DESENSITIZATION BY NORADRENALINE OF RESPONSES TO STIMULATION OF PRE‐ AND POSTSYNAPTIC ADRENOCEPTORS

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Abstract
1 The effect of exposing isolated preparations of rat aortic strip, rat atria and mouse vas deferens to perfusions of Krebs solution containing various concentrations of noradrenaline on their sensitivity to the drug has been determined. 2 The responses evoked by stimulation of postsynaptic adrenoceptors in all the tissues and presynaptic α-adrenoceptors in the mouse vas deferens were diminished by the perfusion of noradrenaline through the organ bath for 30 min. 3 The concentration of noradrenaline required to produce desensitization was higher in the mouse vas deferens than in the other tissues and more was required to desensitize the chronotropic responses than the inotropic responses in rat isolated atria. 4 The inclusion of cocaine (10−5m) in the bathing solution to block uptake1 increased the sensitivity of most tissues to noradrenaline. With the possible exception of the response to stimulation of presynaptic receptors in the mouse vas deferens, desensitization was somewhat increased in its presence. 5 When uptake2 was blocked by oestradiol (10−5m), it was not possible to desensitize the contractor responses of the aortic strip and vas deferens to exogenous noradrenaline, nor the inotropic response of the atria to the drug. However, oestradiol failed to block the desensitization of chronotropic responses and responses to stimulation of presynaptic receptors in the vas deferens. 6 Blockade of monoamine oxidase (MAO) with iproniazid (7.2 × 10−4m) or with pargyline (5 × 10−4 m) did not affect the desensitization process in the aortic strip. 7 Blockade of catechol-O-methyltransferase (COMT) with U-0521 (5.3 × 10−5m) greatly increased desensitization in the aortic strip and desensitization of inotropic responses in the atria. It had no effect on desensitization of chronotropic responses. Its effect on responses in the mouse vas deferens was not determined. 8 The perfusion of methoxamine at concentrations about 1000 times higher than those of noradrenaline also produced desensitization in the aortic strip. 9 The desensitization of presynaptic receptors in the mouse vas deferens was shown to be specific and that of the responses to postsynaptic receptor stimulation to be non-specific. 10 It is concluded that responses to adrenoceptor stimulation may be desensitized by accumulation of noradrenaline inside the cells bearing the receptors and that the desensitization is caused by noradrenaline itself not by a metabolite. Desensitization may also be caused without accumulation of noradrenaline in uptake2 and for some receptors these may not be alternative mechanisms.