Direct Effects of Acute Hypoxia on the Reactivity of Peripheral Vessels of the Chick Embryo

Abstract 348 Poster Session IV, Tuesday, 5/4 (poster 223)

Insults such as hypoxia or malnutrition can cause damage in the vascular system of the embryo which can lead to pathology in the adult life. If this is the case we hypothesize that vessels will respond to stimuli differently than in the normal embryo. In this study the effects of an acute decrease in oxygen tension on arterial reactivity were determined in isolated vessels of the chick embryo.

Isolated femoral artery segments of 19 days old chick embryos were mounted in a myograph for recording of isometric tension. The organ chamber was filled with Krebs-Ringer bicarbonate solution, maintained at 37°C and aerated with 95% O₂, 5% CO₂. Hypoxia was induced by changing the gas mixture to 95% N₂, 5% CO₂. The effect of low oxygen tension (pO₂ ≤ 15 mm Hg) for a period of 10 minutes was measured in unstimulated vessels and vessels preconstricted with 1 µM noradrenaline (NA) or 63 mM K⁺. These experiments were also performed in vessels from which the endothelium was removed by rubbing the inside with a hair or by perfusing it with 0.1% Triton-X (2 min) and in intact vessels in the presence of the NO-synthase inhibitor, L-NAME (100 µM).

In unstimulated vessels (n=18) hypoxia induced a very small and transient constriction (0.07 ± 0.10 mN/mm, 3.8% ± 4.5 of maximal K⁺ constriction). No difference was observed after denudation or after blockade of NO-synthase. No response to a decrease in low pO₂ was observed in 11 out of 47 vessels precontricted with NA. 21 Vessels showed a typical and recurring response; an initial small increase of constriction (10.41% ± 7.18) followed by relaxation (42.9% ± 12.2). In denuded vessels the initial increase of constriction was higher compared with intact vessels (18.4% ± 10.5 and 8.8% ± 4.8 respectively, paired t-test, n=10 P ≤ 0.05), and hypnotic relaxation was diminished (9.9% ± 31.6 and 42.7 ± 7.9, t-test, n=10 P ≤ 0.05). L-NAME did not modify the responses to hypoxia in intact vessels. Hypnotic relaxation in intact vessels preconstricted with K⁺ was significantly lower than in vessels preconstricted with NA (17.59% ± 19.93, t-test n=4 P ≤ 0.05).

The effects of low oxygen tension on arterial tone appear to be largely endothelium dependent. Relaxation in response to hypoxia, however, is probably not mediated by NO, but may be induced via the release of an endothelium derived hyperpolarizing factor.