Factors contributing to differences in the regulation of cGMP in isolated porcine pulmonary vessels

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Guanosine 3′,5′-cyclic monophosphate (cGMP) is an important second messenger in many biological systems including vascular smooth muscle where it mediates relaxation. Cellular levels of cGMP are regulated primarily by three enzymes; nitric oxide (NO) synthase, soluble guanylate cyclase, and cGMP-phosphodiesterase. Basal cGMP levels of isolated endothelium intact porcine pulmonary vein are five fold higher than in pulmonary artery. The objective of this study was to investigate possible reasons for this difference. Therefore, we compared NO synthase activity of pulmonary vein with artery and used pharmacologic approaches to compare soluble guanylate cyclase and phosphodiesterase activities in these vessels. NO synthase activities of pulmonary vein and artery were measured by monitoring the conversion of exogenous l-[ 14 C ]arginine to l-[ 14 C ]citrulline and by quantifying NO formation from endogenous l-arginine. Rates (pM/min per mg protein) of basal l-citrulline and NO formation from endothelium intact pulmonary vein (29.0±4.8 and 44±7.1, respectively) were significantly higher than from artery (8.3±2.2 and 17.1±3.3). Western blot analysis indicated higher constitutive NO synthase protein in the vein than in artery. N-nitro- l-arginine (0–100 μM), a potent inhibitor of NO synthase, induced contractions of the pulmonary vein which were significantly higher than those of the artery. N-nitro- l-arginine (5 and 20 μM) in the presence of phosphodiesterase inhibitors, decreased basal cGMP levels of endothelium intact blood vessels. In endothelium denuded pulmonary vein and artery, basal cGMP levels were not different from each other, but increased significantly following stimulation of soluble guanylate cyclase with exogenous NO. In the presence of both non-specific and specific cGMP phosphodiesterase inhibitors, exogenous NO-induced cGMP levels of endothelium denuded tissues were not significantly different from each other. However, in the absence of the phosphodiesterase inhibitors, exogenous NO-induced cGMP was significantly less in the artery than in the vein. These results suggest that (I) the intact porcine pulmonary vein contains higher levels of NO synthase activity than pulmonary artery, and that (II) the soluble guanylate cyclase activities in pulmonary vein and artery are equally responsive to NO, and finally (III) pulmonary artery expresses greater phosphodiesterase activity than vein. Higher NO synthase and lower phosphodiesterase activity may explain the greater accumulation of cGMP in the pulmonary vein compared to the artery.