Changes in pigeon cone photocurrent caused by reduction in extracellular calcium activity.

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Abstract
Photocurrents were recorded from the red spot of the isolated superfused pigeon retina. The technique recorded photovoltage gradients and extracellular fluid resistivity in a direction parallel with the long axes of the receptors. Cone and rod responses were identified, and experiments designed so that only the former were elicited. In the outer portion of the receptor layer, the wave form of the cone photoresponse lacked the initial transient (the nose) seen in the portions of the receptor layer nearer the synapses. It was argued that this observation permits the use of a simple equivalent circuit for the generation of the extracellular photocurrent, to infer membrane properties from extracellular recordings. When the superfusing Ringer was changed to one which has a very low Ca2+ activity (2 .times. 10-7 M) the 1st result was that photoresponses increased in magnitude ( .times. 7.7) but the relationship between light intensity and response amplitude and the light intensity (.sigma.) required to produce a half maximal response remained unchanged. This increase in photocurrent in low Ca2+ also occurred if the superfusing fluid was cooled to 10.degree. C. After 2-3 min, the photoresponses in low Ca2+ began to decrease in amplitude, and the value of .sigma. was progressively reduced, 10-fold in 10 min. During this time, the wave form of the photocurrent altered, the rate of increase and decrease of the responses being slowed. The relationship between peak photocurrent and duration of light flash was modified. The response to a step of light was not well maintained in higher Ca2+, but was well maintained in low Ca2+. In higher Ca2+, the current overshot during recovery from a flash to below the previous dark level. This did not happen in low Ca2+. In low Ca2+, a light adapting background illumination desensitized the cones. All changes in wave form of the response could be accounted for in terms of the membrane non-linearities. The calculated time course of the change in concentration of the internal transmitter was unaffected. The same was true of desensitization, in the dark, following exposure to intense illumination.