On the Interaction Between Voltage-Gated Conductances and Ca2+ Regulation Mechanisms in Retinal Horizontal Cells

Abstract

The horizontal cell is a second-order retinal neuron that is depolarized in the dark and responds to light with graded potential changes. In such a nonspiking neuron, not only the voltage-gated ionic conductances but also Ca2+regulation mechanisms, e.g., the Na+/Ca2+ exchange and the Ca2+ pump, are considered to play important roles in generating the voltage responses. To elucidate how these physiological mechanisms interact and contribute to generating the responses of the horizontal cell, physiological experiments and computer simulations were made. Fura-2 fluorescence measurements made on dissociated carp horizontal cells showed that intracellular Ca2+ concentration ([Ca2+]i) was maintained <100 nM in the resting state and increased with an initial transient to settle at a steady level of ≃600 nM during prolonged applications of l-glutamate (l-glu, 100 μM). A preapplication of caffeine (10 mM) partially suppressed the initial transient of [Ca2+]iinduced by l-glu but did not affect thel-glu-induced steady [Ca2+]i. This suggests that a part of the initial transient can be explained by the Ca2+-induced Ca2+release from the caffeine-sensitive Ca2+ store. The Ca2+ regulation mechanisms and the ionic conductances found in the horizontal cell were described by model equations and incorporated into a hemi-spherical cable model to simulate the isolated horizontal cell. The physiological ranges of parameters of the model equations describing the voltage-gated conductances, the glutamate-gated conductance and the Na+/Ca2+ exchange were estimated by referring to previous experiments. The parameters of the model equation describing the Ca2+ pump were estimated to reproduce the steady levels of [Ca2+]i measured by Fura-2 fluorescence measurements. Using the cable model with these parameters, we have repeated simulations so that the voltage response and [Ca2+]i change induced by l-glu applications were reproduced. The simulation study supports the following conclusions. 1) The Ca2+-dependent inactivation of the voltage-gated Ca2+ conductance has a time constant of ∼2.86 s. 2) The falling phase of the [Ca2+]i transient induced by l-glu is partially due to the inactivation of the voltage-gated Ca2+ conductance. 3) Intracellular Ca2+ is extruded mainly by the Na+/Ca2+ exchange when [Ca2+]i is more than ∼2 μM and by the Ca2+ pump when [Ca2+]i is less than ∼1 μM. 4) In the resting state, the Na+/Ca2+ exchange may operate in the reverse mode to induce Ca2+ influx and the Ca2+ pump extrudes intracellular Ca2+ to counteract the influx. The model equations of physiological mechanisms developed in the present study can be used to elucidate the underlying mechanisms of the light-induced response of the horizontal cell in situ.