Efficacy of different vasodilators on human umbilical arterial smooth muscle under normal and reduced oxygen conditions

The ability of the smooth muscle of the human umbilical artery to relax may vary under physiological and pathological conditions. We investigated the responsiveness of that preparation to relaxation, as well as the influence of reduced oxygen condition on these responses. Rings of human umbilical arteries from full-term Caesarian deliveries were suspended in modified Krebs–Henseleit solutions bubbled with a gas mixture of 95% O2 : 5% CO2 (normal oxygen condition) or 2.5% O2 : 8% CO2 balanced with N2 (reduced oxygen condition). These rings were contracted with potassium chloride, serotonin or the thromboxane A2 mimetic U46619, before being exposed to either the nitrovasodilator sodium nitroprusside, the potassium channel opener levcromakalim or the calcium channel antagonist amlodipine. While sodium nitroprusside elicited relaxation in this blood vessel, the maximal relaxation to the nitric oxide donor was significantly smaller than that induced by levcromakalim and amlodipine. The nature of the constrictor agent used, or changes of oxygen conditions did not significantly affect the relaxation profile of this human blood vessel. These data suggest that the smooth muscle of the human umbilical artery may be less responsive to vasodilators that act via the nitric oxide pathway. Moreover, vascular responses of umbilical arterial smooth muscle to relaxing agents do not alter under hypoxic or different vasoconstricting conditions.