Role of Nitric Oxide and Ca 11-Dependent K 1 Channels in Mediating Heterogeneous Microvascular Responses to Acetylcholine in Different Vascular Beds 1

Endothelium-derived relaxing factors may differentially modulate vascular tone and relaxation in arteries from specific vascular beds. We evaluated the role of nitric oxide (NO), prostacyclin (PGI2) and endothelium-derived hyperpolarizing factor in determining basal tone and acetylcholine (ACh)-induced relaxation of coronary (Cor), skeletal muscle (Ske) and mesenteric (Mes) small arteries (150–250 mm) isolated from male Golden Syrian hamsters (16–17 weeks). Intraluminal diameter (ID) was recorded in vessels maintained at a constant pressure of 40 mm Hg. Charybdotoxin (0.1 mM), a blocker of large Cadependent K channels (BKCa), decreased base-line ID by 33 6 4% and 15 6 4% in Cor and Mes small arteries, respectively. Neither the nitric oxide synthase (NOS) inhibitor, Nvnitro-L-arginine (LNA, 0.1 mM), indomethacin (10 M) nor apamin (0.5 mM), which blocks small Ca-dependent K channels (SKCa), affected ID. Maximal relaxation to ACh was significantly reduced by LNA in Cor arteries preconstricted with the thromboxane A2 analog, U46619. LNA shifted the doseresponse curve to the right without altering maximal relaxation to ACh in Mes arteries and had no effect on relaxation to ACh in Ske arteries relaxation. A high extracellular K concentration (25–50 mM) largely reduced relaxation to ACh in Ske and Mes and abolished relaxation in Cor arteries, whereas indomethacin had no effect on any vessel. Blockade of both BKCa and SKCa channels with a combination of charybdotoxin and apamin abolished relaxation to ACh in Cor, but had no effect in Mes or Ske arteries. Collectively, these results indicate that ACh-induced relaxation is mediated by both NO and an endotheliumderived hyperpolarizing factor that opens K channels independently of NO or PGI2 in Cor and Mes arteries. Relaxation of Ske arteries is completely due to a NO and PGI2-independent opening of K channels. Relaxation to ACh is mediated by KCa channels in Cor arteries, and by other types of K channels in Ske and Mes arteries. Additionally, BKCa channels regulate basal tone in Cor and Mes, but not Ske arteries. These results indicate that arteries of similar size use different mechanisms of endothelium-dependent regulation of vascular tone and relaxation which are dependent on the vascular bed. The endothelium is an important regulator of vascular tone due to synthesis and release of vasodilatory substances including NO, PGI2 and EDHF (Moncada and Vane, 1979; Furchgott and Zawadzki, 1980; Feletou and Vanhoutte, 1988). Acetylcholine is generally used to induce production of EDRFs and may also stimulate release of vasoconstrictor cyclooxygenase products from endothelial cells (Miller and Vanhoutte, 1985; Luscher and Vanhoutte, 1990). After stimulation of muscarinic receptors, endothelial NOS converts L-arginine to NO (Palmer et al., 1988). NO may produce relaxation by decreasing smooth muscle cell Ca levels through a cGMP-dependent pathway or through hyperpolarization due to increased conductance of K channels (Gruetter et al., 1981; Murphy and Brayden, 1995a; Corriu et al., 1996b). PGI2 is produced by the action of prostacyclin synthase on endoperoxides, which are produced by cyclooxygenase. L-Arginine analogs such as LNA inhibit NOS activity, whereas Indo inhibits cyclooxygenase. The enzyme responsible for EDHF production is not known. However, EDHF may be arachidonic acid metabolites of the cytochrome P450 pathway such as epoxyeicosatrienoic acids (Campbell et al., 1996). Relaxation to EDHF has been mediated through increased conductance of Ca-dependent K channels and ATP-sensitive K channels (Cowan et al., 1993; Campbell et al.,