Seasonal acclimatization of rainbow trout induced an inverse relationship between environmental temperature and the capillary:fibre ratio of slow locomotory muscle, which increased from 1.73±0.09 in the summer (18 °C) to 2.50±0.15 in the winter (4 °C). However, the rate of capillary growth (angiogenesis) was exceeded by that of fibre growth at low temperatures such that the extensive fibre hypertrophy found at 4 °C led to a decrease in capillary density, NA(c,f), to 57 % of that found at 11 °C and 85 % of that at 18 °C. Cold-induced angiogenesis resulted in an expanded capillary bed of similar topology to the existing network, with capillary orientation deviating from that of muscle fibres by only 4­7 %. Capillary length density was maximal at 11 °C, JV(c,f)=2421±239 mm-2, which as previously described corresponds to the point when muscle blood flow is highest and the scope for aerobic swimming is greatest, reflecting an integrated response to optimize aerobic performance at intermediate temperatures. In contrast, ventricular NA(c,f) parallels heart rate and hence was highest at 18 °C, while there was no seasonal variation in myocyte diameter. Although a systemic response to seasonal adjustments in humoral factors may occur, the data reported here suggest that angiogenesis is probably stimulated by different mechanical factors in these two muscles.

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