Truncated salamander rod photoreceptors were internally perfused to investigate the action of cytoplasmic Ca2+ on cGMP-activated channels in the outer segment. 2. Switching from 1 microM Ca2+ to 0 Ca2+ increased the cGMP-activated current by a factor of 7.1 +/- 0.5 when measured in the first 60 s after the outer segment was opened to the bath, but only 2-fold after 5 min or more. This was attributed to the loss from the outer segment of a soluble factor required for Ca2+ to inhibit the cGMP-activated channel. 3. Short exposures to 0 Ca2+ caused an irreversible increase in the cGMP-activated current measured in 1 microM Ca2+, indicating that lowering [Ca2+] accelerated the loss of the channel inhibitor from the outer segment. 4. Channel activation occurred with a half-time of 6.7 s on switching to 0 Ca2+. Replacing 1 microM Ca2+ inhibited the current again with a half-time of 11.0 s. 5. The inhibition of the cGMP-activated current by Ca2+ could be described by a Hill curve with half-maximal suppression at 55 +/- 13 nM Ca2+ and a Hill coefficient of 1.4 +/- 0.4. 6. Addition of calmodulin (1 microM), or the calmodulin inhibitors mastoparan and calmidazolium (5 microM), did not alter the action of Ca2+ on the cGMP-activated current. 7. The increased affinity of the cGMP-activated channels in response to a fall in [Ca2+] has the magnitude, speed and Ca2+ dependence to suggest that it will promote recovery of the cGMP-activated current in response to the light-induced fall in [Ca2+] that normally occurs inside the outer segment.
