Flow and thermal responses of aortic baroreceptors

1. The aims of this study were 1) to determine whether the impulse activity in rabbit aortic baroreceptors (BRs) was influenced by changes in nonpulsatile flow through the aortic lumen and 2) to examine the BR and aortic arch responses to changes in temperature. 2. An in vitro aortic arch-aortic nerve preparation was used to record suprathreshold steady-state discharge in a total of 29 single-unit BRs from 12 New Zealand white rabbits. Changes in BR frequency were measured relative to control and were recorded simultaneously with aortic arch pressure, flow, temperature, diameter, and calculated wall shear stress (Sw). 3. With pressure held constant, stair-step increases in flow (3–18 ml/min) constricted the arch and evoked two types of BR responses: activation in most units (15 of 17 BRs tested) and inhibition in 2 units. The activation response appeared closely related to the changes in flow and Sw, but also appeared related to uncontrolled changes in perfusate temperature. 4. When the effects of temperature were examined more closely with pressure and flow held constant, controlled step increases in temperature (between 32 and 42 degrees C) constricted the arch and again evoked two BR responses: activation in 11 of 14 BRs tested and inhibition in 3 units. The Q10 for the activation response was 1.55 +/- 0.08 (mean +/- SE), which was not significantly different from the Q10 for activation when temperature varied with flow (1.65 +/- 0.1, P < 0.05). Thus the effect of temperature on most BRs was directionally and quantitatively similar in the presence and absence of changes in flow. 5. Last, when flow was examined over a wide range (15–515 ml/min) with temperature and pressure held constant, stair-step increases in flow failed to produce any change in BR frequency in each of 15 fibers tested (10 arches). The lack of response was not due to a functionally damaged preparation because the usual BR and aortic arch responses to pressure and to drug-evoked vasoconstriction (norepinephrine) and endothelial-mediated vasodilation (acetylcholine) were intact. 6. These results demonstrate that aortic BRs in rabbits are not sensitive to flow and thus are not likely influenced by fluctuations in cardiac output apart from associated changes in pressure. The aortic BRs are affected directly by variations in temperature and in some cases indirectly by temperature-related vasoconstriction. The effects of temperature may have important implications for neural control of the circulation when core temperature varies because of physiological and environmental stress.

Aortic arch reflex control of total systemic vascular capacity

The ability of the aortic arch baroreceptors to change vascular capacity was measured and, in the same animal, compared with carotid sinus reflex changes in capacity. Seven dogs were anesthetized with pentobarbital sodium and perfused with constant flow. Changes in external reservoir volume reflected reciprocal changes in total systemic vascular capacity and changes in arterial pressure parallel changes in total peripheral resistance. The aortic arch and carotid sinus baroreceptor areas were isolated, and the pressures were controlled separately. With carotid sinus pressure held constant at 125 mmHg, aortic arch pressure was increased and decreased between 225 and 50 mmHg, and the changes in reservoir volume and systemic arterial pressure were measured. Results from increasing and decreasing aortic arch or carotid sinus pressure were not significantly different and were averaged. The mean change in reservoir volume was 1.9 +/- 0.2 ml/kg and the change in mean arterial pressure was 18.7 +/- 3.7 mmHg. The changes in reservoir volume and arterial pressure caused by the aortic arch reflex were not influenced by the level of carotid sinus pressure. Carotid sinus pressure changes between 200 and 50 mmHg at a constant aortic arch pressure caused reservoir volume and arterial pressure to change by 7.2 +/- 0.9 ml/kg and 45.1 +/- 4.1 mmHg, respectively. The level of aortic arch pressure did not modify these responses.