Inhibition of small conductance K⁺-channels attenuated melatonin-induced relaxation of serotonin-contracted rat gastric fundus

The aim of this study was to investigate the effects of melatonin on rat gastric fundus smooth muscle. Melatonin (10^-4 to 10^-3 M) had no effect on the basal tone of gastric smooth muscle. After precontraction with carbachol (10^-6 M) or serotonin (10^-7 M), melatonin caused a concentration dependent inhibitory action. The half maximal effect on serotonin-induced contraction was found with 1.12 ± 0.86 x 10^-5 M of melatonin. Increasing concentrations of melatonin (10^-5 to 10^-3 M) resulted in a right shift of the serotonin concentration response curve (10^-10 to 10^-5 M). This inhibitory effect of melatonin was partially blocked in the presence of apamin (10^-10 to 10^-7 M), a specific blocker of the small conductance calcium-dependent potassium channel, but not in the presence of other potassium channel blockers like charybdotoxin (10^-8 M), glibenclamide (10^-5 M), or tetraethylammonium (ODQ, 10^-4 M). The inhibitory effect was not changed in the presence of the neuronal blocker tetrodotoxin (10^-6 M), the selective P₂-receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (3 x 10^-5 M), the nitric-oxide synthase inhibitor N-nitro-L-arginine (3 x 10^-4 M), or the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (10^-4 M), suggesting that neither the purinergic, nitrergic, nor guanylate cyclase pathways were involved. We further investigated inhibitory responses to electrical field stimulation (EFS) at different frequencies under non-adrenergic, non-cholinergic (NANC) conditions on a serotonin-induced contraction in the presence of melatonin (10^-5 to 10^-4 M). Melatonin significantly reduced these inhibitory NANC responses in higher (8-32 Hz), but not lower (05-4 Hz), frequencies (16 Hz without melatonin, 103 ± 6.3%; melatonin 10^-5 M, 80.4 ± 7.5%; melatonin 10^-4 M, 39.1 ± 17.1%). Melatonin had no effect on contractile responses induced by EFS under basal tone. These results demonstrate that the inhibitory effect of melatonin in rat gastric fundus smooth muscle is apamin sensitive, but is not affected by other potassium channel blockers. This suggests that melatonin may be another transmitter candidate for the apamin sensitive responses within the gastrointestinal tract.