Effect of labetalol and guanethidine on contractile responses to acetylcholine in the rat anococcygeus muscle

The effects of iprindole on noradrenergic transmission in the rat anococcygeus muscle

The effects of iprindole on the contractile responses to field stimulation and to exogenously applied agents have been studied in the rat anococcygeus muscle. In addition the effects on the spontaneous overflow of 3H and that induced by field stimulation (following preloading of the tissue with [3H]-noradrenaline) are reported. Iprindole (10(-7)-5 X 10(-6) M) had no effect on the contractile responses to acetylcholine. Iprindole at 10(-7) or 10(-6) M was equieffective in potentiating the contractile responses to either field stimulation or to noradrenaline. With a higher concentration of iprindole (5 X 10(-6) M) the potentiation was greater. The responses to oxymetazoline were potentiated and inhibited by iprindole at 10(-8) and 10(-7) M, respectively. Iprindole at 10(-6) M potentiated the responses to tyramine. The effects of iprindole at 10(-7) and 5 X 10(-6) M on the responses to field stimulation and to noradrenaline were not altered by the presence of ouabain (5 X 10(-5) M), neostigmine (10(-6) M) or yohimbine (10(-7) M). In the presence of desipramine (10(-6) M), iprindole at 10(-7) M potentiated the contractile responses to field stimulation and to noradrenaline. The higher concentrations of iprindole (10(-6) and 5 X 10(-6) M) inhibited the contractile responses to field stimulation and to noradrenaline in the presence of desipramine. Iprindole (10(-7) M) had no effect on the relaxant responses to field stimulation. Iprindole at 10(-7) or 10(-6) M had no effect on the spontaneous overflow of 3H or that induced by field stimulation at 2 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of antimuscarinic agents on the contractile responses to cholinomimetics in the rat anococcygeus muscle

1 The effects of antimuscarinic agents alone and in the presence of neostigmine on the contractile responses to exogenously applied cholinomimetics or (-)-noradrenaline were studied in the rat anococcygeus muscle.2 Atropine (1 x 10(-9) -1 x 10(-6)M) alone, in the presence of hexamethonium (1 x 10(-4)M), or phentolamine (1 x 10(-6)M), inhibited responses to acetylcholine but not to (-)-noradrenaline. The inhibitory effect with the higher concentrations of atropine (1 x 10(-8) - x 10(-6)M), was seen as an increase in the slopes of the concentration-response curves. Atropine (1 x 10(-8)M) alone inhibited the responses to methacholine and carbachol without altering the slopes of the concentration-response curves.3 Homatropine (1 x 10(-6)M) alone had no effect on responses to (-)-noradrenaline and inhibited responses to acetylcholine and methacholine. The inhibitory effect on responses to acetylcholine but not to methacholine, included an increase in the slopes of the concentration-response curves.4 Neostigmine (1 x 10(-6)M) alone had no effect on responses to (-)-noradrenaline and potentiated responses to acetylcholine and methacholine. The potentiating effect included an increase in the slopes of the concentration-response curves.5 In the presence of neostigmine (1 x 10(-6)M), atropine (1 x 10(-9)M - 1 x 10(-6)M) caused a parallel concentration-dependent shift of the concentration-response curves to acetylcholine. The pA(2) values, in the presence of neostigmine, were independent of the concentration of atropine and of the agonist (acetylcholine, methacholine, or carbachol) used. In the presence of neostigmine (1 x 10(-6)M), homatropine (1 x 10(-6)M) also failed to alter the slopes of the concentration-response curves to acetylcholine and was approximately 100 times less potent than atropine as an antimuscarinic agent.6 These results illustrate that, in the rat anococcygeus muscle, it is necessary to inhibit acetylcholinesterase before determining the relative potencies of antagonists at muscarinic receptors.

Effect of mianserin on noradrenergic transmission in the rat anococcygeus muscle

1 The effects of mianserin on the accumulation of (-)-[(3)H]-noradrenaline and on contractile responses to field stimulation, exogenously applied (-)-noradrenaline, and to tyramine were studied in the rat anococcygeus muscle.2 Mianserin (10(-9) to 10(-5) M) and nortriptyline (10(-9) to 10(-5) M) inhibited the accumulation of (-)-[(3)H]-noradrenaline. In this aspect mianserin had a similar potency to nortriptyline, the most potent tricyclic antidepressant in inhibiting noradrenaline accumulation in this tissue.3 Mianserin 10(-9) or 10(-8) M alone had no effect on contractile responses to field stimulation and to (-)-noradrenaline but inhibited the responses to tyramine. The responses to field stimulation at low frequencies and to (-)-noradrenaline were potentiated by 10(-7) and 10(-6) M mianserin. It is suggested that the inhibitory effect mianserin has on neuronal accumulation is primarily responsible for these effects. Mianserin 10(-5) M inhibited responses to field stimulation and to (-)-noradrenaline.4 In the presence of nortriptyline (10(-6) M), the contractile responses to field stimulations were potentiated by mianserin (10(-8), 10(-7) and 10(-6) M), 10(-8) M being the most potent in this aspect. Mianserin 10(-8), 10(-7), 10(-6) and 10(-5) M had a similar inhibitory effect on responses to (-)-noradrenaline. In the absence of neuronal uptake, the potentiating effect of mianserin on responses to field stimulation may be due to antagonism at presynaptic alpha-adrenoceptors. In the presence of 10(-6) M nortriptyline, 10(-5) M mianserin abolished responses to field stimulation.5 Following incubation of the tissue in the presence of 6-hydroxydopamine (10(-3) M for 3 h), mianserin (10(-7), 10(-6) and 10(-5) M) nortriptyline (10(-7) and 10(-6) M) and phentolamine (5 x 10(-8) and 5 x 10(-7) M) inhibited contractile responses to (-)-noradrenaline. This illustrates the ability of these agents to inhibit the responses to noradrenaline at a postsynaptic site. The inhibitory effect was dose-related with nortriptyline and phentolamine; this illustrates the ability of these agents to antagonize postsynaptic alpha-adrenoceptors. The inhibitory effect observed with mianserin was not dose-related. This suggests that in addition to its reported ability to antagonize postsynaptic alpha-adrenoceptors, mianserin may have another post-synaptic action at the level of, or distal to, the alpha-adrenoceptor.6 These results illustrate that, in the rat anococcygeus muscle, mianserin is a potent inhibitor of noradrenaline accumulation and may be an antagonist at presynaptic alpha-adrenoceptors. Mianserin also inhibits the responses to exogenously applied noradrenaline in this tissue by an action or actions at the level of, or distal to, the postsynaptic alpha-adrenoceptor.

Effects of pyroxamidine and guanethidine on contractile responses to field stimulation and to noradrenaline in the anococcygeus muscle and vas deferens of the rat

The effects of pyroxamidine (EMD 21192) and guanethidine on contractile responses were studied in the anococcygeus muscle and vas deferens of the rat. Pyroxamidine (10−6 and 10−5 M) and guanethidine (6 times 10−6 and 10−5 M) potentiated the responses to low concentrations of acetylcholine in the rat anococcygeus muscle. Following incubation of the muscle with 6-hydroxydopamine (10−3M for 3 h), pyroxamidine (10−5 M) and guanethidine (10−5 M) had no effect on responses to acetylcholine. This suggests that the potentiating effect of pyroxamidine and guanethidine on responses to acetylcholine is due to the release of subthreshold concentrations of noradrenaline.

In the anococcygeus, pyroxamidine (10−6 and 10−5 M) and guanethidine (10−6 and 10−5 M) inhibited responses to field stimulation and potentiated responses to exogenously applied (−)-noradrenaline. The responses to field stimulation in the vas deferens were also inhibited by 10−5M pyroxamidine and by 10−5 M guanethidine. 10−6 M guanethidine, but not 10−5 M pyroxamidine, potentiated responses to (−)-noradrenaline in the vas deferens. In the presence of nortriptyline (10−6 M), a potent inhibitor of neuronal uptake, the inhibitory effects of pyroxamidine and guanethidine on responses to field stimulation were reduced or reversed and these drugs had no effect on responses to (−)-noradrenaline. This suggests that pyroxamidine is a noradrenergic neuron blocker and that its action is dependent on continued neuronal uptake. Following 6-hydroxydopamine incubation, 10−5 M pyroxamidine and 10−5 M guanethidine inhibited the responses to (−)-noradrenaline in the rat anococcygeus muscle. Thus it seems likely, at high concentrations, that these compounds have postsynaptic blocking activity.

Mianserin potentiates responses to acetylcholine in the rat anococcygeus muscle

In the rat anococcygeus muscle, mianserin (1 and 10 muM) increased the magnitude of the maximal contractile responses to acetylcholine and carbamylcholine without affecting the maximal responses to (--)-noradrenaline. The potentiating effect of mianserin on responses to acetylcholine was maintained in the presence of 5muM phentolamine, following 6-hydroxy dopamine incubation of the tissues (1 mM for 3h) and in the presence of 1 muM nortriptyline. It is suggested that mianserin increases the sensitivity to acetylcholine by an action at the level of, or distal to, the cholinergic receptor.

Release of noradrenaline by labetalol in the rat anococcygeus muscle

The effects of labetalol on the accumulation and spontaneous release of (--)-[3H]noradrenaline, and on contractile responses to exogenously applied (--)-noradrenaline were studied in the isolated anococcygeus muscle of the rat. 1. Labetalol (3 x 10(-7)--10(-4)M) inhibited the accumulation of (--)-[3H]noradrenaline. 2. Labetalol (10(-6)--10(-4)M) and guanethidine (6 x 10(-6) M) increased the spontaneous release of [3H] following incubation of the muscle with (--)-[3H]noradrenaline. Nortriptyline (10(-6) M) had no effect on the spontaneous release of [3H], antagonised the increased release of [3H] produced by 6 x 10(-6) M guanethidine but not that observed with 10(-5) M labetalol. Labetalol (5 x 10(-6) M) markedly increased the loss of tritiated deminated metabolites with little change in the loss of (--)-[3H]noradrenaline. 3. Labetalol (10(-7)--10(-5)M), alone or in the presence of 10(-6) M nortriptyline, had no effect on contractile responses to (--)-noradrenaline. 4. Following pretreatment with 6-hydroxydopamine (10(-3)M for 3 h) to deplete endogenous noradrenaline stores, labetalol (10(-7)--10(-5) M) inhibited responses to exogenously applied (--)-noradrenaline. 5. These results suggest that, in the rat anococcygeus muscle, labetalol is a noradrenaline releasing agent and an alpha-adrenoceptor antogonist.