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

Abstract

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. 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. 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. 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. With very low frequency stimulation yohimbine elevates e.j.p. amplitude only if [Ca]o is below 1.6 mm. These results are not easily compatible with the notion that yohimbine breaks a ‘negative feedback’ control of transmitter release.

Electrical and optical study of nerve impulse-evoked ATP-induced, P2X-receptor-mediated sympathetic neurotransmission at single smooth muscle cells in mouse isolated vas deferens

“…The cause of the variation at the level of the single release site is not presently known. First, the exogenous Ca 2+ concentration strongly influences P DE , which increased from 0.34±0.04 at 1.1 mM Ca 2+ to 0.53±0.03 at 2.1 mM ( Blakeley et al, 1986 ). Second, the stimulus frequency is positively correlated with P DE .…”

Mechanisms Regulating the Expression and Function of Acetylcholine Receptor

“…There is good reason to assume that the two physiological control mechanisms operate by altering the probability of monoquantal release (Stj11rne 1985; however, for the view that they control the' quantal content', Le., the number of quanta the nerve impulse releases from a varicosity, see Blakeley et al 1984Blakeley et al , 1986. Pharmacological block of prejunctional arautorecptors increases the secretory response to nerve impulses at low frequency, by up to 5-fold.…”