inhibitor 2-[(4-methylphenyl)sulfonylcarbamido]-1-(4-nitrobenzyl)pyridinium bromide (2-AP27) is a muscarinic M₂ receptor antagonist

Aminopyridines are known to inhibit acetylcholine-activated K⁺ current (I_KACh) in cardiac myocytes. The aim of this study was to examine the effect of 2-aminopyridine sulfonylcarbamide derivative 2-AP27 on isoprenaline-stimulated L-type Ca²⁺ current (I_CaL) and to identify whether 2-AP27 acts via blocking of muscarinic M₂-receptors in frog cardiomyocytes. The whole-cell configuration of the patch-clamp technique was used to record I_CaL in enzymatically isolated cardiac myocytes. Isoprenaline (0.1 μM), an agonist of β₁-β₂-adrenoreceptors, stimulated the I_CaL up to 475±61% (n=4) (P<0.05) vs. control. Then, in the first series of experiments, carbachol (0.01 μM), an agonist of M2 muscarinic receptors, reduced the stimulatory effect of isoprenaline to 42±15% vs. isoprenaline alone. 2- AP27 (100 μM) alone completely abolished the inhibitory effect of carbachol on isoprenaline-stimulated ICaL, which recovered to 95±5.8% of the effect of isoprenaline. In the second series of experiments, adenosine (1 μM), an agonist of A₁-adenosine receptors, reduced the stimulatory effect of isoprenaline on I_CaL to 56±10% (n=3) (P<0.05). Then 2-AP27 (100 μM) applied in the presence of adenosine, had no effect on I_CaL, which remained at 51±7.9% (n=3) (P<0.05) of the effect of isoprenaline. These results suggest that 2-AP27, a new derivative of 2-AP, containing 4-toluolsulfonylcarbamide instead of amino group and quaternizated nitrogen by 4-nitrobenzylbromide in pyridine ring, is acting as an antagonist of muscarinic M₂ receptors in frog ventricular myocytes.