ANATOMICAL ADAPTATIONS FOR PERIPHERAL OXYGEN-TRANSPORT AT HIGH AND LOW-TEMPERATURES

On acute cold exposure most animals show a reduction in cardiac output and increased blood viscosity that may seriously impair oxygen supply to peripheral tissue. The apparent increase in capillarisation in res ponse to chronic low temperature may be accompanied by significant mus cle atrophy in hibernating mammals, and a rise in capillary density (C D) may then simply reflect a reduced fibre diameter. When hamsters wer e exposed to reduced temperature and photoperiod, capillary to fibre r atio (C:F) of the tibialis anterior muscle was unchanged (at 2.7) and CD increased by 30% (to 1538 mm(-2)) following a similar decrease in f ibre size. A measure of local C:F decreased around glycolytic fibres, but increased around both oxidative fibre types. When corrected for ch anges in fibre size, local CD increased around all fibre types in the cold. Fishes are particularly sensitive to fluctuations in environment al temperature, but in contrast to mammals often maintain activity and undergo muscle hypertrophy. Slow (aerobic) muscle from striped bass a nd goldfish held for eight weeks at seasonal extremes of 5 and 25 degr ees C had a higher C:F in cold vs. warm animals. However, increased fi bre size meant that the functional capacity of the microcirculation wa s maintained rather than increased. Seasonal acclimatisation of trout induced an inverse relationship between environmental temperature and C:F, increasing 40% from summer (18 degrees C) to winter (4 degrees C) . However, extensive hypertrophy at 4 degrees C meant that the size of the capillary bed was maximal at 11 degrees C, corresponding to the p oint of highest muscle blood flow and greatest scope for aerobic swimm ing, reflecting an optimised aerobic performance at intermediate tempe ratures. The consequence of altered fibre composition and capillarisat ion is calculated as an increase in mean fibre PO2 from 1.9 to 4.6 kPa , and in minimum PO2 from 0.6 to 4.2 kPa in 25 degrees v. 5 degrees C- acclimated striped bass, respectively. However, the most dramatic effe ct on intracellular oxygenation results from an altered capillary supp ly with the capillary supply becoming increasingly inadequate at high temperatures.