A. Crema

The effect of catecholamines and sympathetic stimulation on the release of acetylcholine from the guinea-pig colon

1 In isolated guinea‐pig terminal colon, the effect of sympathetic stimulation on contraction and acetylcholine release elicited by pelvic and transmural stimulation was investigated. 2 Sympathetic stimulation reduced the nerve‐mediated contractile responses more than those produced by added acetylcholine. 3 Sympathetic stimulation also reduced the acetylcholine released during pelvic and transmural stimulation at low frequency. The inhibitory effect on acetylcholine released from resting colons is concealed by the simultaneous release of acetylcholine in considerable amounts from stimulated periarterial nerves which probably contain parasympathetic fibres. 4 The inhibitory effect of endogenous and exogenous catecholamines prevails when cholinergic neurones fire at low rates. It was confirmed that adrenaline is more active than noradrenaline. 5 The release of acetylcholine from unstimulated colons was for the most part maintained by nerve‐conducted activity, because tetrodotoxin was able to reduce it to about one‐tenth. 6 It is suggested that the sympathetic control of gastrointestinal tone and motility is exerted through two different routes: inhibition of intramural cholinergic plexuses and direct relaxation of smooth muscle cells. 7 The possible site and mechanism of action of catecholamines on intramural cholinergic structures is briefly discussed.

Some observations on the intrinsic nervous mechanism in Hirschsprung's disease

Both at rest and during transmural stimulation acetylcholine output from isolated longitudinal and circular muscle strips is significantly higher in the spastic segment than in the proximal dilated bowel. No difference has been found in the tissue concentration of acetylcholine between ganglionic and aganglionic specimens. The pattern of response to transmural stimulation is also similar in the spastic and dilated bowel. However, after cholinergic and adrenergic blockade transmural stimulation fails to induce relaxation in aganglionic specimens, as it does in normal colon. The hypotheses are advanced that the increase in acetylcholine output may be partly dependent on a failure of the intrinsic modulating mechanisms and that an alteration of the non-adrenergic inhibitory neurons may be involved in the motor disturbances of the aganglionic tract.

Presence of a non-adrenergic inhibitory system in the human colon.

The presence in the intestinal wall of intramural non-adrenergic inhibitor neurones has been postulated by many workers. Burnstock, Campbell, Bennett, and Holman (1964) have reported that in the taenia coli of the guinea pig the inhibitory responses to transmural stimulation, which persist in the presence of bretylium and guanethidine, are mediated by intrinsic nerves which are distinct from the sympathetic and parasympathetic system. According to Burnstock, Campbell, and Rand (1966) the taenia coli of the guinea pig is also innervated by intramural inhibitory nerves with their cell bodies in Auerbachs plexus. These nerves could be excited by electrical stimulation of the taenia or by the application of ganglion-stimulating drugs. The intramural inhibitory nerves had different properties from sympathetic adrenergic nerves. Relaxation to stimulation was maximal at about 5 pulses/sec and was not blocked by bretylium, guanethidine, or DMPP. Bennett (1966) and Bennett, Burnstock, and Holman (1966) presented evidence for the existence in the guinea-pig taenia coli of inhibitory nerves distinct from sympathetic perivascular nerves. The evidence was based on the observation that the pattern of the inhibitory junction potential was not changed by antiadrenergic agents and the hyperpolarization produced by intramural stimulation was different from that produced by perivascular stimulation. Electrophysiological evidence of intestinal inhibitor nerves was reported also by Bulbring and Tomita (1967). The existence of non-adrenergic inhibitor nerves in the stomach wall has been shown by Martinson (1965a and b), by Campbell (1966), and by Bulbring and Gershon (1967). An inhibition resulting from activation of non-adrenergic inhibitory neurones by transmural stimulation has recently been described in rabbit ileum by Day and Warren (1968). All the results mentioned above were obtained from animal specimens. The present paper deals with similar studies in the human colon.

Vagal non‐adrenergic inhibition of guinea‐pig stomach

1. The effect of vagal and sympathetic stimulation on the mechanical and electrical activity (intracellular recording) of the body of the guinea‐pig stomach was investigated in vitro.

Action of catecholamines on release of acetylcholine from human taenia coli

Abstract The amount of acetylcholine released from longitudinal strips of human taenia coli increased with frequency of stimulation; in contrast there was an inverse relationship between stimulation rate and output per stimulus. The reduction of acetylcholine release by tetrodotoxin was greater during stimulation than at rest. Noradrenaline 1 × 10−6 g/ml (but not isoprenaline 1 × 10−6 g/ml) reduced significantly acetylcholine release from preparations stimulated at a frequency of 2/sec. No effect was observed on the resting release. It was concluded that in human taenia coli catecholamines modulate the activity of cholinergic neurons probably through α-receptors.

Further evidence for the presence of non-adrenergic inhibitory structures in the guinea-pig colon

Abstract A pharmacological analysis was carried out of the responses of guinea-pig isolated terminal colon to transmural, parasyinpathetic and sympathetic electrical stimulation. The contraction produced by stimulating the pelvic nerves was abolished by atropine and the inhibition produced by stimulating the periarterial nerves was suppressed by dibenamine plus propranolol and by adrenergic blocking agents. After complete pharmacological blockade of extrinsic sympathetic and parasympathetic supply, transmural stimulation produced a pure inhibitory response particularly when low frequencies (1–2/sec) were used. This response was abolished by tetrodotoxin. Some physiologically occurring substances were tested with the aim of mimicking such inhibition, but this was unsuccessful. 5-Hydroxy-tryptamine displayed complex effects, mediated through the intramural nervous structures; moreover it evoked a non-adrenergic inhibitory response which was abolished by tetrodotoxin. The presence of non-adrenergic inhibitory structures was confirmed in this part of the gastrointestinal tract. The possible nature of the contractile response which follows the primary inhibition due to transmural stimulation is discussed. It is suggested that the possible release of excitatory and inhibitory substances, different from the so-called “neurotransmitters”, is a consequence of physical and chemical stimulation of the intestinal wall.

Antagonism of atropine towards endogenous and exogenous acetylcholine before and after sympathetic system blockade in the isolated distal guinea-pig colon

Abstract M.DEL TACCA, S.LECCHINI, G.M.FRIGO, A.CREMA and G.BENZI, Antagonism of atropine towards endogenous and exogenous acetylcholine before and after sympathetic system blockade in the isolated distal guinea-pig colon, European J. Phannacol. 4 (1968) 188–197. The antagonism of atropine towards the contraction of the isolated guinea-pig colon caused by acetylcholine, pelvic nerves, and transmural stimulation has been investigated. Atropine was more active against exogenous than endogenous acetylcholine. On the other hand, transmural stimulation was more sensitive than pelvic reserves stimulation. After sympathetic blockade (induced by pretreating the animals with reserpine or a-methyl-p-tyrosine, by surgical sympathetic denervation, or by adding bretylium to the bath) the potency of atropine towards exogenous acetylcholine increased, while it was decreased about 100 times against pelvic and transmural stimulation. Acetylcholine content and release in reserpinized animals did not differ from that of controls.

On the Ability of Caerulein to Increase Propulsive Activity in the Isolated Small and Large Intestine

Summary1.Caerulein increased the efficiency of the peristaltic reflex both in the large and small intestine. Such an effect could be evidenced by measuring the velocity of propulsion of a solid bolus in the colon and the amount of ejected fluid in the fatigued ileum. The effect on propulsion was related to the concentrations employed and log dose-response curves could be constructed.2.Caerulein was able to counteract the inhibitory effect of mild cooling (25 to 28‡ C) and of stimulation of the periarterial nerves while it did not affect the blockade of propulsion brought about by strong cooling (18–21‡ C) or by high frequency transmural stimulation. Moreover it increased the accelerating effect of the pelvic nerves and of low frequency transmural stimulations.3.Both in the colon and in the ileum caerulein did not restore the propulsion impaired by tetrodotoxin while it was able to counteract the blockade caused by hexamethonium. The impairment of propulsion produced by atropine was antagonized in the ileum but not in the colon. It is supposed that caerulein affects propulsion by acting on the nervous structures involved in the reflex, possibly at a presynaptic level.4.In many ways the action of caerulein on peristalsis resembles that of the hormone cholecystokinin-pancreozymin. Both favour propulsion at concentrations much lower than those necessary to contract the muscular coats. Their effect on peristalsis does not show tachyphylaxis.

Pharmacological studies of the rabbit and human renal pelvis

SummaryThe responses of the isolated rabbit and human renal pelvis to drugs and to electrical stimulation have been investigated. Regular spontaneous changes in tension occur in the circular smooth muscle, which in the rabbit are synchronous with electrical waves. The responses to electrical stimulation seem to be due to the release of catecholamines from adrenergic nerves. Such nerves have been observed by means of a fluorescence technique. The cholinergic system appears functionally unimportant. The adrenergic responses in human and rabbit seem to be mediated exclusively by α-adrenoceptors.The denervated rabbit organ shows hypersensitivity, mainly to low concentrations of adrenaline. The failure of uptake inhibitors to potentiate the responses to noradrenaline seems in favour of postsynaptic hypersensitivity.

The time course of functional and morphological changes of the guinea-pig colon after “a frigore” denervation of the periarterial sympathetic nerves

SummaryAfter sympathetic denervation of the guinea-pig distal colon the time course and the relationship between morphological and functional changes were studied, by using the fluorescence method and by evaluating the response of longitudinal muscle to periarterial nerve stimulation.The inhibitory response to sympathetic nerve stimulation was completely abolished 48 h after denervation. Tissue catecholamines can no longer be detected in either the intramural vessels or in the intramural plexuses 96 h after denervation, and in no preparation could they be observed again until 24 days after denervation.After 30 days catecholamines stores slowly began to reappear and were completely restored only 100–120 days after denervation. During the whole period of reinnervation the response to periarterial nerve stimulation was lacking.The inhibitory effect could be elicited only 130 days after denervation and before the morphological pattern had been completely restored.