MECHANISMS OF MODULATION BY INTERNAL PROTONS OF CYCLIC NUCLEOTIDE-GATED CHANNELS CLONED FROM SENSORY RECEPTOR-CELLS

We have examined the modulation by internal protons of cyclic nucleoti de-gated (CNG) channels cloned from bovine olfactory receptor cells an d retinal rods. CNG channels Mere studied in excised inside-out membra ne patches from Xenopus laevis oocytes previously injected with the mR NA encoding for the subunit 1 of olfactory or rod channels. Channels w ere activated by cGMP or cAMP, and currents as a function of cyclic nu cleotide concentrations were measured as pH(i) varied between 7.6 and 5.0. Increasing internal proton concentrations caused a partial blocka ge of the single-channel current, consistent with protonation of a sin gle acidic site with a pK(i) of 4.5-4.7, both in rod and in olfactory CNG channels. Channel gating properties were also affected by internal protons. The open probability at law cyclic nucleotide concentrations was greatly increased by lowering pH(i), and the increase was larger when channels were activated by cAMP than by cGMP. Therefore, internal protons affected both channel permeation and gating properties, causi ng a reduction in single-channel current and an increase in open proba bility. These effects are likely to be caused by different titratable groups on the channel.