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1 Division of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia
2 Department of Physiology, Monash University, Clayton, Victoria, Australia
3 Department of Pharmacology, Graduate School of Medical Sciences, Kyushu University, Fukuoka, 812-8582, Japan
Intracellular recordings were made from either sheets or isolated bundles of the circular muscle layer of guinea-pig proximal colon and the responses evoked by stimulating inhibitory nerve fibres were analysed. Inhibitory junction potentials (IJPs), evoked by single stimuli, had two components which could be separated on their pharmacological and temporal characteristics and their voltage sensitivities. The initial component, which was abolished by apamin and reduced in amplitude by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), had a brief time course: its amplitude was changed when the external concentration of potassium ions ([K+]o) was changed. The second component of the IJP had a slower onset than the first component, was abolished by L-nitroarginine (NOLA) and oxadiazolo quinoxalin-1-one (ODQ), an inhibitor of soluble guanylate cyclase: its amplitude was little affected by changing [K+]o and was increased when the membrane potential of the circular layer was hyperpolarized. The observations suggest that the initial component of the IJP results from the release of ATP which triggers an increase in membrane conductance to K+ and that the second component results from the release of nitric oxide which suppresses a background inward current.
(Received 21 March 2004;
accepted after revision 11 June 2004;
first published online 11 June 2004)
Corresponding author G. D. S. Hirst: Division of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia. Email: david.hirst{at}anu.edu.au
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