Mechanism and Pressor Relevance of the Short-Term Cardiovascular and Renin Excitatory Actions of the Selective A2A-Adenosine Receptor Agonists

Selective A 2A adenosine receptor agonists are potent vasodilators that reduce blood pressure and induce marked increments in heart rate and plasma renin activity (PRA). To examine the mechanisms and pressor relevance of these cardiac and renin responses, we measured blood pressure and heart rate by telemetry and PRA in separate sets of spontaneously hypertensive rats (SHRs), which were given i.p. 2-hexynyl-5-methylcarboxamidoadenosine (2HE-NECA, 0.01 mg/kg) and 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680, 0.1 mg/kg) alone and after pretreatment with the β 1 -adrenoceptor blocking agent atenolol (100 mg/kg). The effects of 2HE-NECA (0.003 mg/kg) also were examined after pretreatment with the angiotensin-converting enzyme (ACE) inhibitor spirapril (3 mg/kg). Both A 2A agonists induced marked reductions in blood pressure, associated with significant increments in heart rate and in PRA. Atenolol reduced blood pressure to the same extent as did the A 2A agonists and markedly decreased heart rate and PRA. Pretreatment with atenolol entirely prevented the increase in heart rate and in PRA induced by the two A 2A agonists but potentiated only slightly their antihypertensive effect. Spirapril alone reduced blood pressure and increased PRA and when given before 2HE-NECA potentiated its depressor and renin-stimulating effects by 44% and 69%, respectively. These results suggest that the increase in heart rate and in PRA induced by A 2A agonists is the result of a reflex increase in sympathetic activity triggered by the decrease in blood pressure rather than of a direct stimulating effect on cardiac and renal A 2A -adenosine receptors; the reactive activation of the renin-angiotensin system elicited by these compounds may contribute to blunting their antihypertensive effect.