Gating of cyclic nucleotide-gated (CNGA1) channels by cGMP jumps and depolarizing voltage steps

¹ Friedrich-Schiller University Jena
² Forschungsinstitut für Molekulare Pharmakologie Berlin

^* To whom correspondence should be addressed. E-mail: klaus.benndorf{at}mti.uni-jena.de.

Submitted on December 1, 2005
Revised on January 7, 2006
Accepted on 23 January 2006

	Abstract

We expressed homotetrameric CNGA1 channels in Xenopus oocytes and studied activation by photolysis-induced jumps of the cGMP concentration and by voltage steps. cGMP jumps to increasing concentrations up to the EC₅₀ value of 46.5 µM decelerate the activation gating, indicative that even at concentrations of cGMP much lower than EC₅₀ binding is not rate limiting. Above the EC₅₀ value, activation by cGMP jumps is again accelerated to the higher concentrations. At the same cGMP concentration, the speed of the activation gating by depolarizing voltage steps is roughly similar to that by cGMP jumps. Permeating ions passing the pore more slowly (Rb⁺>K⁺>Na⁺) slow down the activation time course. At the single-channel level, cGMP jumps to high concentrations cause openings directly to the main open level without passing sublevels. From these results it is concluded that at both low and high cGMP the gating of homotetrameric CNGA1 channels is not rate limited by the cGMP binding but by conformational changes of the channel which are voltage dependent and include movements in the pore region.

Key Words: cGMP binding, flash photolysis, ion current, patch clamp, pore gating