Skeletal muscle capillary supply in a fish that lacks respiratory pigments.

The relationship between capillary density and mitochondrial volume density has been investigated for the slow trunk muscle of the haemoglobinless 'Icefish', Chaenocephalus aceratus Lönnberg. In general, for teleost slow muscles capillary densities, NA(c,f), are positively correlated with mitochondrial volume densities, Vv(mit,f). However, the capillary density for the slow trunk muscle of the Icefish is low (544 +/- 24 capillaries/mm2) when compared with teleosts of similar mitochondrial volume densities (0.34 +/- 0.01). For example, for slow muscle fibres in Saithe (Pollachius virens) Vv(mit,f) is 0.35 +/- 0.02 but the capillary density is 4507 capillaries/mm2. Estimates of capillary surface and volume densities for the Icefish (198 and 0.035 cm-1, respectively) are in the range reported in the literature for other teleost species. Thus, for C. aceratus NA(c,f) provides a relatively poor index of the functional properties of the peripheral exchange vessels. Mean capillary cross-sectional area for C. aceratus (64 +/- 7 micron2) was two to three times greater than that found in most teleost species. The low peripheral resistance reported for the Icefish, C. aceratus is therefore the result of the presence of large bore capillaries, the resulting high flow rates also allowing the maintenance of favourably high capillary - tissue PO2 gradients.