Lyotropic anion effects on vertebrate photoreceptor calcium currents

Purpose: Small, physiologically achievable, reductions in (Cl |0 suppress photoreceptor synaptic transmission and Ic. We examined whether the suppression of photoreceptor Ic. is due to anion effects on extracellular surface charge. Methods: Conventional and nystatin-perforated whole cell patch clamp recordings were obtained from rod and cone photoreceptors using retinal slice and isolated cell preparations prepared from the larval tiger salamander (Ambystoma tigrinum). Except in SO2 experiments, Ic was enhanced with 10 inM Ba or Ca. Results: To test whether anions act at an intracellular site, we compared Ic. amplitudes in recordings obtained with high (110 mM Cl) and low (9 mM Cl, 101 mM CHjSO3) pipette [Cl]. Solitary photoreceptors were triturated to shear off fine processes and thus enhance diffusion into the cell from the patch pipette. The time constant of Ic. rundown during conventional whole cell recording was not accelerated by low pipette [Cl]. Similarly, Ic. amplitude remained fairly constant during nystatin-perforated patch recordings regardless of pipette [Cl]. Anion effects on surface charge should follow the Hofmeister or lyotropic anion series: SO42 > Cl > Br > NO) > I > C1O4. Effects of anion replacement on Ic, amplitude in rods (using Ca or Ba as charge carriers) exhibited the following order: Cl = Br > SO42 > NOy > I > CIO4 (where CIO, caused the greatest suppression), Anion replacement also induced a negative shift in Ic, activation of rods which followed the order: Cl > SO42 > Br > N03 > I' > CIO' (where CIO, caused the largest shift). The dose-dependence of C1O4 effects on activation did not differ significantly from its suppressive effects. However, during application of anion test solutions, the peak of the negative shift preceded maximal suppression. With prolonged perfusion (> 2 min,), suppression continued to increase, but the negative shift subsided. The initial negative shift observed with SO2 reversed after prolonged perfusion resulting in a positive shift of Ic. activation. Conclusions: The insensitivity of ID. amplitude to pipette [Cl] suggests that the predominant site of anion action is extracellular. Consistent with surface charge effects, the effects of lyotropic anions on Ic amplitude and activation generally followed the Hofmeister series. Protons and divalent cations can also modulate photoreceptor ICt. However, it is unlikely that anions exert their effects by altering (K, and K) and anion effects on I_Ca.