Effect of sodium concentration and of atropine on the contractile response of the guinea-pig ileum to potassium ions.

The possibility that the guinea-pig ileum's contractile response to K+-rich solutions is partly mediated by acetylcholine release from the intramural nervous tissue was examined by studying the inhibition of that response by atropine at different values of [Na+]o. In a medium in which the NaCl was replaced by iso-osmotic glucose, the response to high [K+]o was not greatly affected, while the responses to acetylcholine and to other agonists were significantly reduced. In control medium (149 mM Na+), atropine (10(-7) M) partly inhibited the responses to K+-rich solutions and to agonists such as histamine, 5-hydroxytryptamine and bradykinin. When [Na+]o was reduced to 12 mM, by iso-osmotic substitution of glucose for NaCl, the response to high K+ was no longer inhibited by atropine, which still partly inhibited the contractions elicited by the three agonists and totally blocked the response of acetylcholine. It is proposed that atropine's inhibition of the response to high K+ and to agonists is not due to its specific anti-muscarinic effect, but to an unspecific action, which in the case of the agonists is independent of [Na+]o. In addition, the inhibition of the response to high K+ would results from a different Na+-dependent mechanisms, possibly involving the stimulation of the Na-K pump by atropine. This is supported by the observation that this drug partly relaxed ileum preparations that were contracted by ouabain.