Operational sensitivity and acute resetting of aortic baroreceptors in dogs.

Abstract

SUMMARY Stimulus-response curves of aortic baroreceptors constructed by alternately increasing and decreasing pressure from a normal baseline or set-point differ from curves constructed by varying pressure in one direction only from an abnormally high or low pressure. In anesthetized dogs we recorded impulses from aortic baroreceptors with myelinated fibers, using a pressurized reservoir to control mean aortic blood pressure (MABP). After setting MABP to a baseline of 100 mm Hg (normal MABP in unanesthetized dogs), we constructed baroreceptor response curves by alternately decreasing MABP from 100 to 30 mm Hg, and increasing it from 100 to 180 mm Hg, in each case returning MABP to the baseline to obtain hysteresis loops. All baroreceptors were active at 100 mm Hg, their discharge averaging 15-16 impulses/sec. At all pressures above threshold, baroreceptors fired more when pressure was increasing than when pressure was decreasing. This hysteresis caused the steepest part of the response curve constructed in this manner to span the baseline value, demonstrating that, contrary to previous views, aortic baroreceptors signal decreases in pressure below the normal level, as well as increases above it. We also constructed response curves after holding MABP at a "hypertensive" baseline of 125 mm Hg for 20 minutes. "Hypertensive" curves demonstrated reversible resetting, shifting significantly to the right of "normotensive" curves so that baroreceptor threshold increased on average by 7 mm Hg ( P < 0.01). Both hysteresis and short-term resetting probably result from the viscoelastic behavior of wall elements with which baroreceptors are coupled. Circ Res 48: 676-684, 1981 THE ROLE of aortic baroreceptors in regulating blood pressure is controversial. From the results of an electroneurographic study in dogs, Pelletier et al. (1972) concluded that at normal arterial pressure the aortic nerve displays little baroreceptor activity and that, rather than acting as a true buffer system, aortic baroreceptors have a predominantly antihypertensive role. However, two recent studies in conscious dogs indicate that when the carotid sinus nerves are cut the aortic nerves alone can buffer decreases, as well as increases, in arterial pressure (McRitchie et al., 1976;Ito and Scher, 1978). This apparent conflict of evidence about a fundamental aspect of the functions of aortic baroreceptors in dogs suggests that a reexamination of their afferent properties is timely.In studies of the stimulus-response characteristics of arterial baroreceptors, the common practice is either to increase pressure until firing frequency is maximal and then to examine baroreceptor response as pressure is reduced (Fig. 1A), or to reduce pressure below baroreceptor threshold and then to examine the response as pressure is increased (Fig.

Interaction of canine carotid sinus and aortic arch baroreflexes in the control of total peripheral resistance.

“…For this analysis, the direction of change in receptor pressure was included in the factor of step size, and was therefore not found to be significant. Even so, we cannot exclude the possibility that hysteresis exists in the region of maximum gain; in fact, there is evidence in the literature to indicate that this is so (Coleridge et al, 1981).…”

Rapid resetting of rabbit aortic baroreceptors and reflex heart rate responses by directional changes in blood pressure.

“…More recently, resetting has been shown to occur within 10-20 min of a 'steady-state' rise or fall in resting M.A.P. (Salgado & Krieger, 1978; Coleridge, Coleridge, Kaufman & Dangel, 1981; Dorward, Andresen, Burke, Oliver & Korner, 1982;Munch, Andresen & Brown, 1983;Coleridge, Coleridge, Poore, Roberts & Schultz, 1984;Heesch, Thames & Abboud, 1984); once established, it is stable for at least 1-2 h (Dorward et al 1982;Munch et al 1983). Receptor resetting appears to be largely responsible for the resetting of baroreflex responses, which also occurs over a similar time span (Kunze, 1981;Forward et al 1982; Dorward, Riedel, Burke, Gipps & Korner, 1985).…”

Central effect of intravenous phenylephrine on baroreflex control of renal nerves.

“…In fact, because of hysteresis effects, one would expect less baroreceptor discharge at a given pressure when the pressure was falling than when it was rising. 2 Thus, the expected reflex response would be that, as the pressure returned to control, sympathet-ic nerve activity at a given pressure would be higher when the pressure was falling than when it was rising. Put another way, hysteresis in the baroreceptors might be expected to translate into hysteresis in the baroreflex control of sympathetic outflow.…”