The action of insulin on brown adipose tissue in vivo.

1. The action of insulin on the net exchange of glucose, free fatty acids and glycerol by brown adipose tissue of young rabbits was investigated.2. Infusion of insulin (10,000 muu./kg.min I.V. for 10 min) caused a large increase in the uptake of glucose by brown adipose tissue of fed week-old rabbits. The release of fatty acids fell but glycerol release was unchanged.3. The brown adipose tissue of 9-day-old rabbits fasted in a warm environment had a high initial rate of fatty acid and glycerol release and low rates of glucose uptake. Insulin infusions (10,000 and 100 muu./kg.min) greatly reduced fatty acid release but had little or no effect on glucose uptake.4. The brown adipose tissue of 9-day-old rabbits fasted in a cold environment was depleted of fat and took up free fatty acid as well as glucose from the circulation. Infusion of insulin (10,000 muu./kg.min) caused a large increase in glucose uptake accompanied by a reduction in fatty acid uptake.5. The experiments support the view that insulin has a direct effect on fatty acid transport across the cell membrane.