Effect of apamin on the responses to VIP, ATP and NANC neurone stimulation in the rat and cat gastric fundus

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Abstract
1. The influence of apamin on non-adrenergic non-cholinergic (NANC) relaxation and on the effect of the putative transmitters VIP and ATP was examined in the rat and cat gastric fundus. 2. In longitudinal muscle strips of the rat gastric fundus, ATP induced a biphasic effect, relaxation followed by contraction. The relaxant effect of ATP was blocked by apamin, whereas the relaxations induced by VIP and NANC neurone stimulation were not influenced by apamin. 3. In circular and longitudinal muscle strips of the cat gastric fundus, ATP only induced relaxation at high concentrations. The ATP-induced relaxation was increased in the presence of apamin, whereas the VIP-induced and NANC relaxations were not influenced. 4. It is concluded that the relaxant effect of ATP might be related to activation of apamin-sensitive Ca(2+)-dependent K(+)-channels in the rat gastric fundus. In the cat gastric fundus, apamin did not antagonize the relaxant effect of ATP so that it cannot be used to investigate an ATP involvement in inhibitory NANC neurotransmission in this tissue. No evidence for a presynaptic inhibitory action of apamin on VIP-ergic NANC neurones was obtained.