The action of drugs on the circular muscle strip from the guinea-pig isolated ileum

The circular muscle strip is a new preparation for examining the action of drugs on the circular muscle of the guinea-pig isolated intestine. The preparation differed from the longitudinal muscle in that it was insensitive to drugs which act on autonomic effector tissues but, after inhibition of cholinesterase, it responded readily to choline esters, 5-hydroxytryptamine, histamine and nicotine. This behaviour necessitated the treatment of each strip with the anticholinesterase NN-diisopropylphosphodiamidic fluoride (mipafox) before each experiment. The contractions of the strip by 5-hydroxytryptamine, histamine and nicotine were abolished by procaine, botulinum toxin (Type A), morphine and hemicholinium, whilst the actions of acetylcholine and methacholine were unaffected. Contractions of the strip in response to each of the drugs were abolished by atropine and hyoscine. The action of nicotine was specifically antagonized by hexamethonium, that of 5-hydroxytryptamine by desensitization of the tissue to 5-hydroxytryptamine, and that of histamine either by desensitization of the tissue to histamine or by mepyramine. It is postulated that 5-hydroxytryptamine, histamine and nicotine stimulate specific receptor sites within the intramural nerve plexuses of the guinea-pig isolated ileum. Finally, botulinum toxin (Type A), morphine or hemicholinum, acting on the neuronal elements of the intramural plexuses, depressed the contractions of the circular muscle strip due to histamine or nicotine more readily than those due to 5-hydroxytryptamine.

The action of substances which block sympathetic postganglionic nervous transmission

The substances which block sympathetic postganglionic transmission, xylocholine, bretylium and guanethidine, also block neuromuscular and sympathetic ganglionic transmission. To see if these last properties were related to the sympathetic blocking property, phenyltrimethylammonium, which blocks the neuromuscular junction (Riker, 1953), was used. It blocked the inhibition of the rabbit ileum produced by stimulating the periarterial nerves in the mesentery, though with higher concentrations the effect of stimulation was initially increased. The action was not modified by the presence of hyoscine. The blocking action was exerted on the response to stimulation of the highest frequency first, and on the response to stimulation of the lowest frequency last. This relation of block to stimulus frequency is similar to that at the neuromuscular junction when tubocurarine is used. Nine compounds have now been shown to block responses to sympathetic postganglionic stimulation, and seven of these are onium compounds. They are, however, mon-onium compounds, and not bis-onium compounds like hexamethonium and decamethonium, so that they can probably enter the postganglionic fibre, which bis-onium compounds (having a charged group at each end of the molecule) may not be able to do. Since these mon-onium compounds have some blocking action at neuromuscular junctions and at sympathetic ganglia, their block of postganglionic transmission may be essentially similar to that by hexamethonium at ganglia and to that by decamethonium at neuromuscular junctions. It is known that acetylcholine releases noradrenaline from sympathetic postganglionic terminations, and xylocholine and bretylium block this release in the vessels of the rabbit ear and in the rabbit isolated atria.

The role of the gut in albumin catabolism. I. Studies in the jejunoileectomized rabbit

I¹³¹-albumin metabolic studies were carried out in 5 rabbits before, 3 weeks after, and several months after removal of 70 to 90 per cent of the jejunum and ileum. A sixth animal was studied before and 11 weeks after a sham operation. During postoperative experiments, the animals were found to be in a highly unsteady state with large losses of albumin from the vascular compartment. Despite these losses, the plasma albumin concentration was maintained at a relatively constant level. No decrease in the albumin efflux occurred following nearly complete jejuno-ileectomy. The data suggest that albumin catabolism is a first order process.

An analysis of the direct and indirect actions of drugs on the isolated guinea-pig ileum

The responses of the isolated guinea-pig ileum to coaxial stimulation of its nerves, to histamine, acetylcholine, bradykinin, nicotine, tetramethylammonium, 1,1-dimethyl-4-phenyl-piperazinium iodide and 5-hydroxytryptamine were studied, before and during anoxia, cooling, or exposure to hyoscine, phenoxybenzamine hydrochloride, morphine or hexamethonium. Dose ratios were used to determine the amount of block induced by these procedures. With the response to coaxial nerve stimulation as an indication of the excitability of the nervous tissue, it was found that anoxia or cooling abolished the response to single shocks. Under these conditions the response of the ileum to histamine, acetylcholine and bradykinin was hardly affected, indicating a direct action of these substances on the muscle fibres. The effects of nicotine, tetramethylammonium, dimethylphenylpiperazinium and 5-hydroxytryptamine were reduced to various degrees, and we have concluded that their main actions are indirect, through stimulation of cholinergic nerve fibres. When these indirect actions were prevented, increasing the dose revealed a direct action, a larger increase in dose being required for 5-hydroxytryptamine and dimethylphenylpiperazinium than for tetramethylammonium and nicotine. Exposure of the ileum to hyoscine and phenoxybenzamine showed that these direct actions of nicotine and tetramethylammonium were not only on acetylcholine receptors but also on receptors insensitive to hyoscine but sensitive to phenoxybenzamine. The main action of 5-hydroxytryptamine was on nervous elements, yet treatment of the ileum with phenoxybenzamine gave a higher dose ratio for 5-hydroxytryptamine than did treatment with morphine. The meaning of this result is discussed in relation to the general belief that receptors sensitive to morphine are in nervous tissue and receptors sensitive to phenoxybenzamine are in smooth muscle. We have concluded that morphine is only a partial antagonist of 5-hydroxytryptamine receptors in nervous tissue and that phenoxybenzamine antagonizes more 5-hydroxytryptamine receptors than those in smooth muscle.

Antagonism of acetylcholine by adrenaline antagonists

Phenoxybenzamine antagonized the inhibitory action of acetylcholine on the guinea-pig isolated atrium. The antagonism was slow in onset, very slowly reversible, and could be overcome by increased concentrations of acetylcholine. In contrast, atropine inhibited the action of acetylcholine quickly, and the effect disappeared soon after withdrawal. The pA(10) of phenoxybenzamine (2 hr of contact) was 6.8, and that of atropine (30 min of contact) was 8.4. In the presence of atropine phenoxybenzamine did not exert a slowly reversible antagonism, and the dose-ratio of acetylcholine returned to normal soon after withdrawal of both drugs. Phenoxybenzamine also antagonized acetylcholine in the guinea-pig isolated ileum, but with higher concentrations acetylcholine did not overcome the antagonism. The pA(10) (60 min of contact) was 6.6. The pA(10) of chlorpromazine in the atrium (2 hr of contact) and ileum (60 min of contact) was 5.9. Phentolamine, 2-diethylaminomethylbenzo-1,4-dioxan hydrochloride (883 F), and yohimbine antagonized acetylcholine in the atrium and ileum but required higher concentrations than chlorpromazine.

Potentiation of the response to histamine by picolylamines

The response of the isolated ileum of the guinea-pig to histamine is potentiated in the presence of 2-, 3- or 4-picolylamine. These compounds have been found to inhibit the histaminase and/or "diamine oxidase" of pig kidney. The three corresponding picolylmethylamines did not potentiate the response of the ileum to histamine; they were without significant affinity for the pig kidney oxidase. It is suggested that the potentiating action of the three primary amines is due to their inhibitory action on histaminase. The responses of the ileum to acetylcholine and 5-hydroxytryptamine were not potentiated.

THE ACTION OF TRIETHYLCHOLINE ON THE PERISTALTIC REFLEX OF THE GUINEA-PIG ILEUM

The action of triethylcholine on the peristaltic reflex of the guinea-pig isolated ileum has been studied. When perfused through the lumen in low concentrations triethylcholine first stimulated and then inhibited peristalsis. High concentrations gradually inhibited or abolished the reflex without prior stimulation. These inhibitory effects were reversed by choline, in the continued presence of triethylcholine. Similar responses followed application of triethylcholine to the serosal surface of the gut, except that high concentrations produced an immediate abolition of peristalsis which was not restored by choline. It is suggested that the effects of intraluminal triethylcholine are due either to enhancement of acetylcholine release or to reduction of acetylcholine synthesis, or to a combination of these factors, and that the results are evidence that acetylcholine is a humoral transmitter involved in the initiation of peristalsis.

POPULATION DYNAMICS OF INTESTINAL EPITHELIA IN THE RAT TWO MONTHS AFTER PARTIAL RESECTION OF THE ILEUM

Sprague-Dawley rats that had been subjected 2 months previously to partial resection (10 per cent) of the small intestine and an equal number of control rats were injected with tritiated thymidine and sacrificed at intervals during the subsequent 16 hours. Segments of duodenum, jejunum and ileum were prestained by the Feulgen technique and radioautographed. The proportion of crypt cells bearing labeled nuclei, the percentage of labeled crypt cells in mitosis and the appearance of labeled crypt cells on the villi were determined. Comparison of control and resected rats showed that (a) the proportion of intestinal crypt cells incorporating thymidine was considerably greater and uniformly high throughout the shortened intestine, (b) the life cycle of crypt cells was slightly reduced, and was uniform throughout the shortened intestine, and (c) the time during which cells were retained in crypts was markedly reduced. On the basis of persistent, generalized increase in the production of crypt cells, and on prior evidence that the epithelial cells of shortened intestine continue to have a brief life span and evidence of metabolic immaturity, the existence of a humoral factor, tentatively called "intestinal epithelial growth hormone," is postulated.

THE INTERACTION BETWEEN ACTIVE SODIUM AND ACTIVE SUGAR TRANSPORT

The addition of actively transported sugars to the solution bathing the mucosal surface of an in vitro preparation of distal rabbit ileum results in a rapid increase in the transmural potential difference, the short-circuit current, and the rate of active Na transport from mucosa to serosa. These effects are dependent upon the active transport of the sugar per se and are independent of the metabolic fate of the transported sugar. Furthermore, they are inhibited both by low concentrations of phlorizin in the mucosal solution and by low concentrations of ouabain in the serosal solution. The increase in the short-circuit current, ΔI_sc, requires the presence of Na in the perfusion medium and its magnitude is a linear function of the Na concentration. On the other hand, ΔI_sc is a saturable function of the mucosal sugar concentration which is consistent with Michaelis-Menten kinetics suggesting that the increase in active Na transport is stoichiometrically related to the rate of active sugar transport. An interpretation of these findings in terms of a hypothetical model for intestinal Na and sugar transport is presented.

EFFECT OF INTESTINAL RESECTION ON BILE SALT ABSORPTION IN DOGS

The efficiency of intestinal absorption of bile salts was evaluated by studying the rate of disappearance of radioactivity from the bile of dogs after the intravenous administration of sodium taurocholate-24-C14. Bile was sampled through an indwelling tube in the gall bladder. One day after a high-fat meal normal dogs retained 48% of the radioactivity; dogs with resection of the jejunum retained 48%, whereas those with resection of the ileum retained only 3% in the bile. This is consistent with previous observations that the ileum is the site of bile salt absorption in vitro and in anesthetized animals. Animals with resection of the ileum exhibited significant steatorrhea; however, three-fourths of the ingested fat was absorbed in spite of almost complete failure to absorb bile salts. This indicates that fat and bile salts are not normally absorbed together. Elimination of enterohepatic circulation of bile salts by resection of the ileum contributes to the observed steatorrhea.

SOME OBSERVATIONS ON THE PHARMACOLOGY OF ALPHA-METHYLDOPA

alpha-Methyldopa in high concentrations impaired the responses of rabbit isolated ileum and guinea-pig isolated vas deferens to stimulation of the sympathetic nerves and to noradrenaline, but these preparations taken from animals previously treated with alpha-methyldopa showed no sign of impairment. Contractions of the cat nictitating membrane were reduced but not abolished by alpha-methyldopa. In cats, dogs and rats, pressor responses to noradrenaline were usually slightly increased by alpha-methyldopa. Pressor responses to tyramine were not affected consistently. alpha-Methyldopa, alpha-methyldopamine and alpha-methylnoradrenaline behaved like dopa, dopamine and noradrenaline respectively in restoring the responses of tissues from reserpine-treated animals to stimulation of the sympathetic nerves to the rabbit ileum, the guinea-pig vas deferens and the cat nictitating membrane and in restoring responses to tyramine of the cat blood pressure and nictitating membrane, and the rat blood pressure. The potency of alpha-methylnoradrenaline relative to noradrenaline ranged from one-half to one-ninth on various preparations. The results are discussed in relation to the antihypertensive action of alpha-methyldopa.

THE GANGLION BLOCKING ACTION OF PROCAINAMIDE

In cats and rabbits procainamide (20 to 50 mg, intravenously) produced a fall of blood pressure of 20 to 50 mm Hg which reached a maximal effect within 1 min and lasted for about 5 min. Procainamide reduced the pressor responses to nicotine and to carotid arterial occlusion and reduced the depressor response to vagal stimulation, but did not antagonize the actions of adrenaline or noradrenaline on blood vessels. The contractions of the nictitating membrane to stimulation of the preganglionic cervical sympathetic nerve were partially or completely blocked by 20 to 50 mg of procainamide given intravenously. The ganglion blocking effect was more abrupt in onset and more slow to recover than that due to hexamethonium and had about 1/250th of the activity of the latter. Procainamide (1 mg) reduced the acetylcholine output of the perfused superior cervical ganglion to below 30% of the control value and blocked transmission completely. Small doses (10 mug) reduced the acetylcholine output but hardly affected ganglionic transmission. Procainamide, injected into the perfused superior cervical ganglion, blocked contractions elicited by stimulation of the preganglionic cervical sympathetic nerve for a longer period than those produced by acetylcholine injected into the perfusion circuit to the ganglion; the reverse was true for hexamethonium. Procainamide reduced the size of action potentials recorded from the superior cervical ganglion without altering the resting potential of the ganglion. The ganglion blocking activities of procainamide and hexamethonium often potentiated each other, especially when the preparation had been set up for several hours. On the guinea-pig isolated ileum preparation, procainamide (0.5x10(-4) g/ml.) antagonized responses due to acetylcholine, histamine and, most effectively, to nicotine. On the isolated heart, procainamide (1 mg) almost abolished the bradycardia produced by acetylcholine; 10 mg slowed and weakened the heart, while 100 mg stopped it. We conclude that procainamide, like procaine, blocks ganglionic transmission by (1) depressing the release of acetylcholine from preganglionic nerve endings; and (2) competing, with the acetylcholine which is released, for receptor sites on the ganglion cells. The amounts required to produce significant effects in vivo and in vitro are comparable. The methods available for detecting this type of ganglion blocking action are discussed.

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.

THE EFFECT OF INTRA(CEREBRO)VENTRICULAR RESERPINE ON THE ACETYLCHOLINE CONTENT OF THE HEART, ILEUM AND HYPOTHALAMUS OF THE DOG

The effect of injection of reserpine into the cerebral ventricles on the acetylcholine contents of the sino-atrial node, ileum and hypothalamus of the dog was studied in ten dogs. Another group of five dogs served as a control. The effect of intravenous administration of reserpine, in the same dose as given intracerebroventricularly, was also studied on the acetylcholine content of these tissues in five dogs. General sedation, bradycardia, miosis, salivation, emesis and purgation were looked for. Tissues were removed 1 hr after administration of reserpine for estimation of acetylcholine content, which was increased in all the tissues studied. The increase in the peripheral tissues was greater than in the hypothalamus. The increase in the acetylcholine content was not quantitatively related to the other effects of reserpine. The increase in the acetylcholine content of the sino-atrial node and the ileum and also the peripheral effects observed on intracerebroventricular administration of reserpine can be attributed to its central action. With the same dose of reserpine given intravenously the acetylcholine content of the sino-atrial node was significantly increased, while that of the hypothalamus and ileum was not.

SOME EFFECTS OF LONG CHAIN POLYMETHYLENE BISONIUM SALTS ON JUNCTIONAL TRANSMISSION IN THE PERIPHERAL NERVOUS SYSTEM

A survey has been made of the effects on junctional transmission of the complete series of polymethylene bis-trimethylammonium (BTM) and bis-triethylammonium (BTE) salts from the decamethylene compounds (BTM 10 and BTE 10) to those with twenty-one methylene groups in the chain. These were tested for their ability to cause contracture of the isolated chick biventer cervicis preparation, and for their ability to block the twitch responses of this preparation, those of the rat isolated diaphragm preparation, and those of the cat tibialis anterior preparation. They were also tested for their ability to block transmission in the cat superior cervical ganglion, to block the actions of acetylcholine on the guinea-pig isolated ileum, and for ability to inhibit the hydrolysis of acetylcholine by acetylcholinesterase. Their electrical conductivity has been measured in aqueous solution. Ability to cause contracture of the chick biventer cervicis is confined to the compounds BTM 10 to 15; BTE 10, 11 and 12 have some weak activity but the other BTE compounds, and the BTM compounds with more than fifteen methylene groups, have virtually no activity. In the BTE series both neuromuscular blocking and ganglion-blocking activities increase with chain length up to a maximum in the region of BTE 15 to 17 and then decline. In the BTM series ganglion-blocking activity increases with chain length in much the same way as in the BTE series, though the maximum activity is at a slightly longer chain length. At the neuromuscular junction an increase in chain length beyond BTM 10 leads to a decline in activity but this returns to some extent at longer chain lengths, reaching a second maximum at BTM 18, above which it declines further. At the ganglion BTE 16 is only slightly more active than BTM 16 and about five-times as active as hexamethonium; at the neuromuscular junction in the cat BTE 16 is about five-times as active as BTM 16 and about eight-times as active as (+)-tubocurarine. The affinity of the BTE compounds for the postganglionic acetylcholine receptors of the guinea-pig ileum reaches a maximum at BTE 14 but does not decline significantly with further increase in chain length. Anticholinesterase activity, likewise, does not alter significantly between BTM 12 and BTM 21 and the activity of the compounds in the BTE series appears to be similar. This property could conceivably be modifying the actions of some of the intermediate compounds but is not likely to be affecting those of the more active ones. The conductivity experiments indicate that micelle formation could be limiting the actions of the compounds with 20 or 21 methylene groups, but is not likely to be affecting those of the other compounds. The results suggest that there is a regular increase with chain length of the affinity of these compounds for the receptors in the ganglia and at the neuromuscular junction but that efficacy in causing contracture is limited to compounds with three methyl groups in the cationic head and a chain of about ten methylene groups. The connexion between this ability to depolarize and the ability to block transmission by desensitization is discussed.