ACTIVATION OF F-CHANNELS BY CAMP ANALOGS IN MACROPATCHES FROM RABBIT SINOATRIAL NODE MYOCYTES

1. The action of the two diastereometric phosphorothioate derivatives of cAMP, Rp-cAMPs and Sp-cAMPs, was investigated on hyperpolarization- activated 'pacemaker' current (i(f)) recorded in inside-out macropatch es from rabbit sino-atrial(SA) node myocytes. 2. When superfused on th e intracellular side of f-channels at the concentration of 10 mu M, bo th cAMP derivatives accelerated i(f) activation; their action was mode rately less pronounced than that due to the same concentration of cAMP . 3. The measurement of the i(f) conductance-voltage relation by volta ge ramp protocols indicated that both cAMP analogues shift the activat ion curve of i(f) to more positive voltages with no change in maximal (fully activated) conductance. 4. Dose-response relationships of the s hift of the i(f) activation curve showed that both Rp-cAMPs and Sp-cAM Ps act as agonists in the cAMP-dependent direct f-channel activation. Fitting data to the Hill equation resulted in maximal shifts of 9.6 an d 9.5 mV, apparent dissociation constants of 0.82 and 5.4 mu M, and Hi ll coefficients of 0.82 and 1.12 for Sp-cAMPs and Rp-cAMPs, respective ly. 5. The activating action of Rp-cAMPs, a known antagonist of cAMP i n the activation of cAMP-dependent protein kinase, confirms previously established evidence that f-channel activation does not involve phosp horylation. These results also suggest that the cAMP binding site of f -channels may be structurally similar to the cyclic nucleotide binding site of olfactory receptor channels.