Modulation by internal protons of native cyclic nucleotide-gated channels from retinal rods

Ion channels directly activated by cyclic nucleotides are present in the plasma membrane of retinal rod outer segments. These channels call be modulated by several factors including internal pH (pH(i)). Native cyclic nucleotide-gated channels were studied in excised membrane patches from the outer of the salamander. Channels were activated by cGMP or cAMP and currents as a function of voltage and cyclic nucleotide concentrations were measured as pH(i) was varied between 7.6 and 5.0. Increasing internal proton concentrations reduced the current activated by cGMP without modifying the concentration (K-1/2) of cGMP necessary for half-activation of the maximal current. This effect could be well described as a reduction of single-channel current by protonation of a single acidic residue with a pK(1) of 5.1. When channels were activated by cAMP a more complex phenomenon was observed. K-1/2 for cAMP decreased by increasing internal prc,ton concentration whereas maximal currents activated by cAMP increased by lowering pH(i) fi om 7.6 to 5.7-5.5 and then decreased from pH(i) 5.5 to 5.0. This behavior was attributed both to a reduction in single-channel current as measured with cGMP and to an increase in channel open probability induced by the binding of three protons to sites with a PK2 of 6.