FLOW AND THERMAL RESPONSES OF AORTIC BARORECEPTORS

1. The aims of this study were 1) to determine whether the impulse act ivity in rabbit aortic baroreceptors (BRs) was influenced by changes i n nonpulsatile flow through the aortic lumen and 2) to examine the BR and aortic arch responses to changes in temperature. 2. An in vitro ao rtic arch-aortic nerve preparation was used to record suprathreshold s teady-state discharge in a total of 29 single-unit BRs from 12 New Zea land white rabbits. Changes in BR frequency were measured relative to control and were recorded simultaneously with aortic arch pressure, fl ow, temperature, diameter, and calculated wall shear stress (Sw). 3. W ith 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 ac tivation response appeared closely related to the changes in flow and Sw, but also appeared related to uncontrolled changes in perfusate tem perature. 4. When the effects of temperature were examined more closel y with pressure and flow held constant, controlled step increases in t emperature (between 32 and 42-degrees-C) constricted the arch and agai n evoked two BR responses: activation in 11 of 14 BRs tested and inhib ition in 3 units. The Q10 for the activation response was 1.55 +/- 0.0 8 (mean +/- SE), which was not significantly different from the Q10 fo r 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) wi th temperature and pressure held constant, stair-step increases in flo w failed to produce any change in BR frequency in each of 15 fibers te sted ( 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 endo thelial-mediated vasodilation (acetylcholine) were intact. 6. These re sults 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 st ress.