Age-related changes in adenosine-mediated relaxation of coronary and aortic smooth muscle

Abstract

We tested whether adenosine mediates nitric oxide (NO)-dependent and NO-independent dilation in coronary and aortic smooth muscle and whether age selectively impairs NO-dependent adenosine relaxation. Responses to adenosine and the relatively nonselective analog 5 ′-N-ethylcarboxamidoadenosine (NECA) were studied in coronary vessels and aortas from immature (1–2 mo), mature (3–4 mo), and moderately aged (12–18 mo) Wistar and Sprague-Dawley rats. Adenosine and NECA induced biphasic concentration-dependent coronary vasodilation, with data supporting high-sensitivity (pEC50 = 5.2–5.8) and low-sensitivity (pEC50 = 2.3–2.4) adenosine sites. Although sensitivity to adenosine and NECA was unaltered by age, response magnitude declined significantly. Treatment with 50 μM N G-nitro-l-arginine methyl ester (l-NAME) markedly inhibited the high-sensitivity site, although response magnitude still declined with age. Aortic sensitivity to adenosine declined with age (pEC50 = 4.7 ± 0.2, 3.5 ± 0.2, and 2.9 ± 0.1 in immature, mature, and moderately aged aortas, respectively), and the adenosine receptor transduction maximum also decreased (16.1 ± 0.8, 12.9 ± 0.7, and 9.6 ± 0.7 mN/mm2 in immature, mature, and moderately aged aortas, respectively). l-NAME decreased aortic sensitivity to adenosine in immature and mature tissues but was ineffective in the moderately aged aorta. Data collectively indicate that 1) adenosine mediates NO-dependent and NO-independent coronary and aortic relaxation, 2) maturation and aging reduce NO-independent and NO-dependent adenosine responses, and 3) the age-related decline in aortic response also involves a reduction in the adenosine receptor transduction maximum.