The effects of phosphodiesterase inhibitors and lanthanum ions on the light‐sensitive current of toad retinal rods.

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Abstract
The light‐sensitive current of isolated toad rods was recorded using the method of Yau, McNaughton & Hodgkin (1981) and the effects of the phosphodiesterase inhibitor 3‐isobutyl‐1‐methylxanthine (IBMX) and of La ions were examined. IBMX caused an increase in the light‐sensitive current and a prolongation of the time course of the response. A small inward current which may reflect the operation of an Na‐Ca exchange pump was also increased in IBMX. With low doses of IBMX the time course of the dim flash response could be mapped onto that in Ringer solution by a linear transformation of the time scale. Light adaptation had opposite effects to those of IBMX on the time course of the dim flash response, and a steady background light could exactly neutralize the effects of IBMX on the time course. Light adaptation had the additional effect of strongly reducing the amplitude of the dim flash response. La ions caused a rapid inhibition (t1/2 less than 1 s) followed by slow inhibition (t1/2 approximately equal to 30 s) of the light‐sensitive current. In low [Ca] the rapid inhibition became more prominent, perhaps because of a competition between La and Ca for a blocking site near the light‐sensitive channel. The time constants of the falling phases of responses to both bright and dim flashes were slowed by La. The dim flash response could be fitted by a model in which a single time constant in the chain underlying the flash response is slowed by La. La reduced the rate of activation of light‐sensitive current in response to a reduction of external [Ca]. A concentration of La sufficient to block the activation of current in low [Ca] did not prevent the activation of current in IBMX. Light‐sensitive currents carried by Mn, Ca or Mg in the absence of Na and in the presence of IBMX were inhibited by La. An outward current observed in the absence of permeable ions was inhibited by La. The effects of La on the time course of the response and on the rate of activation of current when [Ca] is reduced are consistent with an inhibition of the Ca pump. La ions also have a high affinity for the light‐sensitive channel and can block current carried by another ion.