Functional characterization of the adenosine receptor mediating inhibition of peristalsis in the rat jejunum

Abstract

1 The non-selective adenosine agonist, 5'-N-ethylcarboxamidoadenosine (NECA), is a potent inhibitor of morphine withdrawal diarrhoea in rats. More recently we found that NECA exerts its antidiarrhoeal effect by inhibiting secretion in both the jejunum and ileum and also by inhibiting peristalsis in the ileum. The specific aim of this study was to characterize the receptor in the rat jejunum mediating inhibition of peristalsis via functional studies using a range of metabolically stable adenosine analogues based on the pharmacological criteria of relative agonist and antagonist potencies. 2 Peristalsis in the rat isolated jejunum was achieved by raising the pressure to between 7-11 cmH2O for 3 min followed by a 3 min rest period (pressure at zero). The mean rate of peristalsis during inflation was 7.3 + 0.1 peristaltic waves per 3 min and this rate remained consistent for up to 30 min, in 5 separate tissues. The inhibitory effects of the adenosine analogues were quantified by expressing their effects as a % reduction in the mean number of peristaltic contractions derived from the control tissues. 3 The rank order of agonist potency to reduce the rate of peristalsis was: N6-cyclopentyladenosine (CPA) > NECA > R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) > chloroadenosine (2-CADO) > S-PIA > 2-phenylaminoadenosine (CV-1808). This order complies well with the rank order of agonist potency that represents the activation of the Al receptor subtype (CPA> R-PIA = CHA = > NECA> 2-CADO > S-PIA > CV-1808). 4 The selective A1 adenosine antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) and the nonselective adenosine antagonist 8-phenyltheophylline (8-PT) at their respective concentrations of 10 nM and 2 gM caused parallel rightward shifts in the concentration-response curve to the non-selective Al/A2 agonist NECA. DPCPX was significantly more potent at inhibiting NECA than 8-PT as revealed by their apparent pA2 values; DPCPX (9.5) and 8-PT (7.26). The high affinity of DPCPX relative to that of 8-PT suggests the presence of an Al and not an A2B receptor. In addition, the high affinity of DPCPX (pA2:9.37) against the selective Al agonist CPA, further confirms the presence of the Al receptor subtype. 5 In this study we found that the Al adenosine receptor is involved in regulating in vitro peristalsis which is different from the adenosine receptor regulating inhibition of secretion (A2B) in the same region of intestine of the same species. We propose that A2B adenosine agonists could be of clinical value in the management of diarrhoea that is due to microbiological organisms where antimotility effects are not desired.

Modulatory effect of adenosine receptors on the ascending and descending neural reflex responses of rat ileum

“…The fact that the specific A 1 antagonist DPCPX was able to attenuate the reflex response in the preparations used contrasts with the findings of Hancock and Coupar [31]. Using a different preparation, with reflex stimulation by fluid inflation and recording of longitudinal muscle contraction and of volume expulsion, they were unable to find any influence of the A 1 antagonist DPCPX on the peristaltic reflex response [31]. This difference might be due to different experimental set-ups (electrical vs. fluid inflation), to the use of different muscle layers to record contractile activity (longitudinal vs. circular smooth muscle), or possibly to the fact that different regions were investigated (ileum vs. jejunum).…”

“…The ascending contractile response is increased by the A 1 agonist CPA at lower concentrations, whereas it is attenuated at higher concentrations. This finding is different compared to studies focused on the peristaltic reflex and peristalsis, in which activation of A 1 receptors causes an attenuation of the ascending contraction in guinea-pig ileum [30] or a decrease in peristalsis in rat jejunum [31] but direct comparison in this case is hampered by the different setups, different species and the different and higher agonist concentration used. The experimental set-up used in the present study also showed an attenuation of the ascending contractions, but only at high concentrations – an effect that may be nonspecific, whereas at lower concentrations the effect of CPA has to be regarded as a specific A 1 receptor-mediated effect.…”

“…Since in the present experimental set-up, the A 1 receptor antagonist DPCPX when given alone also attenuates the myenteric reflex responses, these data suggest that activation of A 1 receptors by endogenously released adenosine under our experimental conditions stimulates the myenteric reflex responses. The fact that the specific A 1 antagonist DPCPX was able to attenuate the reflex response in the preparations used contrasts with the findings of Hancock and Coupar [31]. Using a different preparation, with reflex stimulation by fluid inflation and recording of longitudinal muscle contraction and of volume expulsion, they were unable to find any influence of the A 1 antagonist DPCPX on the peristaltic reflex response [31].…”

Pharmacological modulation of adenosine system: Novel options for treatment of inflammatory bowel diseases

“…These findings support previous in vivo data showing that DPCPX, a selective A 1 receptor antagonist, markedly enhanced gut propulsion. A 2a receptors mediate facilitatory effects on acetylcholine release in rat ileum, while A 2b receptor stimulation results in a marked inhibition of peristaltic activity in rat small bowel 84–86. Although there is evidence of A 3 receptor expression in rat jejunum,87 data on their possible role in the regulation of gut motility are lacking.…”

Purine, Kynurenine, Neopterin and Lipid Peroxidation Levels in Inflammatory Bowel Disease

“…In addition, the efflux of adenosine from cells is normally increased in the face of damage, and it might be anticipated that increased local levels of adenosine might arise in inflammatory disease states. The increased adenosine could then modify neuronal activity by acting directly on the cell membrane to depolarise or hyperpolarise [1,2,5,6,41] or to modify the release of enteric neurotransmitters, with A1 receptors depressing and A2A receptors enhancing the release of classical and non-cholinergic, non-adrenergic transmitters [2,16]. There is also accumulating evidence for the effects of adenosine on both ion secretion [8] and cell proliferation and survival [18] in the intestinal epithelium, leading some authors to propose that adenosine may be an endogenous regulator of these functions and may be involved in the aetiology of bowel disorders.…”