1. Noradrenaline and adrenaline reduce the output of acetylcholine by the guinea-pig ileum longitudinal strip by up to 80%, both in resting conditions and after stimulation. The effect is graded with dose, and is detectable with noradrenaline 2 x 10-v g/ml. Adrenaline is approximately 4 times as active as noradrenaline, and its action after being washed out is more persistent. 2. If resting output is high, both amines have a proportionately greater effect and their action, as dosage is increased, is to reduce resting output to a basal level, relatively constant from strip to strip, of about 10 ng/g/min. 3. With stimulation, the effect of the amine is greater at low frequencies, when the output per volley is high, than at high frequencies. The effect is reduced by increasing the number of shocks delivered. There thus appears to be a basal output per volley, of the order of 1-2 ng/g/volley, which can be reached either by relatively rapid stimulation, by prolonged stimulation, or by treatment with these amines. 4. If noradrenaline is applied during continued stimulation at 40/min, the depression of acetylcholine output during its presence is followed by an augmented output when the drug is withdrawn. The magnitude of this " overshoot" increases with the duration of noradrenaline exposure. 5. Phenylephrine 4 Fg/ml. and amphetamine 20 ,ug/ml. reduced the acetylcholine output, but isoprenaline 1 jg/ml., dopamine 1 ug/ml. and methoxamine 10 gg/ml. were ineffective.6. Phenoxybenzamine reduced the resting output and increased the stimulation output. Of the two other blocking agents examined, phentolamine had no effect on either resting or stimulation output and ergotamine transiently reduced stimulation output. The effect of phenoxybenzamine was not due to a reaction with either adrenoceptive or muscarinic receptors. 7. Phenoxybenzamine, phentolamine and ergotamine abolished the effect of adrenaline and noradrenaline on both resting output and on output in response to stimulation. 8. In strips obtained from animals treated with reserpine and guanethidine, a rise in resting acetylcholine output and in stimulation output at low frequencies was found. In these conditions, noradrenaline was still effective.

SUMMMARY1. Strips of longitudinal muscle can be obtained from guinea-pig ileum either retaining or free from Auerbach's plexus.2. The denervated strip is unresponsive to electrical stimulation by brief shocks, whether given singly or in trains; it also fails to respond to nicotine or dimethylphenylpiperazinium iodide (DMPP), and eserine causes no spasm.3. Denervated strips neither contain detectable acetylcholine ( < 0 4 ng/ mg), nor release it spont&neously (< 5 pg/mg/min) or in response to stimulation (< 31 pg/mg/min). The acetylcholine metabolism of the innervated strip is therefore that of the adherent Auerbach's plexus. Innervated strips had a mean acetylcholine content of 28 ng/mg, a mean resting output of 94 pg/mg/min and an output in response to stimulation at 10 c/s of 700-1200 pg/mg/min.4. By comparing the responses of innervated and denervated strips it was concluded that arecoline, methylfurmethide, a,8l-ethylal-y-trimethylammonium propanediol iodide (2268 F), muscarine, histamine, tremorine, oxytocin, and substance P, like acetylcholine, act primarily on the smooth muscle directly; and that angiotensin, barium, potassium, m-bromophenyl choline ether and 5-hydroxytryptamine have a progressively increasing proportionate effect on the nerve plexus. Nicotine and DMPP were inactive in the absence of the plexus.5. The longitudinal muscle with its accompanying plexus contains about one quarter of the acetylcholine of the whole ileum, and is responsible for about one fifth of the output to electrical stimulation.6. The volley output of acetylcholine by the innervated strip declines sharply as rate of stimulation increases. Output of acetylcholine was reduced by morphine and by cocaine, particularly when resting or when stimulated at low rates.

Morphine depresses the twitch and tetanus of stimulated guinea-pig ileum by reducing acetylcholine released from cholinergic nerve endings. Acetylcholine output per shock falls to roughly the same residual amount at varying stimulation rates. Since normal output per shock declines with increasing stimulus frequency, the proportionate effect of morphine diminishes as stimulus frequency rises. Acute " tolerance" to morphine and a state of " morphinedependence" can be produced. Phenadoxone, dihydromorphinone, metopon, methadone, and heroin are more active, codeine and pethidine less active, than morphine. Nalorphine also depresses the twitch and can desensitize the gut both to itself and to morphine.Despite the widespread use of morphine there is still no agreement as to the mode of action, either when used as an analgesic or for depressing the activity of the intestine. It has been suggested that its anticholinesterase activity causes the central actions ; but it is a relatively feeble inhibitor of cholinesterase, and much more active inhibitors do not reproduce its effects. On the intestine, attention has focused on morphine's ability either to depress propulsion, or to increase the tone of the gut. An investigation of the action of opiates on intestinal loops in the dog by Vaughan Williams and Streeten (1950Streeten ( , 1951 has clarified some apparent contradictions in previous experimental results, and shown how the stimulant action of morphine could lead to a failure of transport of intestinal contents. Recent work by the Schaumanns (Schaumann, Giovannini and Jochum, 1952;Schaumann, 1955) and by Kosterlitz and Robinson (1955) In this paper the effect of morphine (and related drugs) on the twitch of the intestine and on its acetylcholine output is described.A short account of some of the results obtained with morphine was given elsewhere (Paton, 1956).METHODS The experiments were all made on strips of guineapig ileum suspended in Krebs solution, bubbled with 95 % 02 and 5 % C02. Rectangular current pulses usually of 1 msec. duration, and of sufficient strength to produce a maximal response to a single shock, were applied to the electrodes; the intraluminal electrode was made the anode. When necessary, repetitive stimulation was used to produce a partially or completely fused tetanus for a period of a few seconds or a minute. The contractions of the gut were recorded either on a smoked drum or by a light inkrecorder. The bath was kept at 35 to 370 C. The capacity of the bath in which the gut and electrodes were immersed was normally 50 ml.The assay of acetylcholine output was conducted on another strip of guinea-pig ileum treated with 5 jug./l. of neostigmine and 10 mg./l. of morphine. As soon as the first observation was made that morphine reduced the output of acetylcholine by the gut,

SUMMARY1. The margin of safety for neuromuscular transmission in the tibialis and sartorius muscles of the cat has been determined by measuring the ratio by which end-plate depolarization produced by succinylcholine, decamethonium, octamethonium or iodocholine is antagonized, in the presence of neuromuscular block produced by tubocurarine, gallamine or DF-596. The estimate of the margin of safety was independent of the particular drugs chosen for the measurement.2. To produce threshold block to indirect stimulation once every 10 sec, a fractional occupancy by the antagonist of 0-76 + 0.05 (S.D.) was required; for nearly complete block, an occupancy of 0-917 + 0-16 (S.D.) was required. These figures correspond to factors of safety of 4*1 and 12 for the most sensitive and the most resistant groups of fibres respectively.3. The interaction between the agonists and the antagonists, when tested over a wide range of dosage, did not conform with the conditions of full competitive equilibrium. It was concluded that this arose, not because of some interfering non-competitive process, but because, during the relatively brief exposure to agonist, the equilibrium between the antagonist and the receptors is not significantly disturbed. An analysis of this condition of quasi-equilibrium is given. A correction downwards of the direct estimates of the margin of safety is required, but this proves to be small, about 8 %., and may not be significant.4. The safety factor diminished when the motor nerve had been cut more than 5 hr; it is suggested that this represents an early sign of nerve degeneration.5. With dog sartorius muscle, results similar to those in the cat were obtained. But for deep block in the rabbit, the safety factor was only about 4.6. The existence of a substantial margin of safety influences considerably the interpretation of the time course of action of blocking drugs, and

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