Quantitative Study of Capillary Supply to the Skeletal Muscles of Crucian Carp Carassius carassius L.: Effects of Hypoxic Acclimation

In aerated water at 15 C the routine oxygen consumption (V̇o2) of crucian carp (Carassius carassius L.) was 75.7 ml O2·kg⁻¹h-1. On exposure to progressive hypoxia in closed respirometers, V̇o2 fell parallel with the decrease in water Po₂(PWo₂). Fish were acclimated to PWo₂ 1.5 kPa for 6 wk. Following hypoxia acclimation, carp maintained near constant V̇o2 until PWo2, fell to below 3.5-5.8 kPa. V̇o2 at PWo2 1.5 kPa was significantly higher for fish acclimated to hypoxic water (34.0 ml O2 · kg⁻¹h⁻¹) than for fish held in aerated water (16.2 ml O2 · kg⁻¹h⁻¹). The volume densities of mitochondria (Vvmt,f) and surface (Svc,f) and volume (Vvc,f) densities of capillaries were determined for fast and slow myotomal muscles using low-power electron micrographs. These parameters provide information on the structural limits relating to oxygen demand and supply to the swimming muscles. Mitochondrial volume densities for normoxic carp were 0. 15 for slow and 0.02 for fast muscles. The surface density of capillaries was 6.4 times higher for slow than for fast muscle, reflecting the greater potential for aerobic metabolism. Muscle capillary density was unchanged following acclimation to hypoxia, whereas slow fiber mitochondrial volume density increased by 67%. It is suggested that the increased slow-muscle Vvmt,f is an adaptation to increase the utilization of circulating oxygen stores at low PWo2. This would be an advantage since increased O2 extraction by the muscles would reduce venous Po2 and hence increase the rate of O2 transfer across the gills.