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Functional characterization of the adenosine receptor mediating inhibition of peristalsis in the rat jejunum
Author(s) -
Hancock Debra L.,
Coupar Ian M.
Publication year - 1995
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1995.tb14995.x
Subject(s) - adenosine , agonist , peristalsis , medicine , jejunum , endocrinology , adenosine receptor , antagonist , ileum , adenosine a1 receptor , chemistry , pharmacology , biology , receptor
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 cmH 2 O 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: N 6 ‐cyclopentyladenosine (CPA) > NECA > R (−)−N 6 ‐(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 A 1 receptor subtype (CPA > R ‐PIA = CHA = > NECA > 2‐CADO > S‐PIA > CV‐1808). 4 The selective A 1 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 μ m caused parallel rightward shifts in the concentration‐response curve to the non‐selective A l /A 2 agonist NECA. DPCPX was significantly more potent at inhibiting NECA than 8‐PT as revealed by their apparent pA 2 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 A 1 and not an A 2B receptor. In addition, the high affinity of DPCPX (pA 2 :9.37) against the selective A 2 agonist CPA, further confirms the presence of the A 1 receptor subtype. 5 In this study we found that the A 1 adenosine receptor is involved in regulating in vitro peristalsis which is different from the adenosine receptor regulating inhibition of secretion (A 2B ) in the same region of intestine of the same species. We propose that A 2B adenosine agonists could be of clinical value in the management of diarrhoea that is due to microbiological organisms where antimotility effects are not desired.