Proceedings of The Physiological Society

University College Cork (2004) J Physiol 560P, C11

Communications

CAROTID BAROREFLEX REGULATION OF VASCULAR RESISTANCE IN LOWLAND AND HIGH ALTITUDE ANAESTHETIZED DOGS.

Moore,Jonathan P; Rivera-Chira,Maria ; Macarlupu,Jose Luis ; Myers,David S; Hainsworth,Roger ; Drinkhill,Mark J;

1. Institute for Cardiovascular Research, University of Leeds, Leeds, United Kingdom. 2. Department of Biological Sciences & Physiology, Universidad Peruana Cayetano Heredia, Lima, Peru.


  • Table 1. Group average values.

    * P < 0.04 versus lowland ambient air (Unpaired t test); # P < 0.03 versus high altitude ambient air (Paired t test)

In this study we compared carotid baroreflex function in dogs living at high altitude (4338 m, PB = 450 mmHg; n = 8) with that in lowland animals (n = 6). Dogs were anaesthetized with α-chloralose (100 mg kg-1 i.v.) and artificially ventilated with a gas mixture to produce arterial oxygen tension typical of the prevailing ambient barometric pressure i.e. either normoxia (lowland) or hypoxia (high altitude). Following vagotomy to eliminate other baroreceptor reflexes, the pressure perfusing the vascularly isolated carotid sinuses was changed in a stepwise manner. Vascular responses were determined from changes in perfusion pressure to a vascularly isolated hind limb (constant flow). Stimulus-response curves were defined during carotid perfusion with blood equilibrated (a) with gases similar to ambient (lowland or high altitude) and (b) hyperoxic gases. Sigmoid functions were applied to the curves and indicators of carotid baroreflex function were determined: i.e. the maximal slope (equivalent to peak gain) and the corresponding carotid pressure (equivalent to ‘set point’). Blood samples were taken at regular intervals for determination of carotid and systemic arterial blood gases and pH. Animals were killed by exsanguination following a lethal dose of anaesthetic. The results are summarized in Table 1. The maximum slopes of the relationship between perfusion pressure change and carotid sinus pressure were not different either between groups of dogs (high altitude versus lowland) or during different carotid perfusates (ambient or hyperoxic). The ‘set point’ was lower in the high altitude than in lowland dogs, during perfusion with ambient blood gases. Also in the high altitude dogs changing the perfusate from ambient (hypoxic) blood to hyperoxic blood caused a significant increase in the ‘set point’. The results indicate that compared to lowland dogs, in the high altitude animals the carotid stimulus-response curve is displaced to the left, i.e. to lower pressure. The effect may be partly attributed to perfusion of the carotid sinuses with hypoxic blood because unlike in the lowland animals in the high altitude dogs hyperoxia significantly increases the ‘set point’.

Where applicable, experiments conform with Society ethical requirements