[Regulation of muscarinic potassium channels]. / Régulation des canaux potassiques muscariniques.

Inwardly rectifying muscarinic potassium channels are directly activated by M2 muscarinic receptors in heart via a Pertussis toxin-sensitive G-protein. An intracellular second messenger is not involved in their activation. These channels undergo rapid (in about 30 seconds) and short-term desensitization. This desensitization is reversible in about 5 minutes. The molecular events underlying muscarinic potassium channels desensitization are still under study and apparently involve a phosphorylation/dephosphorylation process of one the proteins, i.e. the receptor, the G-protein or the channel itself.

Regulation of the cardiac delayed rectifier K current by neurotransmitters and magnesium.

The delayed rectifier K current plays an important role in cardiac electrophysiology: It is involved in the repolarization of the action potential and in frequency-dependent changes in action potential duration and waveform. The delayed rectifier current IK is regulated by the autonomic nervous system: Beta-adrenergic agonists increase IK. This increase is due to an increase in the maximally activatable current as well as a shift of the activation curve to more negative potentials. Thus, in response to sympathetic nerve stimulation, the action potential would be expected to repolarize more rapidly as a result of activation of more IK current and its activation at more negative potentials. Single-channel analysis suggests that the increase in IK is due to an increase in the availability of IK channels to respond to depolarization. IK is also regulated by internal free Mg2+. When the internal solution contains high [Mg2+], IK decreases, whereas low [Mg2+] results in an increase in current. The effect of Mg2+ is not detectably voltage dependent, suggesting that the mechanism of Mg2+ action involves an allosteric or enzymatic effect. Mg2+ also affects the rate of washout of the response to beta-adrenergic agonists, suggesting that Mg2+ may be affecting the activity of a protein phosphatase.

Effects of Mg2+ on basal and beta-adrenergic-stimulated delayed rectifier potassium current in frog atrial myocytes.

1. The effects of internal Mg2+ ions on the delayed rectifier potassium current (IK) of bull-frog atrial myocytes were studied using the whole-cell configuration of the patch-clamp technique with a perfusable patch electrode. 2. Initial variations in IK amplitude were dependent on [Mg2+]i. With [Mg2+] greater than 1 mM, the amplitude of IK usually decreased after initiating the whole-cell recording configuration (run-down); with [Mg2+]i less than 1 mM, IK usually increased (run-up). Mg2+ blocked IK with an apparent half-maximal effect of 0.6 mM [Mg2+]i. 3. The basal free [Mg2+]i, indicated by the amplitude of IK before run-up or run-down, was estimated from the relationship between [Mg2+]i and IK to be 0.8 mM. 4. The amplitude of both the activation curve and the instantaneous voltage-current relationship was decreased by increasing [Mg2+]i. Under these conditions, the voltage dependence of IK was not affected. 5. The rate of activation of the current at +40 mV was slowed by increasing [Mg2+]i with little effect on the rate of deactivation at -50 mV. This is in contrast to the effects of isoprenaline, which speeded activation and slowed deactivation. 6. Isoprenaline increased IK on average by about 2.5 pA/pF, whether IK had previously run down or not, and regardless of [Mg2+]i. The reversibility of isoprenaline was partially inhibited at [Mg2+]i less than 1 mM. 7. It is concluded that Mg2+ affects IK via several mechanisms that might include a Mg(2+)-dependent phosphatase.

Single delayed rectifier channels in frog atrial cells. Effects of beta-adrenergic stimulation.

The patch-clamp technique with two pipettes was used to record single delayed K+ channels (cell-attached electrode) and to control the potential and the composition of the intracellular compartment (whole-cell electrode). With 30 microM cAMP in the cell and physiological potassium concentrations inside and outside the patch, a channel carrying an outward current was characterized. Its open probability was very low and the channel was recorded in only 5% of patches under control conditions. Increasing intracellular cAMP increased the probability of finding a channel in a patch 10-fold. The channel had the characteristics expected of a delayed rectifier channel. The time-course of its ensemble average resembled the whole-cell current in the same cell. The current-voltage relationship exhibited inward rectification, with a slope conductance of 20 pS in the linear portion and a reversal potential close to EK. Both the open- and the closed-time distributions were described by the sum of two exponentials, suggesting a complicated gating scheme involving two closed states and two open states. The beta-adrenergic stimulation did not change the conductance of the channel, but increased its probability of opening.

Modulation of the delayed rectifier potassium current in frog cardiomyocytes by beta-adrenergic agonists and magnesium.

1. The regulation of IK and ICa were studied in single cells isolated from bull-frog atrium using the whole-cell configuration of the patch clamp and a perfused patch pipette. 2. IK was increased approximately 50-100% and ICa was increased approximately 6-10 times by 1 microM-isoprenaline, 5 microM-forskolin, or internal perfusion with 30 microM-cyclic AMP. The effects of cyclic AMP and isoprenaline were not additive. The shape of the concentration-response curves and the EC50 values for the effects of cyclic AMP on ICa and on IK were very similar (2.3 microM for IK and 1.7 microM for ICa). 3. Elevation of intracellular cyclic AMP had a similar effect on IK regardless of whether ICa was blocked with Cd2+ or not. Increasing ICa with dihydropyridine Ca2+ channel agonists had no effect on IK amplitude. 4. Isoprenaline or cyclic AMP caused an increase in the fully-activated IK and also shifted the activation curves to more negative potentials in most cells. The shift in the activation curve was reversible and was also observed when ICa was blocked with Cd2+. The rate of activation of IK was increased and the rate of deactivation of IK was slowed by isoprenaline. 5. After breaking the membrane patch and initiating whole-cell recording, IK ran down with time in about 50% of the cells examined when the intracellular solution contained 1 mM [Mg2+]. In contrast, when the solution contained 0.3 mM [Mg2+], rundown was almost never observed. Internal perfusion with increasing concentrations of [Mg2+] caused reversible decreases in the maximum amplitude of IK and shifted the IK activation curve slightly to more negative potentials, but had negligible effects upon the shape or the curvature of the fully activated current-voltage relationship.