance pathways close and the Na+-K+ pump together with the Na+-K+-2Cl- cotransporter operate as a KCl pump, restoring the intracellular KCl lost initially after start of stimulation and secretion stops.
mesh_terms: D000818:Animals; D001485:Basement Membrane / Q000378:metabolism; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability; D002712:Chlorides / Q000378:metabolism; D005088:Exocrine Glands / Q000166:cytology / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D007765:Lacrimal Apparatus / Q000378:metabolism; D008564:Membrane Potentials; D008839:Microelectrodes; D010179:Pancreas / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D012469:Salivary Glands / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D014882:Water-Electrolyte Balance
chemical_list: D002712:Chlorides; D007473:Ion Channels; D012964:Sodium; D011188:Potassium; D002118:Calcium