Baroreflex gain and time of pressure decay at different body temperatures in the tegu lizard, Salvator merianae

Ectotherms may experience large body temperature (T(b)) variations. Higher T(b) have been reported to increase baroreflex sensitivity in ectotherm tetrapods. At lower T(b), pulse interval (PI) increases and diastolic pressure decays for longer, possibly resulting in lower end-diastolic pressures and mean arterial pressures (P(m)). Additionally, compensatory baroreflex-related heart rate modulation (i.e. the cardiac branch of the baroreflex response) is delayed due to increased PI. Thus, low T(b) is potentially detrimental, leading to cardiovascular malfunctioning. This raises the question on how P(m) is regulated in such an adverse condition. We investigated the baroreflex compensations that enables tegu lizards, Salvator merianae, to maintain blood pressure homeostasis in a wide T(b) range. Lizards had their femoral artery cannulated and pressure signals recorded at 15°C, 25°C and 35°C. We used the sequence method to analyse the heart rate baroreflex-related corrections to spontaneous pressure fluctuations at each temperature. Vascular adjustments (i.e. the peripheral branch) were assessed by calculating the time constant for arterial pressure decay (τ)—resultant from the action of both vascular resistance and compliance—by fitting the diastolic pressure descent to the two-element Windkessel equation. We observed that at lower T(b), lizards increased baroreflex gain at the operating point (G(op)) and τ, indicating that the diastolic pressure decays at a slower rate. G(op) normalized to P(m) and PI, as well as the ratio τ/PI, did not change, indicating that both baroreflex gain and rate of pressure decay are adjusted according to PI lengthening. Consequently, pressure parameters and the oscillatory power fraction (an index of wasted cardiac energy) were unaltered by T(b), indicating that both G(op) and τ modulation are crucial for cardiovascular homeostasis.