Activation of f‐channels by cAMP analogues in macropatches from rabbit sino‐atrial node myocytes

Abstract

The action of the two diastereometric phosphorothioate derivatives of cAMP, Rp‐cAMPs and Sp‐cAMPs, was investigated on hyperpolarization‐activated ‘pacemaker’ current (if) recorded in inside‐out macropatches from rabbit sino‐atrial (SA) node myocytes. When superfused on the intracellular side of f‐channels at the concentration of 10 μm, both cAMP derivatives accelerated if activation; their action was moderately less pronounced than that due to the same concentration of cAMP. The measurement of the if conductance‐voltage relation by voltage ramp protocols indicated that both cAMP analogues shift the activation curve of if to more positive voltages with no change in maximal (fully activated) conductance. Dose–response relationships of the shift of the if activation curve showed that both Rp‐cAMPs and Sp‐cAMPs act as agonists in the cAMP‐dependent direct f‐channel activation. Fitting data to the Hill equation resulted in maximal shifts of 9.6 and 9.5 mV, apparent dissociation constants of 0.82 and 5.4 μm, and Hill coefficients of 0.82 and 1.12 for Sp‐cAMPs and Rp‐cAMPs, respectively. The activating action of Rp‐cAMPs, a known antagonist of cAMP in the activation of cAMP‐dependent protein kinase, confirms previously established evidence that f‐channel activation does not involve phosphorylation. 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.

Hyperpolarization‐activated cationic currents (Ih) in neurones of the trigeminal mesencephalic nucleus of the rat

“…Numerous studies have shown that elevation of cAMP by transmitter receptors coupled to Gs proteins can shift the voltage dependence of Ih in neurones and in cardiac myocytes (see Pape, 1996, for review). Recent data in cardiac myocytes and sensory neurones show that these actions may be the direct effects of cAMP on Ih channels (DiFrancesco & Tortora, 1991;Ingram & Williams, 1996;Bois et al 1997), and the overall contribution of protein kinases and the direct effect of cAMP on Ih in many cell types remains unclear. In initial studies on MNV neurones we have demonstrated only a modest effect of cAMP elevation using forskolin, amounting to a 5 mV depolarizing shift in the V½ for Ih activation (Khakh & Henderson, 1997a).…”

Hyperpolarization-Activated Cation Channels: From Genes to Function

“…It has been long known that native I h as well as heterologously expressed HCN currents display a rapid rundown when measured in excised patches or during prolonged whole cell recordings (43,85,118,119,122,227). This effect is caused by an ϳ30 to 50 mV hyperpolarizing shift of the activation curve.…”

Developmental changes in functional expression and β-adrenergic regulation of If in the heart of mouse embryo

“…In the adult mammalian heart, the binding of -adrenergic agonist toadrenergic receptor ( -AR) is coupled to an intracellular cascade by the stimulatory G protein, Gs. Agonist occupancy activates AC to increase intracellular concentrations of cAMP, which stimulates fchannels directly in sino-atrial node [25], [26]. But in Purkinje fibres, instead of the direct interaction between cAMP and f-channels, the phosphorylation of the channels is involved [27], [28].…”