Effect of yohimbine on the contractile response of the mouse vas deferens to short, low frequency trains of nerve impulses.

A study was undertaken to ascertain why in the mouse vas deferens excitatory junction potentials facilitate whereas contractions depress during low frequency stimulation. In a set of contraction studies, where a conditioning stimulus was followed 2 s later by a test stimulus, depression in the first phase of contraction was observed only at high stimulus strengths when many nerves are activated leading to a large secretion. Low stimulus strengths did not produce depression, suggesting that the autoinhibitory effect due to released noradrenaline is absent. At supramaximal stimulus strengths short, low frequency (0.1-0.5 Hz) trains gave varying degrees of depression of the first phase at each stimulus frequency. This depression was reversed by yohimbine (10 microM), indicating that the level of autoinhibition depends on the total amount of noradrenaline secreted. In the absence of autoinhibition, facilitation could be predicted in terms of the residual Ca2+ hypothesis.