Calcium mediates the light-induced decrease in maintained K+ current in Limulus ventral photoreceptors.
In addition to increasing the conductance to sodium, light reduces the maintained voltage-dependent potassium current (iK) in Limulus ventral photoreceptors. We have investigated the mechanism underlying this long-lasting decrease in ik. Intracellular injection of calcium produced a similar reduction of the voltage-dependent outward current. This reduction was not due to an activation of the voltage-dependent inward current (iin) because calcium injection reduced the outward current even under conditions where iin was blocked with Ni2+, and because calcium injection produced a decrease in conductance, as measured from the slope of the instantaneous i-V curve. The effect of light on ik could be blocked by injection of the calcium buffer EGTA (pCa 7.1) to an intracellular concentration of 50-70 mM. Even larger injections of the pH buffer MOPS (100-200 mM) did not reduce the effect of light on ik. These experiments show that intracellular free calcium (Cai2+) can reduce ik. Furthermore, since Cai2+ is known to increase in light, our results are consistent with the hypothesis that calcium is the internal transmitter for the light-induced decrease in ik.
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D006737:Horseshoe Crabs / Q000502:physiology*; D007277:Injections, Jet; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D008027:Light; D009025:Morpholines / Q000494:pharmacology; D010786:Photoreceptor Cells / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*
D002021:Buffers; D007473:Ion Channels; D009025:Morpholines; C008550:morpholinopropane sulfonic acid; D004533:Egtazic Acid; D011188:Potassium; D002118:Calcium