On the interaction between voltage-gated conductances and Ca(2+) regulation mechanisms in retinal horizontal cells

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 Ca(2+) regulation mechanisms, e.g., the Na(+)/Ca(2+) exchange and the Ca(2+) 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 Ca(2+) concentration ([Ca(2+)]i) was maintained <100 nM in the resting state and increased with an initial transient to settle at a steady level of approximately 600 nM during prolonged applications of L-glutamate (L-glu, 100 microM). A preapplication of caffeine (10 mM) partially suppressed the initial transient of [Ca(2+)]i induced by L-glu but did not affect the L-glu-induced steady [Ca(2+)]i. This suggests that a part of the initial transient can be explained by the Ca(2+)-induced Ca(2+) release from the caffeine-sensitive Ca(2+) store. The Ca(2+) 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(+)/Ca(2+) exchange were estimated by referring to previous experiments. The parameters of the model equation describing the Ca(2+) pump were estimated to reproduce the steady levels of [Ca(2+)]i measured by Fura-2 fluorescence measurements. Using the cable model with these parameters, we have repeated simulations so that the voltage response and [Ca(2+)]i change induced by L-glu applications were reproduced. The simulation study supports the following conclusions. 1) The Ca(2+)-dependent inactivation of the voltage-gated Ca(2+) conductance has a time constant of approximately 2.86 s. 2) The falling phase of the [Ca(2+)]i transient induced by L-glu is partially due to the inactivation of the voltage-gated Ca(2+) conductance. 3) Intracellular Ca(2+) is extruded mainly by the Na(+)/Ca(2+) exchange when [Ca(2+)]i is more than approximately 2 microM and by the Ca(2+) pump when [Ca(2+)]i is less than approximately 1 microM. 4) In the resting state, the Na(+)/Ca(2+) exchange may operate in the reverse mode to induce Ca(2+) influx and the Ca(2+) pump extrudes intracellular Ca(2+) 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.