Homologous sensitisation of embryonic chick atrial myocytes to adenosine: Mediation by adenosine A₁ receptor and guanine nucleotide binding protein

Objective: The aim was to characterise the process and the mechanisms of sensitisation of the atrial myocyte to adenosine receptor agonist. Methods: The ability of adenosine A₁ receptor to mediate inhibition of adenylyl cyclase activity and myocyte contractility was determined in atrial myocytes cultured from 14 d chick embryos. Under conditions in which the myocytes were sensitised to the effects of A₁ agonist, changes in the levels of adenosine A₁ receptor and pertussis toxin sensitive G proteins were determined and correlated with alterations in the adenylyl cyclase activity and contractile responses of the myocyte to the A₁ agonist. Results: Removal of adenosine from the culture medium with adenosine deaminase resulted in an enhanced ability of the adenosine A₁ receptor agonist R-N⁶-(2-phenylisopropyl)-adenosine to exert a direct, negative inotropic effect and to inhibit isoprenaline stimulated adenylyl cyclase activity. The increase in the extent of maximum inhibition of adenylyl cyclase activity and of myocyte contractility was 215(30), n=5, and 90(10)%, n=14, respectively. Binding of the antagonist radioligand [³H]-8-cyclopentyl-1,3-dipropylxanthine in membranes from myocytes pre-exposed to adenosine deaminase showed a 70% increase in the adenosine A₁ receptor density and a 54% increase in the proportion of the high affinity adenosine A₁ receptor: control 33(5)%, n=5, versus sensitised 49(3)%, n=5, p<0.01. The increase in the total number of adenosine receptors and the proportion of the high affinity form was associated with a similar increase in the level of pertussis toxin sensitive G protein(s), as determined by pertussis toxin mediated ³²P-ADP ribosylation and by immunoblotting. Prior exposure of the culture to the adenosine receptor antagonist 8-(p-sulphophenyl) theophylline also led to similar results. Conclusions: The data indicate that the increased level of pertussis toxin sensitive G protein(s) results in an enhanced coupling to form the high affinity adenosine A₁ receptor, that newly formed high affinity receptors are linked to an enhanced sensitivity of atrial myocytes to A₁ adenosine agonist stimulation, and that upregulation of the G protein is a mechanism mediating the homologous sensitisation of cardiac adenosine A₁ receptor pathway.Cardiovascular Research 1993;27:102-110.