European Journal of Pharmacology - Molecular Pharmacology Section, 244 (1993) 223-230 223 '9 1993 Elsevier Science Publishers B.V. All rights reserved 0922-4106/93/$06.00 EJPMOL 90403 Affinity of central adenosine A 1 receptors is decreased in spontaneously hypertensive rats Alexandra Matias ~, Franz Josef Zimmer, Anna Lorenzen, Roger Keil and Ulrich Schwabe Pharmakologisches lnstitut der Unit'ersitiit Heidelberg. bn Neuenheimer Feld 366. 6900 Heidelberg, Germany Received 27 July 1992, revised MS received 14 October 1992. accepted 20 October 1992 Functional defects in purinergic neurotransmission have been related to the development of arterial hypertension in spontaneously hypertensive rats. In order to elucidate the molecular basis of this perturbation, we have directly characterized adenosine A 1 receptors using radioligand binding to rat brain membranes of Wistar Kyoto (WKY) and stroke-prone sponta- neously hypertensive rats (SHRSP). Saturation studies with [3H]l,3-dipropylcyclopentylxanthine ([3H]DPCPX) showed a lower affinity in both 5- and 48-week-old SHRSP in comparison with age-matched WKY. Similarly, competition experiments with [3H]DPCPX showed lower affinity of R-N6-phenylisopropyladenosine for the low-affinity binding site in 5- and 48-week-old SHRSP in comparison with WKY. In both studies, the difference in Ko values was abolished by guanosine-5'-triphosphate in 5-week-old rats and mitigated in 48-week-old animals. No differences in Bmax values were observed in 5-week-old rats, whereas in 48-week-old SHRSP the number of receptors was significantly higher in comparison with age-matched WKY. Saturation experiments with the A l-selective agonist [3H]2-chloro-N6-cyclopentyladenosine ([3H]CCPA) demonstrated a higher affinity in 5-week-old SHRSP, whereas in 48-week-old hypertensive animals it was lower than in control WKY rats. No difference in receptor number was detected in comparison with age-matched WKY. In conclusion, our data demonstrated a diminished affinity of central adenosine A~ receptors for antagonists and for the low affinity statc of the agonist binding site in genetically hypertensive rats. This might be due to structural changes of the receptor protein, to an altered G protein or defcctive receptor-G protein coupling in arterial hypertension. Adenosine A t receptors; Hypertension; G proteins 1. Introduction Extracellular adenosine, acting via membrane-bound receptors, is a ubiquitous modulator of cellular activity. Its first reported action was the ability to lower sys- temic blood pressure by vasodilatory effects on blood vessels (Drury and Szent-Gy6rgi, 1929). The important role of adenosine in the central nervous system has early been recognized when the effects of intracere- broventricular administration of adenosine were stud- ied (Feldberg and Sherwood, 1954). In the brain, adenosine has been shown to elicit potent effects on various biochemical and electrophysi- ological processes. It acts either directly via presynaptic inhibition of neurotransmitter release (Hedquist and Correspondence to: U. Schwabe, Pharmakologisches Institut tier Universifiit Heidelberg, Im Neuenheimer Feld 366, D-6900 Heidel- berg, Germany. Tel.: +49 6221 56 3902; Fax: +49 6221 53 3944. 1 Fellow of the Alexander von Humboldt-Stiftung, on leave from Department of Pharmacology and Therapeutics, Faculty of Medicine, P-4200 Porto, Portugal. Fredholm, 1976; Williams, 1984) or indirectly via ac- tions on postsynaptic sites, reducing neuronal excitabil- ity (Phillis and Wu, 1981), activating K + channels, inhibiting Ca 2+ influx (Dolphin et al., 1986; Michaelis, 1988; Linden and Delahunty, 1989) and dilating blood vessels (Jackisch et al., 1985; Schubert and Kreutzberg, 1987; Dunwiddie and Fredholm, 1989; Linden, 1991). Defective adenosine receptor mechanisms have been proposed to be involved in the pathophysiology of essential hypertension. Studies in which adenosine was injected in the area postrema and the nucleus tractus solitarii have shown that the hypotensive response to adenosine was markedly attenuated in spontaneously hypertensive rats (SHR) when compared to Wistar Kyoto (WKY) rats (Robertson et al., 1988), suggesting that the affinity or the number of central adenosine receptors in the hypertensive rats are decreased. Similarly, several reports provide evidence that a malfunction of adenosine A 1 receptors in blood vessels of SHR may be involved in the development of primary hypertension (Kamikawa et al., 1980; Kubo and Su, 1983; Jackson, 1987; Illes et al., 1989; Green et al.,