Absence of early resetting of coronary baroreceptors in anaesthetized dogs

Abstract

1. Both carotid and aortic arch baroreceptors have been shown to reset after as little as 20 min exposure to a different conditioning pressure; the mid-point of the stimulus-response curve is displaced towards the conditioning pressure. 2. Coronary baroreceptors operate over much lower pressures and induce slower reflex vasoconstriction than the other baroreceptors and this investigation was designed to determine whether their resetting characteristics are also different. 3. In chloralose anaesthetized dogs, a perfusion circuit allowed independent control of pressures distending carotid, aortic and coronary baroreceptors. Stimulus-response curves were obtained for carotid and coronary baroreceptors after maintaining the distending pressure at 60 or 180 mmHg for 20 min. 4. Neither the magnitude of the responses nor the baroreceptor pressure corresponding to 50 % of the response (BP50) of the coronary curves was changed by the conditioning regime. In contrast, conditioning carotid baroreceptors with the same regime produced significant shifts in the BP50 towards the conditioning pressure. 5. No changes were obtained after conditioning the coronary baroreceptors at 60 or 120 mmHg for 40 min. 6. These results confirm early resetting of carotid baroreceptors but show that coronary baroreceptors do not reset over a period of at least 40 min.

Figure 1. Experimental preparation

A large cannula inserted into the aortic arch, with the aorta tied onto it just distal to the coronary ostea and distal to the left subclavian artery, allows control of aortic root pressure and creates a pouch outside the cannula containing aortic baroreceptors. Blood is drained from the left atria into reservoir D. Blood from this reservoir is pumped to the main reservoir, A. Blood from A is pumped into pressurized reservoirs B and C to maintain constant levels of blood in these reservoirs which supply the carotid sinuses and the aortic arch. Carotid and aortic arch blood is drained from catheters in the lingual arteries and in the brachiocephalic artery (passed down the right common carotid artery). Blood from reservoir A was pumped into the descending aorta, isolated hindlimb and cephalic circulation at constant flow. Abbreviations: CP, constant pressure; SG, strain gauge transducer; P, pump.

Figure 2

Group carotid curves fitted to a third order polynomial where the carotid baroreceptors were conditioned at 60 mmHg () and 180 mmHg () for 20 min (n= 6).

Figure 3

Group coronary curves fitted to a third order polynomial where coronary baroreceptors had been conditioned at 60 mmHg () and 180 mmHg () for 20 min (n= 6).

Figure 4

Group coronary curves fitted to a third order polynomial where coronary baroreceptors had been conditioned at 60 mmHg () and 120 mmHg () for 40 min (n= 6).