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

Ectotherms may experience large body temperature (Tb) variations. Higher Tb have been reported to increase baroreflex sensitivity in ectotherm tetrapods. At lower Tb, pulse interval (PI) increases and diastolic pressure decays for longer, possibly resulting in lower end-diastolic pressures and mean arterial pressures (Pm). Additionally, compensatory baroreflex-related heart rate modulation (i.e. the cardiac branch of the baroreflex response) is delayed due to increased PI. Thus, low Tb is potentially detrimental, leading to cardiovascular malfunctioning. This raises the question on how Pm 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 Tb 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 Tb, lizards increased baroreflex gain at the operating point (Gop) and τ, indicating that the diastolic pressure decays at a slower rate. Gop normalized to Pm 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 Tb, indicating that both Gop and τ modulation are crucial for cardiovascular homeostasis.

Evaluation of the sequence method as a tool to assess spontaneous baroreflex in reptiles.

The sequence method is an alternative to the traditional pharmacological approach (i.e., the Oxford technique) used to calculate baroreflex gain (G) in mammals. Although the sequence method assesses baroreflex by measuring spontaneous events of blood pressure regulation, the pharmacological method relies on the injection of vasoactive drugs that impact the baroreflex mechanism itself. The sequence method might be relevant for dynamic measurement of baroreflex modulation but it was never validated for vertebrates with low heart rate. Hence, we tested the sequence method in three species of reptiles and compared the results with those provided by the classic pharmacological method. G was similar between both methods and values correlated when parameters for the sequence method were set at delay 0 or 1 (i.e., the baroreflex system responds immediately to blood pressure changes or after 1 heartbeat). Calculation of the baroreflex effectiveness index was adequate at a minimum of 300 cycles and a delay of 1 for the three species. Therefore, the sequence method has been validated to investigate baroreflex regulation in reptiles, enabling studies during dynamic alterations in homeostasis.