In anaesthetized and curarized rabbits, the cervical sympathetic nerve (CSN) stimulation induces in jaw elevator muscles a tension response which can be mimicked by the intravenous injection of adrenaline, noradrenaline and phenylephrine. This response, previously described and attributed to the contraction of muscle spindle fibres, is entirely mediated by α-adrenoceptors. The administration of phenoxybenzamine (2.5-3.5 mg/kg) markedly inhibits the responses to the sympathetic stimulation and to the injection of adrenergic agonists. Rauwolscine (1 mg/kg) reduces the development of tension induced by both CSN stimulation and noradrenaline injection without significantly affecting the response to phenylephrine. These data suggest the presence of postsynaptic α1- and α2-madrenoceptors in intrafusal muscle fibres. Moreover, the possibility that α2-adrenoceptors may also have an extrasynaptic location is entertained.
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