Interactions between the effects of yohimbine, clonidine and [Ca]o on the electrical response of the mouse vas deferens

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Abstract
1 Excitatory junction potentials (e.j.ps) were recorded from mouse vas deferens and resolved into families of ‘discrete events’ (d.es) reflecting intermittent release of packets of transmitter from one or a few sites. Within families d.es vary in amplitude between a few preferred values unaffected by any treatments used in these experiments. 2 As [Ca]o is raised from 1.1 to 4.0 mm there is a rise in d.e. amplitude due to an increase in the frequency of large events and a decrease in that of small. 3 At all [Ca]o clonidine reduces d.e. amplitude by increasing failures and small events and decreasing large events. Yohimbine has opposite effects. Both drug effects are concentration-dependent in the range 5 × 10−9-10−6 m. 4 As [Ca]o is raised from 1.1 to 4.0 mm, and therefore more natural agonist is released, clonidine becomes more effective at altering d.e. amplitude whereas yohimbine becomes less so. 5 With very low frequency stimulation yohimbine elevates e.j.p. amplitude only if [Ca]o is below 1.6 mm. 6 These results are not easily compatible with the notion that yohimbine breaks a ‘negative feedback’ control of transmitter release.