European Journal of Pharmacology, 159 (1989) 33-40 33 Elsevier EJP 50587 Relaxing effects of adenosine in coronary artery in calcium-free medium Mudumbi V. Ramagopal 1, Mikio Nakazawa 2 and S. Jamal Mustafa * Department of Pharmacology, School of Medicine, East Carolina University. Greenville, NC 27858-4354, U.S.A. Received 3 March 1988, revised MS received 4 August 1988, accepted 11 October 1988 Adenosine relaxes the coronary arteries of various species through A 2 receptors. The mechanism(s) by which adenosine causes relaxation of the coronary smooth muscle through changes in intracellular Ca levels is not understood. Therefore, the aim of this study was to evaluate the vasodilatory action of adenosine in normal and in Ca-free medium and the effect of adenosine on 45Ca efflux in Ca-free medium. Prostaglandin F2, (10 5 M) was used to induce tone in bovine coronary artery rings both in the presence and absence of Ca. Adenosine, 5'-N-ethyl- carboxamide adenosine (NECA) and N6-L-phenyl-isopropyl adenosine (L-PIA) produced concentration-dependent relaxation of the coronary artery rings when precontracted with prostaglandin F2~. Both in normal and Ca-free medium, the order of potency for adenosine analogs (NECA > L-PIA > adenosine) was similar and 8-phenyltheophyl- line antagonized the relaxation response to adenosine and its analogs. In Ca-free medium, the concentration-response curves for adenosine and its analogs were shifted to the right, in a parallel fashion, from the curves obtained in Ca-containing solution. Removal of extracellular Ca reduced the maximum responses and slowed the rate of relaxation of adenosine and its analogs and the K B values for 8-phenyltheophylline were significantly increased. In Ca-free medium, adenosine was without an effect on 4SCa efflux in the presence of prostaglandin F2,. The data suggest that the coronary vasodilatory action of adenosine has both extracelhilar Ca-dependent and -independent components. Adenosine receptors; Ca 2 +; Coronary circulation; Smooth muscle (vascular); Prostaglandin 1=2, ~ 1. Introduction Adenosine plays an important role in the regu- lation of myocardial blood flow (Berne, 1980). This nucleoside dilates various blood vessels in- cluding canine (Kusachi et al., 1983), bovine (Mustafa and Askar, 1985) and human (Rama- gopal et al., 1988) coronary arteries through A 2 adenosine receptors. The mechanism(s) by which 1 Present address: Department of Molecular Pharmacology, Glaxo Inc., 5, Moore Drive, Research Triangle Park, NC, U.S.A. 2 Present address: Department of Pharmacology, Nigata Uni- versity School of Medicine, No. 757 Asahimachi Dori 1, Nigata, Japan. * To whom all correspondence should be addressed: Depart- ment of Pharmacology, School of Medicine, East Carolina University, Greenville, NC 27858-4354, U.S.A. adenosine causes the relaxation of the coronary blood vessels after its binding to the extracellular membrane site is not well understood. It has been suggested that the coronary vasodilatory action of adenosine might involve the inhibition of Ca in- flux (Harder et al., 1979; Fenton et al., 1982). However, in Ca-free medium, adenosine inhibited norepinephrine-induced contractions in dog saphenous vein and rabbit femoral artery (Ver- haeghe, 1977; Young and Merrill, 1983) indicating that adenosine may induce relaxation partly through extracellular Ca-independent mechanism. Bradley and Morgan (1985) reported that in ferret portal vein low doses of adenosine decrease cyto- solic ionized Ca, whereas higher doses of adeno- sine desensitize the smooth muscle myofilaments. These studies indicated that adenosine relaxes the smooth muscle by inhibition of Ca influx and/or by lowering the intracellular Ca level. The lower- 0014o2999/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)