In a recent investigation (Biilbring & Lin, 1958) it was found that small amounts of 5-hydroxytryptamine (5-HT) were continuously released into the lumen of an isolated loop of intestine. The quantity released was directly related to the height of intraluminal pressure. Experiments carried out to ascertain whether 5-HT was liberated as a result of pressure on the mucosa, or as a result of peristaltic activity which was evoked by raising the pressure, were not entirely conclusive. When peristaltic activity was abolished either by blocking mucosal sensory receptors with procaine, or by blocking intramural ganglia with hexamethonium, the correlation between pressure and 5-HT release was still observed. However, the extent to which 5-HT release was augmented on raising the intraluminal pressure was less. It was therefore concluded that muscular activity contributed to the mechanism releasing 5-HT, probably by causing, during a peristaltic wave, a considerable additional rise in pressure on the mucosa.The present work was carried out in order to investigate further the mechanism of 5-HT release. METHODS Experiments in vivoGuinea-pigs were used throughout. The animal was anaesthetized by an intravenous injection of a mixture of 0.6% chloralose + 10% urethane (5 ml./kg). One hour after the injection the abdomen was opened and a piece of ileum close to the caecum, 5-7 cm long, was cannulated at both ends. Tyrode solution was passed through the lumen as in the method described by Biilbring & Lin (1958) for isolated loops of intestine. The fluid flowed from a Marriotte bottle, the height of which could be changed, through a narrow tube at a slow rate into the oral end of the intestine. This was connected to a vertical glass tube acting as a water manometer, the pressure being recorded, by air transmission, with a float recorder. The caudal end of the intestine was connected to a T-piece, one arm of which led to a valve. As a rubber valve constituted a certain resistance we used a float valve made of Perspex, as shown in Fig. 1. Fluid entered at A and lifted the conical Perspex float, which was, however, held in a vertical position by the side arm projecting across the tube. The fluid left the tube from the side arm B. The height of the water column in the tube was maintained by the outlet of the syphon tube. 5-HT RELEASE BY INTESTINAL PRESSURE 19When the side arm of the T-piece was closed fluid flowed only as a result of active peristaltic propulsion. In the absence of peristalsis fluid was collected from the side arm of the T-piece. The animal lay on its back and the cut edges of the body wall were lifted up to make a pool. The whole abdominal cavity with the perfused loop of intestine covered by a piece of muslin was bathed by a continuous drip of Tyrode solution at 370 C. Though the two cannulae were held rigidly, the intestine was not stretched between them and could move freely.Experiments in vitro A few experiments were carried out on isolated loops of intestine using the method described by Bilbring & Lin (1958)...

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In isolated guinea-pig intestine 5-hydroxytryptamine increased the longitudinal muscle contractions in response to acetylcholine while the ganglionic action of nicotine was first facilitated and then blocked. Phenyldiguanide, veratrine, veratridine and protoveratrine, like 5-hydroxytryptamine, depressed the response to nicotine, leaving that to acetylcholine unaffected.The sensory stimulants, like 5-hydroxytryptamine, facilitated the peristaltic reflex when applied to the mucosa, and abolished it when applied to the serosa. Preceding the block, the initial effect of low concentrations of 5-hydroxytryptamine applied to the serosa was a short stimulation of peristalsis.Concentrations of 5-hydroxytryptamine which had an approximately equal stimulant action (mucosal 1 to 4 x 10-', serosal 2 to 8 X 10-8) were tested when various parts of the reflex arc were blocked. During block by procaine introduced into the lumen, mucosal application of 5-hydroxytryptamine re-established peristalsis, but serosal application of 5-hydroxytryptamine had no effect. During block by hexamethonium or atropine present in the bath, 5-hydroxytryptamine restored peristalsis more effectively by serosal application than by mucosal application. During block by serosal application of 5-hydroxytryptamine, morphine, phenoxybenzamine or dihydroergotamine, mucosal application of 5-hydroxytryptamine restored the peristaltic reflex while serosal application had no effect. During block by 2-bromo-lysergic acid diethylamide or lysergic acid diethylamide acting from the serosal surface, 5-hydroxytryptamine had no effect whether acting on the mucosal or on the serosal surface.It is concluded that 5-hydroxytryptamine facilitates the peristaltic reflex at two sites: when introduced into the lumen it stimulates mucosal sensory receptors; when acting from the serosal surface it sensitizes the muscle to the transmitter acetylcholine. There is also a transient stimulant action on the ganglia which is soon followed by inhibition; this indicates that 5-hydroxytryptamine applied to the serosa abolishes peristalsis by ganglion block.Two different actions of 5-hydroxytryptamine on the peristaltic reflex have recently been observed on isolated intestine of the guinea-pig. If 5-hydroxytryptamine was applied to the serosal surface, it inhibited peristalsis (Kosterlitz and Robinson, 1957; Ginzel, 1957); if it was applied to the mucosal surface it facilitated the peristaltic reflex (Bulbring and Lin, 1958;Lembeck, 1958). The stimulant action could be explained by the effect of 5-hydroxytryptamine on sensory receptors in the mucosa. The mechanism of the inhibitory action, however, when 5-hydroxytryptamine was applied to the serosa was not clear.It is known that a variety of substances stimulate not only chemoreceptors but also tension and stretch receptors in various organs (Dawes and Comroe, 1954; Iggo, 1957;Paintal, 1954Paintal, , 1957a and b). In the present investigation the action of some sensory stimulants, phenyldiguanide and veratrum alkaloids, has been compared w...

1. Intraluminal pressure was recorded from the isolated guinea-pig and mouse stomach with the vagus and sympathetic nerves attached.2. The response to vagal stimulation, which consists of an excitatory and an inhibitory component, resembled the response to 5-hydroxytryptamine (5-HT), which has no direct action on the muscle but acts on intrinsic excitatory and inhibitory ganglia.3. In the presence of hyoscine, the effect of vagal stimulation, of nicotinic compounds and of 5-HT were all purely relaxant. Competitive block of ganglionic receptors for acetylcholine reduced the vagal relaxation without antagonizing 5-HT. Specific desensitization of ganglionic receptors for 5-HT reduced the vagal relaxation without antagonizing nicotinic compounds.4. During the early phase of the blocking action of nicotine, responses to vagal stimulation and to 5-HT were both abolished. As the non-specific antagonism changed to the later phase of specific antagonism to acetylcholine, the inhibitory (but not the excitatory) component of the vagal response recovered partially, in parallel with the recovery of the relaxant effect of 5-HT.5. The vagal inhibitory effect was completely abolished only when competitive block of acetylcholine receptors was combined with desensitization of 5-HT receptors.6. Stimulation of the mouse stomach (after asphyxiation of the mucosa and exclusion of the luminal content) in the presence of hyoscine caused the release of 5-HT; this release was blocked by tetrodotoxin.7. The results, together with previous observations that 5-HT is contained within preganglionic nerve fibres in the myenteric plexus, are consistent with the hypothesis that 5-HT, with acetylcholine, may be a neurotransmitter in the vagal inhibitory innervation of the stomach.

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SUMMARY1. Spontaneous spike activity and action potentials evoked by external field stimulation were recorded, intracellularly and with the double sucrose gap method, from the smooth muscle of guinea-pig taenia coli.2. Replacement of external NaCl with sucrose (leaving 10 mM-Na in the buffer) caused hyperpolarization and stopped spontaneous activity within 10 min. Spikes could, however, be evoked for 2-3 hr. The amplitude, the overshoot and rate of rise of the spike were increased.3. In 10 mM-[Na]o the intracellular Na concentration was reduced from 35 to 24 mm, shifting the Na-equilibrium potential from + 34 to -22 mV.4. Excess Ca (12-5 mM) caused hyperpolarization and increased membrane conductance. The amplitude and the rate of rise of the spike were increased, the threshold was raised and the latency of the spike evoked by threshold stimulation became shorter.5. The effect of reducing the external Ca concentration depended on the Na concentration present, being greater with higher external [Na]o. When the membrane was depolarized and spikes deteriorated in low Ca (0-2-0*5 mm) reduction of Na to 10 mm caused repolarization and recovery of the action potential.6. Mn (0-5-1*0 mm) blocked spontaneous spike discharge after 20 min. Higher concentrations (more than 2X0 mm) were required to block the evoked action potential.7. The results indicate that the smooth muscle spike in taenia is due to Ca-entry and that Na influences spike activity indirectly by competing with Ca in controlling the membrane potential.

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The main differences between the electrical properties of the smooth muscle cells of taenia coli and those of skeletal muscle cells are as follows: In the taenia the membrane potential is lower; the spike amplitude is variable; the maximum rate of rise and fall of the spike is lower, and the cells are capable of generating spontaneous spikes as well as propagating