A cyclic GMP-sensitive conductance has recently been observed with patch-clamp recording in excised inside-out patches of plasma membrane from frog and toad rod outer segments1,2. This conductance has properties suggesting that it is probably the light-sensitive conductance involved in visual transduction1,2. We now report a similar conductance in the outer segment membrane of catfish cones. Cyclic GMP showed positive cooperatively in opening this conductance, with a Hill coefficient of 1.6-3.0 and a half-saturating cGMP concentration of 35-70 μM. Cyclic AMP at 1 mM, or changing Ca concentration (in the presence of Mg), had little effect on the conductance. In physiological solutions the cGMP-induced current had a reversal potential near +10 mV; the current amplitude increased roughly exponentially with membrane potential in both depolarizing and hyperpolarizing directions. Our results suggest that cGMP is also the internal transmitter for phototransduction in cones.
ASJC Scopus subject areas