Electrophysiological Characterization of Cardiac Muscarinic Acetylcholine Receptors: Different Subtypes Mediate Different Potassium Currents

Abstract

To characterize electrophysiologically the K⁺ currents mediated by various mAChR subtypes, we performed detailed whole-cell patch-clamp studies in canine atrial myocytes. I_KACh was induced by 1 mM ACh (acetylcholine) or by arecaidine but-2-ynyl ester tosylate (100 nM, an M₂ receptor selective agonist) and was blocked by methoctramine (20 nM, an M₂ receptor selective antagonist). Tetramethylammonium (0.5 mM) activated a K⁺ conductance with delayed rectifying properties (I_KM3) and the currents were highly sensitive to 4-diphenylacetoxy-N-methylpiperidine methiodide (2 nM, an M₃ receptor inhibitor). 4-aminopyridine (1 mM) induced a delayed rectifier-like current (I_K4AP) which was selectively suppressed by tropicamide (200 nM, an M₄ receptor blocker). The current waveforms, I-V relationships, steady-state voltage-dependence, kinetics and pharmacological properties of these three currents were different from one another and distinct from the classical delayed rectifier K⁺ currents (I_Kr and I_Ks). Both I_KACh and I_K4AP were sensitive to pertussis ntoxin, whereas I_KM3 was not. Isoproterenol (1 mM) markedly depressed I_KM3, but increased I_K4AP and did not alter I_KACh. The effects of isoproterenol were reversed by propranolol (1 mM); and ACh completely suppressed I_KM3 and I_K4AP. The results suggest that the K⁺ currents mediated by different subtypes of mAChR represent different populations of K⁺ channels and that the cholinergic regulation of the heart’s electrical function is a consequence of activating multiple mAChRs linked to different effector systems with potentially varying signal transduction.

Negative inotropic and chronotropic effects on the guinea pig atrium of extracts obtained from Averrhoa carambola L. leaves

“…In heart muscle, stimulation of opioid (µ, κ, and δ) or muscarinic (M 2 ) receptors may mediate a reduction in cardiac contractility (16)(17)(18)(19), a result that can also be obtained by activating the ß 3 -adrenoceptor. This effect involves the nitric oxide synthase pathway (20,21).…”

Atrial tachycardia induces remodelling of muscarinic receptors and their coupled potassium currents in canine left atrial and pulmonary vein cardiomyocytes

“…These studies have provided functional, pharmacological and molecular evidence suggesting that I KM2 , I KM3 and I KM4 represent currents that are coupled to the three corresponding subtypes of mAChRs. APD is strikingly shortened by choline, an effect reversible by washout or the application of 4-DAMP, an M 3 receptor blocker (Shi et al, 1999b(Shi et al, , 2003. 4-AP also produces resting membrane potential hyperpolarization and action potential shortening, consistent with enhanced K þ conductance (Navarro-Polanco and SanchezChapula, 1997).…”

“…Representative I KM4 recordings are illustrated in Figure 5a and have typical characteristics (Shi et al, 2003). I KM4 step current was significantly reduced by atrial tachypacing (Figure 5b), with a mean overall reduction of 4872%.…”

Pharmacological Conversion of a Cardiac Inward Rectifier into an Outward Rectifier Potassium Channel

“…Based on their sensitivity to muscarinic antagonists and to pretreatment with pertussis toxin, it was suggested that the current induced by 4-AP was mediated by M 4 muscarinic receptor (Shi et al, 1999a(Shi et al, , 2003. The current evoked by choline, tetramethylammonium, and the muscarinic agonist pilocarpine (Pilo) was proposed to be mediated by the M 3 muscarinic receptor, and the current was named I KM3 (Shi et al, 1999a(Shi et al, ,b, 2003Wang et al, 1999Wang et al, , 2004.…”