title: Endoplasmic reticulum potassium-hydrogen exchanger and small conductance calcium-activated potassium channel activities are essential for ER calcium uptake in neurons and cardiomyocytes.
abstract: Calcium pumping into the endoplasmic reticulum (ER) lumen is thought to be coupled to a countertransport of protons through sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) and the members of the ClC family of chloride channels. However, pH in the ER lumen remains neutral, which suggests a mechanism responsible for proton re-entry. We studied whether cation-proton exchangers could act as routes for such a re-entry. ER Ca(2+) uptake was measured in permeabilized immortalized hypothalamic neurons, primary rat cortical neurons and mouse cardiac fibers. Replacement of K(+) in the uptake solution with Na(+) or tetraethylammonium led to a strong inhibition of Ca(2+) uptake in neurons and cardiomyocytes. Furthermore, inhibitors of the potassium-proton exchanger (quinine or propranolol) but not of the sodium-proton exchanger reduced ER Ca(2+) uptake by 56-82%. Externally added nigericin, a potassium-proton exchanger, attenuated the inhibitory effect of propranolol. Inhibitors of small conductance calcium-sensitive K(+) (SK(Ca)) channels (UCL 1684, dequalinium) blocked the uptake of Ca(2+) by the ER in all preparations by 48-94%, whereas inhibitors of other K(+) channels (IK(Ca), BK(Ca) and K(ATP)) had no effect. Fluorescence microscopy and western blot analysis revealed the presence of both SK(Ca) channels and the potassium-proton exchanger leucine zipper-EF-hand-containing transmembrane protein 1 (LETM1) in ER in situ and in the purified ER fraction. The data obtained demonstrate that SK(Ca) channels and LETM1 reside in the ER membrane and that their activity is essential for ER Ca(2+) uptake.
authors.affiliation: Department of Pharmacology, Centre of Excellence for Translational Medicine, University of Tartu, Ravila 19, Tartu EE-51014,  Estonia.
authors.forename: Malle
authors.initials: M
authors.lastname: Kuum
authors.forename: Vladimir
authors.initials: V
authors.lastname: Veksler
authors.forename: Joanna
authors.initials: J
authors.lastname: Liiv
authors.forename: Renee
authors.initials: R
authors.lastname: Ventura-Clapier
authors.forename: Allen
authors.initials: A
authors.lastname: Kaasik
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D017136:Ion Transport / Q000187:drug effects; D051379:Mice; D008954:Models, Biological; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009550:Nigericin / Q000494:pharmacology; D027961:Potassium-Hydrogen Antiporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011433:Propranolol / Q000494:pharmacology; D051381:Rats; D051640:Small-Conductance Calcium-Activated Potassium Channels / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D027961:Potassium-Hydrogen Antiporters; D051640:Small-Conductance Calcium-Activated Potassium Channels; D011433:Propranolol; D009550:Nigericin; D002118:Calcium