Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors

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 HEPESbased 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 [Ca ²⁺]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 pHi 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.