Insulin administration induces capillary growth in brown adipose tissue of heat-exposed rats

1. An apparent effect of insulin administration on enlargement of interscapular brown adipose tissue (BAT) was found in heat-exposed rats, but not in warm-adapted or cold-acclimated rats. 2. BAT extracts from the heat-acclimated/insulin-treated (HI) rats notably increased the capillary growth in an in vitro angiogenesis model in which microvascular fragments and myofibroblastic (Mf) cells isolated from lipid tissues were grown in co-culture, although a direct effect of insulin was not high. 3. BAT extracts from the HI rats stimulated the production of endothelial cell growth factor and collagen by Mf cells. 4. It is probable that an increased angiogenic activity contributes to the capillary growth and tissue growth in BAT of HI rats.