Pharmacological properties of the atropine-resistant contraction of the carp (Cyprinus carpio) intestinal bulb induced by transmural stimulation

Abstract

1. The pharmacological properties of the atropine-resistant contraction of the carp intestinal bulb induced by transmural stimulation were investigated. 2. In the presence of atropine (1 microM), transmural stimulation caused frequency-dependent (2-50 Hz) contraction which was abolished by tetrodotoxin (780 nM). 3. The atropine-resistant contraction was not decreased by tubocurarine (5 microM), hexamethonium (100 microM), carteolol (5 microM) and phentolamine (5.4 microM). 4. In vitro pretreatment with guanethidine (10 microM for 1 hr) markedly decreased the noradrenaline and adrenaline contents of the carp intestinal bulb. The atropine-resistant contraction was not affected by pretreatment with guanethidine. 5. Diphenhydramine (1 microM), methysergide (3 microM) and naloxone (1 microM) did not decrease the atropine-resistant contraction, indicating that histamine, 5-hydroxytryptamine and opioid peptides were not involved in the atropine-resistant response. 6. These results indicate that a non-cholinergic, non-adrenergic excitatory nerve is present in the carp intestinal bulb. The neurotransmitter mediating the excitatory response could not be identified.