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.