Relaxation of smooth muscle following contraction elicited by sympathetic nerve stimulation in vivo

Methoxyphenamine inhibits basal and histamine-induced nasal congestion in anaesthetized rats

1. Nasal resistance in anaesthetized rats was assessed by measuring air overflow during ventilation of the nasal passages at constant pressure. Nasal basal resistance was reduced in a dose-dependent manner by methoxyphenamine hydrochloride (0.01-30 mg kg-1, i.v.), pseudoephedrine hydrochloride (0.03-3 mg kg-1, i.v.) and adrenaline bitartrate (0.01-3 micrograms kg-1, i.v.). Both methoxyphenamine and pseudoephedrine were less potent and less efficacious than adrenaline but caused longer-lasting responses. 2. Nasal congestion induced by histamine (0.2% nebulised solution passed into the nasal passages for 15 s) was inhibited by i.v. administration of methoxyphenamine, pseudoephedrine, adrenaline, methoxamine or tyramine: the ID50s against 0.2% histamine-induced nasal congestion were 1.16 (95% confidence limits; 0.5, 1.8) mg kg-1, 0.25 (0.19, 0.33) mg kg-1, 0.037 (0.018, 0.06) micrograms kg-1, 8.12 (6.74, 9.65) micrograms kg-1 and 30.6 (26.1, 35.8) micrograms kg-1 respectively. 3. The inhibitory effects of both methoxyphenamine and tyramine on histamine-induced nasal congestion were reduced after administration of desmethylimipramine (0.1 and 1 mg kg-1, i.v.) or prazosin (0.1 and 0.3 mg kg-1, i.v.). Similarly, the inhibitory effects of methoxamine were reduced after prazosin (0.1 and 0.3 mg kg-1). 4. These results indicate that methoxyphenamine (1 mg kg-1, i.v.) inhibits histamine-induced nasal congestion in the rat. This action, at least in part, is probably indirect being mediated by release of neuronal noradrenaline which then acts on alpha 1-adrenoceptors.

Sympathetic vascular control of the pig nasal mucosa: adrenoceptor mechanisms in blood flow and volume control

1. The adrenoceptor mechanisms influencing the total blood flow, volume and superficial blood flow in the nasal mucosa of pigs anaesthetized with pentobarbitone have been characterized by use of various agonists and antagonists. 2. Local intra-arterial bolus injection of the selective alpha 1-agonist phenylephrine, the selective alpha 2-agonist UK 14.304, the mixed alpha 1/alpha 2-agonist oxymetazoline and the mixed alpha/beta-agonists noradrenaline (NA) and adrenaline induced dosed-related reduction of nasal arterial blood flow (BF), nasal mucosal volume (V, reflecting capacitance vessel function) and the laser Doppler flowmetry signal (LDF, reflecting superficial movement of blood cells). The rank order of alpha-agonist potency regarding BF reduction was UK 14.304 greater than oxymetazoline greater than phenylephrine = adrenaline. For the volume response the potency order was UK 14.304 greater than oxymetazoline = NA = adrenaline greater than phenylephrine while for the reduction of the LDF signal the potency was UK 14.304 = NA = adrenaline greater than oxymetazoline greater than phenylephrine. The selective beta 2-agonist terbutaline caused dose-dependent increase of BF whereas only a small augmentation of the V was obtained upon the highest dose (40 nmol) while no modification of the LDF signal was observed. 3. After pretreatment with the selective alpha 1-antagonist prazosin, the response to phenylephrine was abolished while the selective alpha 2-antagonist idazoxan attenuated the effect of UK 14.304. After pretreatment with alpha-antagonists, both NA and adrenaline caused biphasic effects with constriction followed by vasodilatation for BF, but not for V or LDF. This vasodilatation was blocked by the beta-antagonist propranolol. 4. The reduction in nasal BF and V upon sympathetic nerve stimulation was attenuated both by prazosin and idazoxan. Propranolol enhanced the remaining reduction of BF but not of V in the presence of alpha-antagonists. 5. It is concluded that alpha 2-adrenoceptor mechanisms in the pig nasal mucosa are dominating for the BF, V and LDF responses to exogenous agonists. alpha 1-Adrenoceptors also seem to be involved in the sympathetic control of BF, V and LDF. Activation of beta 2-receptors increases mainly BF and does not influence the LDF signal.

The influence of an extraneuronal compartment on the relaxation of the cat nictitating membrane in vivo

1 Contractions of the cat nictitating membrane were elicited on stimulation of the internal carotid nerve, and the effects were studied of desipramine and two inhibitors of catechol-O-methyltransferase, U-0521 and pyrogallol, on the subsequent relaxation of the muscle. 2 The relaxation of the nictitating membrane occurred in at least two phases. The late phase of relaxation was prolonged after increase in the period of nerve stimulation and the duration of this phase was further prolonged after treatment with pyrogallol. 3 After inhibition of neuronal uptake of noradrenaline with desipramine both the early and late phases of relaxation were increased in duration, and subsequent administration of pyrogallol or U-0521 caused a further increase in the duration of the late phase of relaxation. 4 The results suggest that the late phase of relaxation of the nictitating membrane is influenced by efflux of noradrenaline from an extraneuronal pool.