The plasma membrane of retinal rod outer segments contains a cyclic GMP-activated conductance1-6 which appears to be the light-sensitive conductance involved in phototransduction7. Recently, it has been found that this conductance is partially blocked by Mg2+ (refs 3, 8, 9) and Ca2+ (refs 3, 4, 8-11) at physiological concentrations, thus possibly accounting for the absence of observable single-channel activity in excised membrane patches1,3,8 and for the unusually small apparent unit conductance deduced from noise measurements on intact cells 4,10,12,13. We now report that, as expected from this idea, single cGMP-activated channel activity can be detected from an excised rod membrane patch in the absence of divalent cations. The most prominent unitary current had a mean conductance of ∼25 pS. Both individual channel openings (mean open time ∼1 ms) and short bursts of openings (mean burst duration of about a few milliseconds) were observed. In addition, there were smaller events which probably represented other states of the conductance. The mean current increased with the third power of cGMP concentration, suggesting that there are at least three cGMP-binding sites on the channel molecule. With 0.2 mM Mg2+ in the cGMP-containing solution, a flickering block of the open channel was observed; the effect of Ca2+ was similar. The results resolve a puzzle about the light-sensitive conductance by demonstrating that it is an aqueous pore rather than a carrier.
ASJC Scopus subject areas