The electrical and mechanical basis of co-transmission in some vascular and non-vascular smooth muscles

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Abstract
1 The intracellularly-recorded electrical and mechanical responses to field stimulation of intramural nerves in three sympathetically-innervated smooth muscles-the mouse vas deferens, the rabbit ear artery and the rabbit mesenteric bed preparation were investigated. 2 In each tissue there was evidence for co-transmission involving noradrenaline (NA) and adenosine 5''-triphosphate (ATP) or a closely related nucleotide. 3 The electrical response in each tissue consisted of excitatory junction potentials (ejps) which were abolished by .alpha., .beta.-methylene ATP (.alpha., .beta.MeATP, 1-10 .times. 10-6 M), suggesting that they were mediated by ATP. Only in the rabbit ear artery was there an additional electrical event mediated by NA. This took the form of a small slow membrane depolarization which followed the ejps and which was antagonized by either of the .alpha.-adrenoreceptor blocking agents phentolamine (1 .times. 10-6 M) or prazosin (1 .times. 10-7 M). 4 In the mouse vas deferens and rabbit mesenteric artery, both transmitter substances (NA and ATP) played a role in the contractile response to field stimulation. In the rabbit ear artery, NA alone appeared to mediate the contractile event. 5 Contractile responses to nerve-released ATP were accompanied by a membrane potential change, whereas those to NA appeared to be mediated largely by a voltage-independent mechanism. 6 In the mouse vas deferns, the ejps and action potentials evoked by field stimulation appeared to be mediated by a discrete increase in permeability to Na+ and K+. 7 In the mouse vas deferens, local application of bradykinin (1-100 .times. 10-7 M) produced a small, slow membrane hyperpolarization. VIP (1-100 .times. 10-7 M), neuropeptide Y (1-100 .times. 10-7 M) substance P (1-100 .times. 10-7 M), somatostatin (1-100 .times. 10-7 M), leu-enkephalin (100 .times. 10-7 M), metenkephalin (1-100 .times. 10-7 M) and bombesin (1-100 .times. 10-7 M), similarly applied, each produced no significant change in membrane potential. None of these peptides appear to be the transmitter mediating the ejp in this tissue.