Lingual blood flow and its hypothalamic control in the dog during panting

Abstract

1. The effects of increased ambient temperature (Ta) on blood-flow and -temperatures of the tongue were studied in the unanaesthetized dog. At Ta of 20 degrees C and a relative humidity (rh) of 30% the mean lingual blood flow was 11 ml-min-1 (0.15 ml-g-1-min-1) and the temperature difference between the lingual artery and vein (deltaTLAV) was 1.0 degrees C. Accordingly, a heat loss of 48.6 J-min-1 was calculated even for the dog breathing with the mouth closed. When Ta was elevated to 38 degrees C at constant rh, panting ensued. In parallel fashion lingual blood flow increased to 60.4 ml-min-1 (0.81 ml-g-1-min-1) in mean and to 74.7 ml-min-1 (0.99 ml-g-1-min-1) at peak rate of thermal tachypnoea (272 breaths-min-1). This flow increase resulted from a decrease in the local vascular resistance since the driving systemic pressure remained constant. It was accompanied by an increase in TLAV to 1.5 degrees C equivalent to a heat loss of 400.7J-min-1 in mean and 496.2J-min-1 at maximum respiratory rate. 2. The preoptic/anterior hypothalamic (PO/AH) region was heated with a water perfused thermode in urethane anaesthetized dogs at constant body temperature in order to study the relationship in time between the increase in lingual blood flow and the onset of thermal panting. Lingual blood flow was found to be 20 ml-min-1 at a respiratory rate of 60 breaths/min. During hypothalamic heating both respiratory rate and lingual blood flow increased markedly. At maximum respiratory rates (244 breaths-min-1) lingual blood flow reached a level of 60 ml-min-1. When perfusion of the thermode was stopped, both respiratory rate and lingual blood flow synchronously returned to control values within 5 min. Similar changes did not occur in dogs in which a ventilatory response failed to be elicited during hypothalamic heating. 3. The results suggest that the tongue contributes to the evaporative heat loss mechanism and they confirm the concept that panting, associated with increased lingual blood flow, is induced by a common autonomic outflow pattern which is mediated by the central mechanism controlling thermal homeostasis.