Venous myogenic tone and its regulation through K+ channels depends on chronic intravascular pressure

In this study, we compared the level of myogenic tone and its negative-feedback control through specific K+ channels in two types of human veins (saphenous [SV] and cephalic [CV] veins), which experience different ranges of pressure in vivo. We also investigated whether an experimental model of increased venous pressure in rats exposed to head-up tilt for 2 weeks produced changes similar to those observed in the human veins. Cylindrical vein segments were cannulated, their diameters were measured, and the intraluminal pressure was set at different levels (2 to 30 mm Hg) in vitro. Acetylcholine test showed that during the regular harvesting process 76% of the human SVs exposed for coronary bypass grafts had no functional endothelium. We found significant myogenic tone in the human SV, where the in vivo pressure is high, but it was not present in the human CV, where the in vivo pressure is low. The nonspecific K+ channel antagonist, tetraethylammonium (TEA), decreased the diameter of the human SV but not the CV. Iberiotoxin and 4-aminopyridine, blockers of the Ca(2+)-sensitive (KCa) and voltage-gated K+ (KV) channels, also decreased the diameter of the human SV by 10.2 +/- 4.8% and 19.5 +/- 4.7%, respectively. In the rat SV, significant myogenic tone was found, but TEA had no effect, even after 2 weeks of in vivo pressure increase in the hindlimb by head-up tilt. We conclude that (1) an increased venous myogenic tone correlates with higher chronic intraluminal pressure loads, (2) KCa and KV channels counterregulate the myogenic tone in human, but not in rat, saphenous vein, (3) the counterregulatory effect is more effective at high than at low intraluminal in vitro pressure levels, and (4) its development is probably a long-term process.