The effects of acute and chronic systemic hypoxia on muscle oxygen supply and oxygen consumption in the rat

The aims of the present study were to evaluate how acute systemic hypoxia affects O-2 delivery to skeletal muscle and muscle O-2 consumption ((V) over dot (O2)) of the rat and to establish how these relationships are altered by chronic systemic hypoxia. Thus, the effects of breathing different concentrations of O-2 (air, 12% and 8% O-2) upon oxygen delivery and (V) over dot (O2) were studied in hindlimb muscles of control, normoxic (N) rats and of rats that had been made chronically hypoxic in a chamber at 12% O-2 for 3-4 weeks (CH) rats. Under anaesthesis, arterial blood pressure, femoral blood flow (FBF), arterial O-2 content (C-a,C- (O2)) and venous O-2 content in the efflux from hindlimb were measured. In N rats, changing the inspirate from air to 12% and 8% O-2 for 5 min each, reduced C-a,C- (O2) from 20 +/- 0.3 ml (100 ml)(-1) in air to 13 +/- 1.0 ml (100 ml)(-1) in 8 % O-2. FBF did not change significantly (1.7 +/- 0.1 ml min(-1) in air) so that O-2 delivery to hindlimb muscles fell from 0.28 +/- 0.07 to 0.16 +/- 0.02 ml min(-1) in 8 % O-2. Nevertheless, the (V) over dot (O2) of hindlimb muscles was well maintained: 0.06 +/- 0.02 ml min(-1) in air and 0.08 +/- 0.02 ml min in 8 % O-2. In CH rats breathing 12% O-2, C-a,C- (O2) (23 +/- 1.0 ml (100 ml)(-1)) was comparable to that of N rats breathing air, due to an increase in haematocrit, as were FBF(1.6 +/- 0.2 ml min(-1)) and O-2 delivery (0.39 +/- 0.05 ml min(-1)). However, (V) over dot (O2) was 2.5-fold greater in CH rats (0.16 +/- 0.03 ml min(-1)). As in N rats, FBF was well maintained at 1.7 +/- 0.2 and 1.6 +/- 0.2 ml min(-1) in 8% O-2 and air, respectively. Further, (V) over dot O-2 was also well maintained, at 0.17 +/- 0.02 and 0.12 +/- 0.02 ml min(-2) in 8% O-2 and air, respectively. These results suggest that, contrary to previous reports, muscle (V) over dot (O2) of the rat is independent of O-2 delivery over a wide range of O-2 delivery values. They also suggest that muscle (V) over dot (O2) of CH rats is similarly independent of O-2 delivery. The novel finding that muscle (V) over dot (O2) has a greater absolute value in CH rats can, we propose, be explained by an increase in (V) over dot (O2) of the vasculature rather than of the skeletal muscle fibres and reflects increased biosynthetic activity of the vessel walls and/or vascular remodelling.