A new potassium ion current induced by stimulation of M2-cholinoreceptors in fish atrial myocytes

A novel potassium ion current induced by muscarinic stimulation (IKACh2) is characterized in atrial cardiomyocytes of teleost fishes (crucian carp, Carassius carassius; rainbow trout, Oncorhynchus mykiss) by means of the whole-cell patch-clamp technique. The current is elicited in atrial, but not ventricular, cells by application of carbamylcholine (CCh) in moderate to high concentrations (10(-7)-10(-4) mol l(-1)). It can be distinguished from the classic IKACh, activated by the βγ-subunit of the Gi-protein, because of its low sensitivity to Ba(2+) ions and distinct current-voltage relationship with a very small inward current component. Ni(2+) ions (5 mmol l(-1)) and KB-R7943 (10(-5) mol l(-1)), non-selective blockers of the sodium-calcium exchange current (INCX), strongly reduced and completely abolished, respectively, the IKACh2. Therefore, IKACh2 was initially regarded as a CCh-induced outward component of the INCX. However, the current is not affected by either exclusion of intracellular Na(+) or extracellular Ca(2+), but is completely abolished by intracellular perfusion with K(+)-free solution. Atropine (10(-6) mol l(-1)), a non-selective muscarinic blocker, completely eliminated the IKACh2. A selective antagonist of M2 cholinoreceptors, AF-DX 116 (2×10(-7) mol l(-1)) and an M3 antagonist, 4-DAMP (10(-9) mol l(-1)), decreased IKACh2 by 84.4% and 16.6%, respectively. Pertussis toxin, which irreversibly inhibits Gi-protein coupled to M2 receptors, reduced the current by 95%, when applied into the pipette solution. It is concluded that IKACh2, induced by stimulation of M2 cholinoceptors and subsequent Gi-protein activation, represents a new molecular target for the cardiac parasympathetic innervation.