Cardiovascular effects of adenosine A2 agonists in the conscious spontaneously hypertensive rat: a comparative study of three structurally distinct ligands.

The hemodynamic responses to 5'-N-ethylcarboxamide adenosine (NECA), a nonselective adenosine agonist, were compared to those elicited by the sodium salt of 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido adenosine (CGS 21680C) and N6-2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl adenosine (CGS 24012), two structurally dissimilar selective A2 agonists in conscious spontaneously hypertensive rats (SHR). Dose-related reductions in mean arterial pressure occurred after bolus administration of NECA, CGS 21680C and CGS 24012. Dose-dependent tachycardia was seen with both CGS 21680C and CGS 24012, whereas NECA produced a biphasic response on heart rate. At high doses (3 and 10 micrograms/kg), NECA evoked an immediate and dramatic fall in heart rate, followed by a more gradual and long-lasting tachycardia. At equihypotensive doses, both CGS 21680C and CGS 24012 produced significant increases in cardiac output, but NECA had no effect. Although each of the adenosine agonists reduced total peripheral resistance, the greatest change was produced by CGS 21680C. Hindquarter, renal and mesenteric vascular resistances were significantly reduced by both CGS 21680C and CGS 24012, whereas only mesenteric vascular resistance was reduced with NECA. CGS 24012 reduced renal vascular resistance to the greatest extent and produced a concomitant significant increase in renal blood flow. Marked elevation in plasma renin activity occurred with CGS 24012 and CGS 21680C, whereas no change was seen after NECA. The hemodynamic responses to NECA, CGS 21680C and CGS 24012 were significantly reduced by the adenosine antagonist, 8-(p-sulfophenyl) theophylline, suggesting that these agents act through stimulation of adenosine receptors in the conscious SHR. Furthermore, blockade of the beta adrenergic receptor with metoprolol (1 mg/kg, i.v.) significantly attenuated the increase in heart rate produced by NECA, CGS 21680C and CGS 24012. The cardiovascular pattern of responses to the two selective A2 agonists, CGS 21680C and CGS 24012, are distinct from those of NECA, the nonselective adenosine agonist. The responses to both CGS 21680C and CGS 24012 indicate that systemic vasodilation, with resultant cardioexcitation and stimulation of renin release, are the predominant hemodynamic effects of selective A2 agonists in the conscious SHR. In contrast, the cardiovascular effects produced by NECA are mediated by activation of both A1 and A2 receptors.