Anion modulation of calcium current voltage dependence and amplitude in salamander rods.

Hofmeister anions were used to investigate the ability of Cl(-) replacement to produce inhibition and a hyperpolarizing activation shift in L-type Ca(2+) currents (I(Ca)) of rod photoreceptors. Inhibition of I(Ca) largely followed the Hofmeister sequence: Cl(-)=Br(-)<NO(-)(3)<I(-)<ClO(-)(4) (ClO(-)(4) caused the greatest suppression). Anion-induced hyperpolarizing activation shifts also followed the Hofmeister sequence: Cl(-)<Br(-)<NO(-)(3)<I(-)<ClO(-)(4) (ClO(-)(4) caused the largest shift). Agreement with the Hofmeister sequence suggests that these effects are due to anion interactions at the membrane surface. Hofmeister anions also caused similar hyperpolarizing shifts in the voltage dependence of inwardly rectifying cation currents (I(h)) and outward K(+) currents (I(K)) consistent with the hypothesis that hyperpolarizing shifts arise from anion effects on membrane surface potential. Sulfate and phosphate inhibited rod I(Ca) and phosphate caused a significant leftward activation shift suggesting these anions are strongly adsorbed to the membrane. Because of the overlap between the physiological voltage range and the lower part of the I(Ca) activation curve, anion effects on amplitude and activation may influence synaptic transmission at the first retinal synapse.