title: Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors.
abstract: Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pH(i)) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca²(+)), in salamander photoreceptor inner segments loaded with Ca²(+) and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ∼0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH₄Cl) depolarized photoreceptors and stimulated Ca²(+) release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca²(+)](i). Alkalinization-evoked [Ca²(+)](i) decreases were independent of voltage-operated and store-operated Ca²(+) entry, plasma membrane Ca²(+) extrusion, and Ca²(+) sequestration into internal stores. The [Ca²(+)](i)-suppressive effects of alkalinization were antagonized by the fast Ca²(+) buffer BAPTA, suggesting that pH(i) directly regulates Ca²(+) binding to internal anionic sites. In summary, this data suggest that endogenously produced protons continually modulate the membrane potential, release from Ca²(+) stores, and intracellular Ca²(+) buffering in rod and cone inner segments.
authors.affiliation: Department of Ophthalmology, Moran Eye Center, University of Utah School of Medicine, Salt Lake City, 84132, USA. david.krizaj@hsc.utah.edu
authors.forename: David
authors.initials: D
authors.lastname: Krizaj
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Mercer
authors.forename: Wallace B
authors.initials: WB
authors.lastname: Thoreson
authors.forename: Peter
authors.initials: P
authors.lastname: Barabas
mesh_terms: D000557:Ambystoma; D000643:Ammonium Chloride / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D003600:Cytosol; D006706:Homeostasis / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000502:physiology; D020419:Photoreceptor Cells, Vertebrate / Q000187:drug effects / Q000502:physiology*; D012160:Retina / Q000166:cytology*; D015398:Signal Transduction
chemical_list: D000643:Ammonium Chloride; D002118:Calcium