Winter metabolic depression does not change arterial baroreflex control of heart rate in the tegu lizard Salvator merianae

Abstract

Baroreflex regulation of blood pressure is important for maintaining appropriate tissue perfusion. Although temperature affects heart rate (f(H)) reflex regulation in some reptiles and toads, no data are available on the influence of temperature-independent metabolic states on baroreflex. The South American tegu lizard Salvator merianae exhibits a clear seasonal cycle of activity decreasing f(H) along with winter metabolic downregulation, independent of body temperature. Through pharmacological interventions (phenylephrine and sodium nitroprusside), the baroreflex control of f(H) was studied at similar to 25 degrees C in spring-summer-and winter-acclimated tegus. In winter lizards, resting and minimum fH were lower than in spring-summer animals (respectively, 13.3 +/- 0.82 versus 10.3 +/- 0.81 and 11.2 +/- 0.65 versus 7.97 +/- 0.88 beats min(-1)), while no acclimation differences occurred in resting blood pressure (5.14 +/- 0.38 versus 5.06 +/- 0.56 kPa), baroreflex gain (94.3 +/- 10.7 versus 138.7 +/- 30.3% kPa(-1)) or rate-pressure product (an index ofmyocardial activity). Vagal tone exceeded the sympathetic tone of f(H), especially in the winter group. Therefore, despite the lower f(H), winter acclimation does not diminish the f(H) baroreflex responses or rate-pressure product, possibly because of increased stroke volume that may arise because of heart hypertrophy. Independent of acclimation, f(H) responded more to hypotension than to hypertension. This should imply that tegus, which have no pressure separation within the single heart ventricle, must have other protection mechanisms against pulmonary hypertension or oedema, presumably through lymphatic drainage and/or vagal vasoconstriction of pulmonary artery. Such a predominant f(H) reflex response to hypotension, previously observed in anurans, crocodilians and mammals, may be a common feature of tetrapods.