THE EFFECT OF PHENOXYBENZAMINE AND OF TOLAZOLINE ON THE RESPONSE TO SYMPATHETIC STIMULATION

The effect of phenoxybenzamine has been determined on the physiological response to sympathetic stimulation in two preparations, the rabbit isolated ileum and the guinea-pig isolated vas deferens. In both preparations phenoxybenzamine increased the response to stimulation of low frequency, the response being inhibitory in the one and motor in the other. This increase was large. As the stimulus frequency was raised, phenoxybenzamine caused a progressively smaller increase in the response, and at high frequencies phenoxybenzamine decreased the response. These observations agree with those of earlier workers who showed that antiadrenaline substances have more than one property. They not only block the motor effects of adrenaline and noradrenaline, but at the same time they may increase the response to sympathetic stimulation. The observations which have been made are not consistent with the interpretation which has been placed by others on the effect of phenoxybenzamine on the amount of noradrenaline appearing in the splenic vein following sympathetic stimulation; this interpretation assumes that phenoxybenzamine will decrease the response to sympathetic stimulation at low frequency. The mode of action of phenoxybenzamine is discussed, and fresh evidence that it has an anticholinesterase action is given.

THE SITE OF THE 5-HYDROXYTRYPTAMINE RECEPTOR ON THE INTRAMURAL NERVOUS PLEXUS OF THE GUINEA-PIG ISOLATED ILEUM

Dose/response measurements were made on the guinea-pig isolated ileum with six agonists, acetylcholine, 5-hydroxytryptamine, nicotine, dimethylphenylpiperazinium, choline phenyl ether and histamine. The dose effects were repeated in the presence of each of twelve antagonists and one anticholinesterase. Acetylcholine and histamine were chosen because of their direct mode of action on smooth muscle, nicotine, dimethylphenylpiperazinium and choline phenyl ether were used as examples of drugs that act at the ganglionic acetylcholine receptor. 5-Hydroxytryptamine was the drug investigated. Hyoscine blocked the contractions caused by acetylcholine, 5-hydroxytryptamine and the ganglion-stimulants but left the responses to histamine unchanged. The anticholinesterase N,N'-diisopropylphosphorodiamidic fluoride (mipafox) potentiated all the agonists except histamine. The strength of potentiation decreased in the order 5-hydroxytryptamine, nicotine, dimethylphenylpiperazinium and choline phenyl ether, and acetylcholine. The local anaesthetic procaine inhibited to the same extent contractions elicited by 5-hydroxytryptamine, nicotine, dimethylphenylpiperazinium and choline phenyl ether. These results showed that 5-hydroxytryptamine, like nicotine, choline phenyl ether and dimethylphenylpiperazinium, mediated its response through the nervous plexus. Of those tested 5-hydroxytryptamine was the only specific antagonist to 5-hydroxytryptamine; lysergic acid derivatives produced spasm and prolonged changes in tone; phenoxybenzamine caused non-specific block. The diverse modes of action of a number of ganglion-blocking agents were selectively used. Thus hexamethonium, pentolinium, and nicotine in its competitive phase, blocked contractions due to nicotine, dimethylphenylpiperazinium and choline phenyl ether and left those due to 5-hydroxytryptamine, acetylcholine and histamine unchanged. The depolarizing ganglion-blocking agents, dimethylphenylpiperazinium and nicotine, inhibited the responses to all the indirectly acting drugs. Furthermore, mecamylamine, a drug with a less well-defined mode of action, partially inhibited contractions due to 5-hydroxytryptamine in a concentration that blocked those due to nicotine, dimethylphenylpiperazinium and choline phenyl ether. Pempidine, known to act like mecamylamine, did not antagonize 5-hydroxytryptamine. It is concluded that 5-hydroxytryptamine activates specific receptors sited at the intramural parasympathetic ganglion cells.

THE USE OF ISOLATED ORGANS FOR DETECTING ACTIVE SUBSTANCES IN THE CIRCULATING BLOOD

A method is described for the assay of circulating hormones after their injection or release into the circulation. The method is applicable to cats, dogs and rabbits, and consists of bathing or superfusing isolated smooth muscle preparations in a stream of heparinized arterial blood taken from and returned to the animal at a constant rate. The tone of the smooth muscle preparations was affected by small changes in the concentrations of various amines. Thus increases in blood concentrations of catechol amines can be assayed with the rat stomach strip and chick rectum preparations. The proportions of adrenaline and noradrenaline in a mixture can be determined. Circulating histamine can be assayed on the blood-bathed guinea-pig ileum and bradykinin on the rat duodenum preparations. The uses and limitations of the technique are discussed.

POTENTIATION OF THE ACTION OF BRADYKININ ON SMOOTH MUSCLE BY CHYMOTRYPSIN, CHYMOTRYPSINOGEN AND TRYPSIN

Chymotrypsin, chymotrypsinogen and trypsin sensitized the guinea-pig isolated ileum and rat isolated uterus preparations to the action of bradykinin, whilst the responses to histamine, acetylcholine and 5-hydroxytryptamine were unaffected. Chymotrypsin caused a quick contraction of the guinea-pig ileum which was abolished by mepyramine and therefore probably mediated by histamine. Trypsin contracted the rat uterus as well as the guinea-pig ileum; the latter contraction was slow, resistant to mepyramine and gave rise to tachyphylaxis. It is suggested that isolated smooth muscle preparations should be treated with chymotrypsin for use in the estimation of minute amounts of bradykinin.

AN ANTIHISTAMINE-LIKE SUBSTANCE (OR SUBSTANCES) IN EXTRACTS PREPARED FROM HUMAN TISSUES

A method for the extraction of an antihistamine-like substance (or substances) from human tissues is described. The method has demonstrated the presence of this active principle(s) in a large variety of human tissues, so far without exception. The substance(s) has a wide range of activity in antagonizing the in vitro effects of histamine, 5-hydroxytryptamine and bradykinin and the Schultz-Dale reaction on the guinea-pig ileum and rat uterus preparations.

SOME PHARMACOLOGICAL PROPERTIES OF THE CIRCULAR AND LONGITUDINAL MUSCLE STRIPS FROM THE GUINEA-PIG ISOLATED ILEUM

A circular and a longitudinal muscle strip were prepared from adjacent parts of a guinea-pig ileum and a direct pharmacological comparison made under identical conditions. The longitudinal preparation was sensitive to acetylcholine, methacholine, carbachol, 5-hydroxytryptamine, histamine and nicotine, while the circular preparation was insensitive to 5-hydroxytryptamine, histamine and nicotine, and responded to the choline esters only in high concentrations. Incubation of the preparations with the anticholinesterase, mipafox (NN-diisopropylphosphodiamidic fluoride), sensitized both preparations to the action of acetylcholine; potentiation of the contraction of the longitudinal muscle was 16-times; that of the circular one 4,000-times. The longitudinal muscle was more sensitive than the circular muscle to acetylcholine whether both were treated with mipafox or not. Bradykinin and substance P both stimulated the longitudinal but not the circular muscle, an effect not modified after mipafox. Hyoscine antagonized the responses of the circular muscle strip, treated with mipafox, to acetylcholine and to histamine, but on the longitudinal muscle strip the response to histamine was not affected, the response to acetylcholine being competitively antagonized. Morphine, in the same concentrations on both circular and longitudinal muscle strips, antagonized the stimulant actions of nicotine and to a lesser extent of 5-hydroxytryptamine, but the responses to histamine on the longitudinal muscle strip were not antagonized by morphine which was in contrast to its action on the circular muscle strip. These observations showed that the main differences in the responses of the circular and longitudinal muscle of the guinea-pig ileum to drugs were in the intrinsic properties of the smooth muscle cells. In addition cholinesterase may protect the circular muscle cells. Finally the circular muscle strip preparation proved to be a useful tool to study the action of drugs on the nervous plexuses of the ileum of the guinea-pig.

THE RELEASE OF ACETYLCHOLINE BY SYMPATHETIC NERVE STIMULATION AT DIFFERENT FREQUENCIES

Supramaximal stimulation of the periarterial nerves in the mesentery to the rabbit isolated ileum causes inhibition which is increased when hyoscine is added to the bath. This increase is, however, usually seen only when the stimulus frequency is low, and the addition of hyoscine usually makes no difference when the frequency is 10 shocks/sec or more. The observations suggest that, at low frequency, stimulation releases both acetylcholine and noradrenaline, but that at higher frequencies only noradrenaline is released. Similar observations have been made for the nictitating membrane of the cat. When the postganglionic fibres from the superior cervical ganglion were stimulated supramaximally, the contractions of the membrane increased in size as the frequency rose. In the presence of hyoscine the contractions were smaller, the greatest difference being for the lowest stimulus frequency, the difference diminishing as the frequency rose. These observations are consistent with the view that sympathetic cholinergic fibres in many situations release acetylcholine to act directly only at low frequencies, and that at higher frequencies the acetylcholine is almost entirely used to release noradrenaline.

STIMULANT ACTIONS OF VOLATILE ANAESTHETICS ON SMOOTH MUSCLE

A number of volatile anaesthetics, and some compounds synthesized in the search for new anaesthetics, have been tested on guinea-pig intestinal smooth muscle in vitro. All the compounds produced a contractile response. This effect did not correlate well with convulsant activity in vivo among the compounds tested. Two kinds of stimulant effect were distinguishable: (1) Rapid, transient contractions, abolished by cocaine or lachesine; most of the anaesthetics in clinical use had this action. (2) Slow, sustained contractions, unaffected by cocaine or lachesine; this effect predominated among the fluorinated ring compounds. Hexamethonium and mepyramine did not affect the contractile response to any of the compounds. The first type of effect presumably represents excitation of postganglionic nerve cells, while the second type is a direct action on the muscle cell. The action of perfluorobenzene, which is of the latter kind, was studied further. Adrenaline and lack of calcium diminished the contraction in parallel with the contraction to histamine, which suggests that the cell membrane was the site of action; in contrast to the stimulant action of histamine or acetylcholine, the effect was highly temperature-sensitive, being almost abolished by cooling to 32 degrees C, and enhanced at 40 degrees C. The depressant action of anaesthetics on smooth muscle is affected very little by temperature changes. These findings are discussed in relation to other observations which suggest a stimulant action of volatile anaesthetics on excitable tissues. Protein denaturation is tentatively suggested as a mechanism of action.

INTESTINAL AND HEPATIC TRANSPORT OF CHOLATE AND ORGANIC DYES

The 3-hr cumulative biliary appearance of a bile salt and three representative organic dyes was studied in rats anesthetized with sodium pentobarbital. The substances were administered in one of the following locations: a branch of the portal vein, a 20-cm isolated loop of proximal jejunum, or a similar loop of distal ileum. Average biliary recoveries of 10 mg cholate, bromsulfonphthalein, diiodotetrachlorofluorescein (rose bengal) or phenolsulfonphthalein (phenol red) introduced slowly into the portal circulation were 97, 97, 68, and 23%, respectively. The respective recoveries of 30-mg quantities of these four substances placed in the intestinal loops were 43, 5, 2, and 1% proximally and 90, 5, 5, and 0.4% distally. These observations confirm the presence of a specific intestinal transport mechanism for cholate with maximal activity in the distal ileum. However, the three representative organic dyes are shown not to be carried by the intestinal bile salt transport mechanism and their movement from gut lumen to portal vein blood should be ascribed to a passive process.