Localization of calcium signals by a mobile calcium buffer in frog saccular hair cells

Abstract

A recent study (Roberts, 1993) of saccular hair cells from grass frogs (Rana pipiens) has suggested a mechanism by which the unusually high concentrations of calcium-binding proteins found in certain sensory receptors and neurons, particularly in the auditory system, can influence short-range intracellular calcium signaling. In frog saccular hair cells, the mechanism operates within arrays of calcium channels and calcium-activated potassium channels that are involved in the cells' electrical resonance and synaptic transmission. The present study tests the hypothesis that calbindin-D28k, one of the most abundant proteins in these cells, can serve as a mobile calcium buffer that reduces and localizes changes in the intracellular free-calcium concentration ([Ca2+](i)) by shuttling calcium away from the channel arrays. Based upon theoretical analysis and computer modeling, it is shown that [Ca2+](i) near one or more open channels quickly reaches a steady- state level determined primarily by two properties of the buffer, the mean time (τ(c)) before it captures a free-calcium ion and a replenishment factor (R), which are related to the buffer's diffusional mobility (D(Bu)), association rate constant (k(on)), and concentration (B0) by τ(c) = (k(on)B0)-1 and R = B0D(Bu). Simulation of calcium entry through a channel array showed that ~1.5 mM of a molecule with the diffusional and binding properties expected for calbindin-D28k (B0 ≃ 8 mM calcium-binding sites) is needed to reproduce the previous experimental results. A lower concentration (B0 = 2 mM) was almost completely depleted within the channel array by a modest calcium current (8 pA = 12% of calcium channels open), but still had two important effects: it caused [Ca2+](i) to fall steeply with distance outside the array (space constant < 50 nm), and returned [Ca2+](i) quickly to the resting level after the channels closed. A high concentration of calbindin-D28k can thus influence the cell's electrical resonance and synaptic transmission. Its most important functions may be to localize regions of high [Ca2+](i) and speed the return of [Ca2+](i) toward the resting level.