A quantitative study of the anticholinergic action of several tricyclic antidepressants on the rat isolated fundal strip

1. Investigations were carried out on the antagonism of the action of carbamylcholine chloride on the isolated fundus of the rat stomach by several tricyclic antidepressants.2. The anticholinergic potency of the compounds was in the order: GP 45437>amitriptyline>protriptyline>desmethylimipramine>opipramol. All of the antagonists were less effective than atropine.3. Statistical analysis was carried out to determine whether the theory of competitive antagonism would fit the data obtained.     

Action of bretylium and guanethidine at the neuromuscular junction

Bretylium and guanethidine produced a block of neuromuscular transmission in the rat phrenic nerve diaphragm and cat sciatic gastrocnemius preparations, but had a potentiating effect on acetylcholine-induced contracture of the frog rectus. On the sciatic gastrocnemius preparation of the cat the compounds had a twofold action, consisting of an initial transient, and a delayed but more sustained, block of neuromuscular transmission. Bretylium and guanethidine had no immediate effect on the response of the muscle to direct stimulation, though a weak delayed depressant effect was observed. Intravenous injection of these compounds produced a flaccid paralysis in pigeons. Possible mechanisms of the neuromuscular blocking action of bretylium and guanethidine are discussed. Inhibition of acetylcholine release from motor nerve endings resulting from local anaesthesia by these compounds is suggested as a possible mechanism of their neuromuscular blocking action.     

Comparison of the effects of bretylium, guanethidine and bethanidine on smooth muscle responses to different rates of sympathetic nerve stimulation

The relative effects of bretylium, guanethidine and bethanidine on smooth muscle responses to different rates of sympathetic nerve stimulation have been compared. The responses studied were vasoconstriction in the femoral vascular bed and contraction of the spleen in anaesthetized cats, vasoconstriction in perfused ears of rabbits and inhibition of pendular movements in rabbit isolated ileum preparations. Except in the isolated ileum, the action of bretylium on curves relating the frequency of nerve stimulation and the effect on response was different from that of guanethidine. Whereas bretylium caused relatively greater inhibition of responses to high stimulus frequencies and depressed the slopes of the curves, guanethidine preferentially suppressed responses to low stimulus frequencies and caused roughly parallel shifts of the curves. In each situation tested bethanidine was the most potent of the three blocking agents and in general its effect on frequency/response curves was intermediate between those of bretylium and guanethidine.     

Effect of sympathomimetic amines on the blocking action of guanethidine, bretylium and xylocholine

Experiments were carried out in which the adrenergic neurone blocking activity of xylocholine, bretylium and guanethidine was studied by the use of the inhibitory responses of the isolated rabbit ileum to lumbar sympathetic nerve stimulation, and the contractions of the nictitating membrane of the anaesthetized cat in response to stimulation of the cervical sympathetic nerves. In both these preparations, after blockade of the effects of sympathetic nerve stimulation had been produced with xylocholine, bretylium or guanethicdine, the sympathomimetic amines, dexamphetamine, mephentermine, hydroxyamphetamine, ephedrine and phenethylamine, reversed the blockade; if these amines were given first, then the adrenergic neurone blocking agents were ineffective. Tyramine and dopamine were effective on the isolated rabbit ileum but not on the cat's nictitating membrane. Effective antagonism of the adrenergic neurone blocking drugs was also shown by some substances which inhibit mono-amine oxidase but only those which in addition possess sympathomimetic effects. Thus phenelzine, pheniprazine and tranylcypromine were effective whereas iproniazid and nialamide were not. Since xylocholine, bretylium and guanethidine were all antagonized by the same agents, it seems likely that they all produce sympathetic blockade by a similar mechanism. The possibility is discussed that the sympathomimetic amines which antagonize the adrenergic neurone blocking drugs are competing with these substances for the same receptor sites.     

Mechanism of the positive inotropic responses to bretylium and guanethidine

Isolated, atropinized, rat atria exhibited positive inotropic responses to bretylium, guanethidine and tyramine. These responses were prevented by treatment of the animal with reserpine, or by addition of dichloroisoprenaline to the organ bath. The positive inotropic effects of these compounds on atria from reserpinized animals were restored by incubation of the tissue with noradrenaline. On the basis of these findings it is concluded that the cardiac stimulation by bretylium, guanethidine and tyramine involves the release of catechol amines. The usually reported increase in sensitivity of the myocardium from reserpinized animals to noradrenaline was not observed. The influence of bretylium and guanethidine on cardiac uptake and release of noradrenaline was also studied with the rat. Guanethidine decreased the concentration of catechol amines and inhibited the uptake of exogenous noradrenaline, while bretylium had no effect on either. The decrease in concentration of cardiac catechol amines produced by guanethidine was prevented by treatment of the animal with bretylium or with 1-phenyl-2-hydrazinopropane (pheniprazine), a monoamine oxidase inhibitor.     

Mechanism of the initial adrenergic effects of bretylium and guanethidine

Treatment with phenoxybenzamine and dichloroisoprenaline prevented the rise of blood pressure, contraction of the nictitating membrane and increase in cardiac contractile force produced by intravenous injections of bretylium and guanethidine in anaesthetized or spinal cats. Treatment with cocaine, imipramine or reserpine reduced the sensitivity to bretylium and guanethidine of the spinal cat. In the spinal cat treated with reserpine, sensitivity to the drugs could be restored by an infusion of noradrenaline. Chlorpromazine also blocked the pressor and nictitating membrane responses to bretylium and guanethidine. The drug effects were unaltered 1 hr after bilateral adrenalectomy. During the intravenous infusion of noradrenaline into spinal cats the pressor responses to bretylium and guanethidine were increased, whilst those to adrenaline and to noradrenaline were decreased. Guanethidine (3 to 5 mg/kg) injected intravenously into the cat caused a sudden relaxation of the rat isolated stomach-strip bathed in blood. In seven similar experiments bretylium (3 to 5 mg/kg) relaxed the strip only once; in the other six experiments there was either no effect (four experiments) or an increase in the tone of the strip (two experiments). It is concluded that the initial adrenergic effects of bretylium and guanethidine are mediated, at least in part, through a release of catechol amines from stores in the effector organ.     

The relative activities of some tryptamine analogues on the isolated rat stomach strip preparation

The relative potencies of analogues of tryptamine and 5-hydroxytryptamine have been determined on the rat fundus preparation. This tissue had an amine oxidase activity, which, in the homogenate, was able to inactivate both tryptamine and 5-hydroxytryptamine to about the same degree. Amine oxidase inhibitors potentiated the action of tryptamine and many analogues on the isolated rat fundus preparation, but not the action of 5-hydroxytryptamine or of other hydroxytryptamines. This suggested that, in the isolated organ, the amine oxidase was unable to inactivate 5-hydroxytryptamine, but could inactivate tryptamine, 5-methoxytryptamine and many others. These results may be explained if it is supposed that tryptamine entered the cell, but because of the polar hydroxyl group 5-hydroxytryptamine did not. This hypothesis is supported by the oil/water partition coefficients. The structure/activity of the various tryptamine derivatives is discussed in the light of this assumption.     

Mechanisms of the relaxations induced by 5-hydroxytryptamine on the rat isolated caecum

The mechanism by which 5-HT produces relaxations of the rat caecum has been examined. Propranolol and cocaine markedly attenuated the relaxations whereas tetrodotoxin had no effect. Higher doses of propranolol and cocaine (greater than 10(-6) M), as well as reserpinization, converted the relaxations into contractions. There was a residual relaxation resistant to propranolol, cocaine and reserpine treatment. High doses of 5-HT (greater than 10(-5) M) were thus thought to relax the rat caecum indirectly, through the release of noradrenaline from the tissue by a tyramine-like action. The relaxations do not seem to be due only to the release of noradrenaline.     

Pharmacological properties of a C-fibre response evoked by saphenous nerve stimulation in an isolated spinal cord-nerve preparation of the newborn rat.

1. An isolated spinal cord-peripheral nerve preparation of the newborn rat was developed. In this preparation it is possible to record spinal reflexes from a lumbar ventral root in response to stimulation of the ipsilateral saphenous or obturator nerve. 2. Single shock, weak intensity stimulation of the saphenous nerve induced a fast conducted compound action potential in the L3 dorsal root and a fast depolarizing response in the ipsilateral L3 ventral root. As a stronger stimulus was applied to the saphenous nerve, a slowly conducted compound action potential appeared in the dorsal root and a slow depolarizing ventral root potential (v.r.p.) in the L3 ventral root. 3. Single shock stimulation of the obturator nerve induced a rapidly conducted compound action potential in the L3 dorsal root and monosynaptic and polysynaptic reflexes, with a fast time course, in the ipsilateral L3 ventral root. 4. The slow v.r.p. evoked by saphenous nerve stimulation was depressed by the tachykinin antagonist, [D-Arg1, D-Trp7,9, Leu11] substance P (spantide), 4-16 microM. The response recovered its original shape and size 30-60 min after the removal of this antagonist. 5. The saphenous nerve-evoked slow v.r.p. was depressed by [Met5] enkephalin (0.1-1 microM), dynorphin (1-13)(0.2 microM) and morphine (1-2 microM), and these effects were reversed by naloxone (1 microM). 6. Two endogenous peptides, galanin (1-2 microM) and somatostatin (1-2.5 microM), inhibited the slow v.r.p. evoked by saphenous nerve stimulation, whereas another endogenous peptide, calcitonin gene-related peptide (0.1-0.5 microM), potentiated the slow v.r.p. The slow v.r.p. was also inhibited by gamma-aminobutyric acid (GABA, 20 microM) and muscimol (0.2 microM), and their effects were antagonized by bicuculline (1 microM). 7. The present results suggest that substance P and neurokinin A are involved in the saphenous nerve-evoked C-fibre response in the spinal cord of the newborn rat.