Relative significance of the nitric oxide (NO)/cGMP pathway and K+ channel activation in endothelium-dependent vasodilation in the femoral artery of developing piglets

Mechanisms mediating endothelium-dependent vasodilation were investigated in femoral artery rings from <2-day-old (newborn) and 2-week-old piglets. Based on previous results we hypothesized an age difference in the relative contribution of nitric oxide(NO)-cyclic 3',5'-guanosine monophosphate (cGMP) and K+ channel-activation to acetylcholine (ACh)-induced vasodilation. Changes in vascular tone were studied in organ baths in the absence or presence of NO synthase(NOS) inhibition or K+ channel blockade and the intra-arterial accumulation of cGMP in response to ACh was measured with radioimmunoassay (RIA). In control experiments, relaxant responses to ACh were equal in the two age groups. In the presence of the NOS-inhibitors N-G-monomethyl-L-arginine acetate (L-NMMA, 100 <mu>M) or N-G-nitro-L-aiginine (L-NOARG; 1-100 muM), however, relaxation was significantly more reduced in femoral artery rings from 2-week-old than from newborn, with lower pD(2) values in the older age group. Inhibition of large (BKCa) conductance calcium-sensitive K+ channels with tetraethylammonium chloride (TEA; 1 mM), gave a significant rightward shift in the concentration-response curves to ACh which was of the same magnitude in both age groups. The ACh-induced vasodilation was abolished in both age groups by high K+ (20 mM) in combination with L-NOARG (100 muM). The relative increase in cGMP levels after addition of ACh (10 nM) was significantly larger in rings from newborn compared with 2-week-old piglets (12- vs. four-fold). In summary, sensitivity to NOS inhibition increased with age while the effect of K+ channel blockade with TEA was the same in femoral artery rings from newborn to 2-week-old piglets. Lower sensitivity to NOS inhibition and a larger increase in cGMP in response to ACh could indicate a higher efficacy of the NO/cGMP pathway in this vessel in the newborn piglet.