Microvascular responses to norepinephrine in skeletal muscle of cold-acclimated rats

The microcirculation of the spinotrapezius muscle in normal and cold-acclimated (4 degrees C) rats was observed by an in vivo microscopic technique. The responses of the arterioles and venules in the skeletal muscle microcirculation to topical application of norepinephrine (NE) were recorded by a photomicrographic method. Results show that the skeletal muscle microcirculation possesses both alpha- and beta-adrenergic vascular receptors. Stimulation of the alpha-receptor results in vasoconstriction, and of the beta-receptor, vasodilatation. These microvascular responses are antagonized by specific alpha- and beta-adrenergic blockers. Cold-acclimation (4 degrees C) for 3 wk decreases the responses of the skeletal muscle microvessels to NE stimulation. This diminished sensitivity is due to an attenuation of the alpha-adrenergic receptor mechanism.