Role of the Baroreceptor Reflex in Daily Control of Arterial Blood Pressure and Other Variables in Dogs

Abstract

Normal and sinoaortic baroreceptor-denervated dogs were monitored continuously (24 hours a day) to quantify the role of the baroreceptors in determining the average level and the variability of arterial blood pressure, heart rate, cardiac output, and total peripheral resistance. The frequency of occurrence over 24-hour periods was obtained for each variable using a fiber optic curve-scanning system to read the variables from continuously recorded charts and a digital computer system to plot curves. The results indicate that the degree of hypertension previously reported for this preparation has been highly exaggerated, presumably due to the methods of study. The average 24-hour mean arterial blood pressure was 101.6 mm Hg in normal dogs and only 112.7 mm Hg in baroreceptor-denervated dogs. The normal dogs exhibited narrowly distributed 24-hour frequency distribution curves for blood pressure; in contrast the denervated dogs exhibited curves with twice the 24-hour standard deviation. Similar analysis indicated that the baroreceptors exerted less influence on the daily stabilization of heart rate than they did on arterial blood pressure and that they had very little if any influence on the daily stabilization of cardiac output and total peripheral resistance. Hemodynamic variables during postural changes were studied along with diurnal rhythms. We concluded that the primary function of the baroreceptor reflex is not to set the chronic level of arterial blood pressure but, instead, to minimize variations in systemic arterial blood pressure, whether these variations are caused by postural changes of the animal, excitement, diurnal rhythm, or even spontaneous fluctuations of unknown origin.

Hypertension following denervation of aortic baroreceptors in unanesthetized dogs.

“…In some older studies (Nowak, 1940;Thomas, 1944) and in many of the animals studied by Cowley et aL (1973), denervation produced pressures higher than were seen in our dogs. Taken with our findings, these results argue against the importance of the nonneural "infinite gain" mechanisms that Guyton et al (1974) have emphasized.…”

“…Hypertension has been seen in awake dogs after arterial baroreceptor denervation (Ferrario et al, 1969;Heymans and Neil, 1958;Nowak, 1940;Thomas, 1944). However, in other studies (Cowley et al, 1973;Cowley and Guyton, 1975;Cowley and DeClue, 1976;Heymans and Neil, 1958;Koch and Mattonet, 1934), little or no increase in mean arterial pressure followed baroreceptor nerve section. If baroreceptor denervation produces no sustained rise in mean arterial pressure, despite the decrease in input to the medulla from the baroreceptors, this implies an adaptation of the central nervous system and/or of peripheral motor mechanisms.…”

“…If baroreceptor denervation produces no sustained rise in mean arterial pressure, despite the decrease in input to the medulla from the baroreceptors, this implies an adaptation of the central nervous system and/or of peripheral motor mechanisms. Cowley et aL (1973) believed that some prior studies in which hypertension was produced were suspect because only brief recordings of pressure were taken and because emotional responses might have altered the pressure. In all of the above studies, pressure was more labile in the denervated than in the normal animal.…”

Diurnal blood pressure variation and sympathetic activity

“…9 This hypothesis is based on evidence that in experimental animals, sino-aortic denervation or the surgical ablation of carotid baroreceptors is accompanied by a three-fold increase in blood pressure variability. 10,11 However, diurnal blood pressure variation and circadian blood pressure rhythms depend on the influences of other factors, such as the renin-angiotensin system, 12,13 vasomotor tone 14 and cyclic variations in plasma norepinephrine and epinephrine, as shown previously in studies that measured catecholamines in cerebrovascular regions or in the plasma reservoir 12,[15][16][17] (Figure 2). Direct recording of efferent postganglionic muscle sympathetic nerve traffic can now be accomplished using the microneurographic technique.…”

Behaviorally selective cardiovascular hyperreactivity in spontaneously hypertensive rats. Evidence for hypoemotionality and enhanced appetitive motivation.

“…Cardiovascular responsitivity during behavior was analyzed by the construction of frequency-interval histograms 24 for each behavior. The histograms of AP and HR indicate the incidence with which the cardiovascular variables assume a certain value during select time periods, in this case defined by the occurrence of natural behaviors.…”