Ž . European Journal of Pharmacology 340 1997 243–247 Ca 2q dependence of the response of three adenosine type receptors in rat hepatocytes Raquel Guinzberg a , Antonio Dıaz-Cruz b , Salvador Uribe c , Enrique Pina a, ) ´ ˜ a Department of Biochemistry, School of Medicine, UniÕersidad Nacional Autonoma de Mexico, Apdo. Postal 70-159, 04510 Mexico DF, Mexico ´ ´ b School of Veterinary Medicine, UniÕersidad Nacional Autonoma de Mexico, Apdo. Postal 70-159, 04510 Mexico DF, Mexico ´ ´ c Inst de Fisiologıa Celular, UniÕersidad Nacional Autonoma de Mexico, Apdo. Postal 70-159, 04510 Mexico DF, Mexico ´ ´ ´ Received 21 August 1997; revised 29 September 1997; accepted 30 September 1997 Abstract The effect of three different receptor-specific adenosine agonists on the rate of ureagenesis by isolated rat hepatocytes and the 2q Žw 2q x . w 2q x dependence on the external free Ca concentration Ca were investigated. In the presence of high Ca all adenosine receptor e e w 2q x Ž. agonists increased ureagenesis to similar levels. However, with low Ca the effects of each agonist varied as follows: i the adenosine e 6 w 2q x Ž. A receptor agonist, 2-chloro-N -cyclopentyl-adenosine, increased ureagenesis depending partially on Ca , ii the adenosine receptor 1 e Ž . X A agonist, 2-p- -2-carboxy-ethyl phenethylamino-5 -N-ethylcarboxyamido adenosine hydrochloride, increased ureagenesis indepen- 2 w 2q x Ž . 6 Ž . dently of Ca and iii in contrast, the adenosine receptor A agonist N -2- -4-aminophenyl ethyladenosine, increased ureagenesis e 3 w 2q x only in the presence of high Ca . The adenosine receptor A antagonist, 1-allyl-3,7-dimethyl-8-phenyl xanthine, inhibited the effect of e 1 the adenosine receptor A agonist on ureagenesis, but not the effect of the adenosine A or A receptor agonists. The adenosine A 1 2 3 2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine, inhibited only the effect of the adenosine A receptor agonist. Thus, in addition to 2 A and A type adenosine receptors, rat hepatocytes possess an A -like adenosine receptor which responds to the addition of an 1 2 3 w 2q x adenosine A agonist by accelerating ureagenesis a Ca dependent manner. Moreover, it was observed that in the presence of 3 e 2q w 2q x extracellular Ca each agonist increased Ca and this effect was inhibited by the appropriate specific antagonist. q 1997 Elsevier i Science B.V. Keywords: Adenosine; Adenosine receptor; Ca 2q ; Cell; Cellular metabolism; Ureagenesis 1. Introduction In mammals, the interaction of adenosine with the different adenosine receptors found in the plasma mem- brane of each type of cell results in a large number of physiological effects. There are at least five subtypes of adenosine receptors. These have been classified as: A , 1 A ,A ,A and A . This classification is supported by 2A 2B 3 4 biochemical, pharmacological and molecular cloning evi- Ž . dence Dalziel and Westfall, 1994; Palmer et al., 1995 . In some organs and species, such as mouse and humans, adenosine A receptors have been cloned. However, no 3 reports are available on the physiological response of the adenosine A receptor in the liver. The synthesis of adeno- 3 sine receptor-specific agonists allows the analysis of the cellular response to the stimulation of each specific adeno- ) Corresponding author. Tel.: q52-5-6232403; fax: q52-5-6225630; e-mail: suribe@ifcsun1.ifisiol.unam.mx. sine receptor type. Thus, in this paper, the effects of three adenosine receptor-specific agonists and two adenosine receptor-specific antagonists on the rate of urea synthesis in isolated rat hepatocytes are reported. The dependence of w 2q x the ureagenic response on Ca was investigated in e Ž. order to gain insight into the transduction signal s in- volved for each of these receptors within the liver. 2. Materials and methods All reagents were of the highest purity available com- mercially. Adenosine, quin-2, A23187, bovine serum albu- min type V and dimethylsulfoxide were from Sigma Co., St Louis, MO. Adenosine agonists were chosen according Ž to their affinity for each kind of receptor RBI catalog, . RBI Co, Natick, MA . These were as follows: the adeno- sine A receptor agonist was 2-chloro-N 6 -cyclopentyl- 1 Ž .Ž . adenosine CCPA K s 0.2–0.4 nM ; the adenosine A a 2 0014-2999r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.