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

Abstract

Baroreflex regulation of blood pressure (BP) is important for maintaining appropriate tissue perfusion. Although temperature affects heart rate (fH) 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 fH along with winter metabolic downregulation, independent of body temperature. Through pharmacological interventions (phenylephrine and sodium nitroprusside), the baroreflex control of fH was studied at ∼25°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 vs 10.3±0.81 and 11.2±0.65 vs 7.97±0.88 beats.min−1), while no acclimation differences occurred in resting BP (5.14±0.38 vs 5.06±0.56 kPa), baroreflex gain (94.3±10.7 vs 138.7±30.3 %.kPa−1) and rate-pressure product (an index of myocardial activity). Vagal tone exceeded the sympathetic tone of fH especially in the winter group. Therefore, despite the lower fH, winter acclimation does not diminish the fH baroreflex responses nor rate-pressure product possibly because of increased stroke volume that may arise due to heart hypertrophy. Independent of acclimation, fH 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 fH reflex response to hypothension, previously observed in anurans, crocodilians and mammals, may be a common feature of tetrapods.