We have previously shown that hypoxia inhibits isometric tension of depolarised or agonist-activated small rat mesenteric arteries (Otter & Austin, 1999). Small arteries mounted under isobaric conditions, however, may also develop intrinsic myogenic tone. In arteries from other vascular beds reduced oxygen tension has been shown to inhibit myogenic reactivity (Messina et al. 1992; Loutzenhiser & Parker, 1994), however its effects on myogenic tone of mesenteric arteries in unclear. In this study, therefore, we have investigated the influence of acute severe hypoxia on depolarisation- and agonist-induced contractions, and on myogenic tone, of mesenteric arteries examined under isobaric conditions.

Small mesenteric arteries (150-200 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 (P_O2 = 134.3 ± 6.4 mmHg (mean ± S.E.M.), n = 18; n is the number of animals) and left to equilibriate for 90 min. Intraluminal diameter was continuously monitored and any constriction noted. Vessels were also stimulated with high K⁺ solution (60 mM KCl isosmotically substituted for NaCl) or 10 mM phenylephrine (PE). Hypoxia was achieved by gassing with 95 % N₂-5 % CO₂ and determined as P_O2p < 10 mmHg (time = 10.4 ± 1.2 min, n = 18).

In some arteries (n = 5), warming to 37 °C under normoxic conditions induced a constriction of luminal diameter from 166 ± 5 mm to 112 ± 8 mm (n = 5). In 4/5 arteries 10 min hypoxia reduced this temperature-dependent myogenic tone (mean dilatation was 37 ± 12 % (n = 5)). Addition of high K⁺ solution or PE produced a rapid constriction (from 189 ± 10 mm to 64 ± 12 mm in high K⁺ (n = 5) and 175 ± 9 mm to 70 ± 10 mm in PE (n = 8)). The change in diameter was similar in response to both agents (125.0 ± 5.7 mm (n = 5) for high K⁺ compared with 110.3 ± 7.4 mm (n = 8) for PE). In PE-constricted tissues a 5 min hypoxia caused a 67.0 ± 8.6 % dilatation which was significantly greater than the dilatation observed to hypoxia in arteries constricted by high K⁺ (15.7 ± 2.2% P < 0.05, Student’s unpaired t test).

Thus, temperature-dependent myogenic tone was reduced by acute hypoxia in pressurised rat mesenteric small arteries. Furthermore, hypoxia resulted in a greater dilatation of PE-pre-constricted arteries compared to arteries pre-constricted by high K⁺ solution.

This work was supported by The British Heart Foundation.