Differential desensitization of A1 adenosine receptor-mediated inhibition of cardiac myocyte contractility and adenylate cyclase activity. Relation to the regulation of receptor affinity and density.

Abstract

Effects of chronic exposure of cultured atrial myocytes to R-N6-(2-phenylisopropyl)-adenosine (R-PIA) on the A1 adenosine receptor-mediated inhibition of adenylate cyclase activity and myocyte contractility were examined. Chronic exposure of atrial myocytes cultured from 14-day-old chick embryos to R-PIA desensitized the myocyte to the inhibitory effects of R-PIA on contractility and adenylate cyclase activity in a time- and dose-dependent manner. Desensitization of the negative inotropic response was only partial, whereas the adenosine receptor-mediated inhibition of adenylate cyclase activity was almost completely absent after 24 hours of R-PIA (1 microM) exposure. Furthermore, the contractile response to R-PIA desensitized more slowly than the desensitization of A1 adenosine receptor-mediated inhibition of adenylate cyclase (t1/2 = 11.4 +/- 0.7 hours versus 7.5 +/- 1 hours, mean +/- SEM, n = 12 and 6, respectively). Thus, the two A1 adenosine receptor-linked functional responses desensitized differently in response to chronic exposure of the myocyte to R-PIA. Binding of the antagonist radioligand [3H]-8-cyclopentyl-1,3-dipropylxanthine [( 3H]CPX) in membranes from myocytes preexposed to R-PIA demonstrated a time-dependent decrease in receptor density without any change in the affinity for the antagonist radioligand. Computer analyses of agonist competition with [3H]CPX binding in membranes from control and R-PIA-treated myocytes revealed a conversion of the high-affinity A1 adenosine receptor to a low-affinity form such that after 24 hours of 1 microM R-PIA exposure, all of the receptors were in a low-affinity form.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of adenosine receptors in intact cultured heart cells

“…It is worth mentioning that similar results were obtained by Liang and Donovan [36], in which sustained exposure of cultured chick myocytes to adenosine agonists produced a 40% decrease in the force of contractions, accompanied by a 40% reduction in the receptor number.…”

“…Estimates of adenosine A 1 receptor density in cardiac cell membranes are in the range of 15-30 fmol/mg protein in rat [34], pig [35], guinea pig [22,37], chick embryos [21,36], bovine [38] and diseased human cardiac preparations [39]. There are differences in the density of atria1 adenosine receptors (A 1 ) according to the species: guinea pig > rat > rabbit [37].…”

Protection against ischemic damage by adenosine amine congener, a potent and selective adenosine A1 receptor agonist

“…Substantial evidence obtained from non-neural tissues demonstrates adenosine A 1 receptor downregulation following protracted exposure to their agonists (e.g., Parsons and Stiles, 1987;Longabaugh et al, 1989;Liang and Donovan, 1990). Nonetheless, studies in which cerebral adenosine A 1 receptors have been chronically exposed to either agonists (Von Lubitz et al, 1994a) or non-selective antagonists (Georgiev et al, 1993) provided contradictory data indicating that, contrary to the expectations, chronic treatment with drugs acting at adenosine A 1 receptors does not result in the decrease of the receptor density.…”

A physiological role of the adenosine A 3 receptor: Sustained cardioprotection

“…Atrial and ventricular cells were cultured from chicken embryos 14 days in ovo and maintained in culture as described previously (9,10,15). All experiments were performed on day 3 in culture, at which time the medium was changed to a Hepes-buffered medium containing 139 mM NaCl, 4.7 mM KCl, 0.5 mM MgCl 2 , 0.9 mM CaCl 2 , 5 mM Hepes (N-2-hydroxyethylpiperazine-NЈ-2-ethanesulfonic acid) and 2% fetal bovine serum, pH 7.4, 37°C, before exposing the myocytes to simulated ischemia.…”