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

Abstract

Abstract A novel potassium ion current induced by muscarinic stimulation (IKACh2) is characterized in atrial cardiomyocytes of teleost fishes (crucian carp, Carassius carassius L.; rainbow trout, Oncorhynchus mykiss W.) 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 and high concentrations (10-7 - 10-4 M). It can be distinguished from the classic IKACh, activated by βγ-subunit of the Gi-protein, due to its low sensitivity to Ba2+ ions and distinct current-voltage relationship with a very small inward current component. Ni2+ ions (5 mM) and KB-R7943 (10-5 M), 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 Ca2+, but is completely abolished by intracellular perfusion with K-+ free solution. Atropine (10-6 M), a non-selective muscarinic blocker, completely eliminated the IKACh2. A selective antagonist of M2 cholinoreceptors, AF-DX 116 (2×10-7M) and a M3 antagonist, 4-DAMP (10-9M), 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.