Adenosine and Adenosine Analogues

Adenosine receptors that modulate adenylate cyclase activity have been identified recently in a number of tissues. The purpose of these investigations was to determine the effect of adenosine on ion transport in rabbit ileum in vitro. Adenosine and some of its analogues were found to increase the short circuit current (Isc) and the order of potency was N-ethylcarboxamide-adenosine 2 2-chloroadenosine > phenylisopropyladenosine > adenosine. Purine-intact adenosine analogues had no effect on Isc. The effect of adenosine on Isc was enhanced by deoxycoformycin, an adenosine deaminase inhibitor, and by dipyridamole, an adenosine uptake inhibitor. The increase in Isc induced by 2chloroadenosine was partially reversed in a dose-dependent manner by 8-phenyltheophylline but not by theophylline or isobutylmethylxanthine. 2-Chloroadenosine increased cyclic AMP content, and stimulated net Cl secretion; these effects were partially blocked by 8phenyltheophylline. These results suggest that there is an adenosine receptor on rabbit ileal mucosal cells that stimulates adenylate cyclase, which results in secondary active Cl secretion. Introduction Adenosine, which is produced by all cells, has long been recognized to have numerous metabolic effects in a variety of This work was performed in part during the tenure of Dr. Forrest as an Established Investigator of the American Heart Association. Dr. Dobbins is the recipient of a research Career Development Award AM-00647. Address reprint requests to Dr. Dobbins. Received for publication 9 December 1983 and in revised form 14 May 1984. tissues, including coronary vasodilatation, stimulation of steroidogenesis, stimulation of platelet aggregation, and inhibition of lipolysis (1-7). These and other effects of adenosine are antagonized by methylxanthines and potentiated by adenosine uptake inhibitors, suggesting an external receptor (8-10). Additionally, adenosine analogues that are taken up poorly by cells can have potent metabolic effects (11, 12), further suggesting an external receptor. Present evidence suggests that there are two adenosine receptors that modulate adenylate cyclase activity (Fig. 1) (810). One has a high affinity for adenosine, inhibits adenylate cyclase activity, and is termed the A, receptor by Van Calker et al. (13) and the R, receptor by Londos et al. (8). The other class of receptors has a lower affinity for adenosine, stimulates adenylate cyclase, and has been termed the A2 or Ra' receptor. A lower affinity internal "P site," which inhibits the catalytic unit of adenylate cyclase, has also been postulated (14). Recently, Forrest and collaborators (15-17) demonstrated that methylxanthine-sensitive adenosine receptors mediate chloride secretion and cyclic AMP accumulation in the elasmobranch rectal gland. Spinowitz and Zandunaisky (18) previously found that chloride transport in the amphibian cornea is increased by adenosine. Because secondary active chloride transport is common to several epithelia, including amphibian cornea, elasmobranch rectal gland, and mammalian intestine (19), we studied the effects of adenosine on ion transport in the rabbit ileum. Our studies provide the first evidence for adenosine-mediated cyclic AMP-dependent chloride transport in a mammalian epithelium. Methods Nonfasting male New Zealand white rabbits weighing 2-3 kg were killed with an intravenous air bolus and the terminal ileum was 1. Abbreviations used in this paper: Ado, adenosine; 2-Cl-Ado, 2chloroadenosine; GMP, guanosine monophosphate; IBMX, isobutylmethylxanthine; Isc, short circuit current; NECA, N-ethylcarboxamideadenosine; PIA, phenylisopropyladenosine; 8-PT, 8-phenyltheophylline; R. and Ri, adenosine receptors that stimulate and inhibit adenylate cyclase, respectively. 929 Adenosine Effect on Ileal Ion Transport J. Clin. Invest. © The American Society for Clinical Investigation, Inc. 0021-9738/84/09/0929/07 $ 1.00 Volume 74, September 1984, 929-935 Downloaded from http://www.jci.org on June 26, 2017. https://doi.org/10.1172/JCI111511