Time course and Ca2+ dependence of sensitivity modulation in cyclic GMP-gated currents of intact cone photoreceptors

We determined the Ca2+ dependence and time course of the modulation of ligand sensitivity in cGMP-gated currents of intact cone photoreceptors. In electro-permeabilized single cones isolated from striped bass, we measured outer segment current amplitude as a function of cGMP or 8Br-cGMP concentrations in the presence of various Ca2+ levels. The dependence of current amplitude on nucleotide concentration is well described by the Hill function with values of K-1/2, the ligand concentration that half-saturates current, that, in turn, depend on Ca2+. K-1/2 increases as Ca2+ rises, and this dependence is well described by a modified Michaelis-Menten function, indicating that modulation arises from the interaction of Ca2+ With a single site without apparent cooperativity. K-Ca(m), the Michaelis-Menten constant for Ca2+ concentration is 857 +/- 68 nM for cGMP and 863 +/- 51 for 8Br-cGMP. In single cones under whole-cell voltage clamp, we simultaneously measured changes in membrane current and outer segment free Ca2+ caused by sudden Ca2+ sequestration attained by uncaging diazo-2. In the presence of constant 8Br-cGMP, 15 mu M, Ca2+ concentration decrease was complete within 50 ms and membrane conductance was enhanced 2.33 +/- 0.95-fold with a mean time to peak of 1.25 +/- 0.23 s. We developed a model that assumes channel modulation is a pseudo-first-order process kinetically limited by free Ca2+. Based on the experimentally measured changes in Ca2+ concentration, model simulations match experimental data well by assigning the pseudo-first-order time constant a mean value of 0.40 +/- 0.14 s. Thus, Ca2+-dependent ligand modulation occurs over the concentration range of the normal, dark-adapted cone. Its time course suggests that its functional effects are important in the recovery of the cone photoresponse to a flash of light and during the response to steps of light, when cones adapt.