We have previously shown that acute (5 min) periods of severe hypoxia cause partial dilatation of depolarised or agonist-contracted pressurised rat mesenteric small arteries (Shaw et al. 2003). This study examines the effect of extended periods of severe hypoxia on contractions elicited to various stimuli in pressurised rat mesenteric arteries.

Small mesenteric arteries (100-200 µm internal diameter) were isolated from male Wistar rats (humanely killed by cervical dislocation following stunning), cannulated and mounted on a pressure myograph. Vessels were pressurised to 60 mmHg and perfused with bicarbonate-buffered saline at 37 °C, gassed with 95 % air-5 % CO₂ and left to equilibriate for 30 min. The P_O2 of the superfusate was continuously measured by placing a small O₂ probe (Instech Laboratories) 1-2 mm from the artery (P_O2 = 135.4 ± 3.3 mmHg (mean ± S.E.M.), n = 12 animals). Intralumenal diameter was continuously monitored. Vessels were stimulated with high K⁺ solution (60 mM KCl isosmotically substituted for NaCl), 10 µM phenylephrine (PE) or 10 µM U46619. Severe hypoxia was achieved by gassing with 95 % N₂-5 % CO₂ and determined when superfusate P_O2 < 10 mmHg (time taken = 7.2 ± 0.9 min, n = 12).

Resting diameter of arteries in normoxia was 178.3 ± 11.6 µm (n = 12). Addition of high K⁺ solution, PE or U46619 produced a rapid constriction of similar magnitude in all vessels (changes in diameter were 147.5 ± 16.7 µm (n = 6), 121.7 ± 10.9 µm (n = 3) and 93.3 ± 17.6 µm (n = 3) for high K⁺, PE and U46619, respectively). Severe hypoxia dilated PE pre-constricted arteries by 97.5 ± 1.3 % (time for maximum response = 19.3 ± 4.5 min). Significantly smaller dilatory responses were observed in tissues contracted by high K⁺ (maximum dilatation of 67.6 ± 3.5 %, time = 21.8 ± 3.7 min) or by U46619 (maximum dilatation 66.4 ± 11.0 %, time = 82.0 ± 19.6 min) (P < 0.05, Student’s unpaired t test). Following maximal dilatation, depolarised arteries re-constricted after 2 h in the continued presence of hypoxia to 96.7 ± 8.3 % of the high K⁺ constricted tone in normoxia. However, arteries pre-constricted by either PE or U46619 and subsequently dilated with hypoxia did not re-constrict in the continued presence of lowered oxygenation. Thus, dilatations to long-term, severe hypoxia were greater in PE compared to high K⁺ or U46619 pre-constricted pressurised rat mesenteric arteries. Vessels pre-constricted by high K⁺, but not by PE or U46619, exhibited a biphasic response to lowered oxygen and, with time, re-constricted fully in the continued presence of hypoxia.

This work was supported by the British Heart Foundation.