Cold-induced vasodilatation in the sheep.

Abstract

Experiments were performed to investigate the nature and control of cold-induced vasodilatation (C.I.V.D.) in the extremities of sheep exposed to air temperatures below 0 °C. Two distinct patterns of C.I.V.D. were observed, the 'hunting' phenomenon and continuous proportional control of skin temperature. The different patterns were characteristic of individual sheep and not influenced by cold acclimation. Temperature fluctuations (hunts) were usually asynchronous in different extremities. Infusion of noradrenaline (NA) usually initiated a synchronous rise in skin temperature as blood pressure rose but thereafter reduced the intensity and frequency of hunting. During infusion of NA one sheep regularly exhibited cardiac arrhythmia except during a 5-week period of cold acclimation. Following section of the left vagosympathetic trunk, the sympathectomized left ear remained close to +40 °C at air temperatures of −25 °C. Infusion of NA for 100 min reduced temperature of the sympathectomized ear to about +15 °C after which point regular hunting ensued. Angiotensin and vasopressin did not reduce temperature in the sympathectomized ear. The results indicate that sympathetic nervous control is essential to cold vasoconstriction in the sheep and suggest that it is the dominant factor regulating skin temperature during the process of C.I.V.D.