The pharmacology and dynamics of the light-induced Calcium signal in Limulus ventral photoreceptors
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The Limulus ventral photoreceptor is a model system for understanding inositol (1,4,5) trisphosphate (IP3) induced calcium (Ca2+) release and the light response of invertebrate photoreceptors. Light-induced Ca2+ release via the phosphoinositide cascade is thought to activate the photocurrent in Limulus. The pharmacology and dynamics of the light-induced Ca2+ signal was investigated. Application of 1-100 μM 2 - aminoethoxydiphenyl borate (2-APB), a non-specific IP3 receptor inhibitor, reversibly inhibited the photocurrent in a concentration-dependent manner, acting on at least two processes thought to mediate the visual cascade. 2-APB reversibly inhibited both light and IP3-induced calcium release, consistent with its role as an inhibitor of the IP3 receptor. 2-APB also reversibly inhibited the activation of depolarizing current flow through the plasma membrane by released calcium ions. In addition, 100 μM 2-APB reversibly inhibited voltage-activated potassium currents. This lack of specificity of 2-APB's action in Limulus suggests that the effects of 2-APB need to be interpreted with caution. Dynamics: The light-induced Ca2+ release begins at the light sensitive plasma membrane of the R lobe. Consistent with its role in mediating light responses, this Ca2+ signal rises to its peak with a similar time course to that of the electrical signals. Then the Ca2+ signal spreads into the interior of the cell with two phases. The initial fast phase is a diffusion driven process that needs both the diffusion of IP3 and of Ca2+ released by IP3, since both manipulations that alter the apparent diffusion coefficient (D) of Ca2+ ions or those that change the life span of IP3 molecules can change the spread of the fast phase. A model of simple diffusion of IP3 molecules (or Ca2+ ions) and a diffusion model for IP3 and Ca2+ released by IP3 were constructed using estimated D values of Ca2+ and IP3 inside cells obtained through the Graham's Law. Simulation results indicate that diffusion of IP3 and released Ca2+ is both necessary and sufficient to determine the initial fast phase. The expected spread of excitation following absorption of one photon can be predicted by this model.