Effects of prazosin on autonomic control of circulation in essential hypertension

Gender-modulated endogenous baseline neuropeptide Y Y1-receptor activation in the hindlimb of Sprague-Dawley rats

This study examined the effect of neuropeptide Y Y(1)-receptor blockade both alone, and in interaction with alpha(1)-adrenoceptor antagonism, on basal hindlimb vascular conductance in male and female Sprague-Dawley rats. Hindlimb vascular conductance was measured during infusion of BIBP3226 (Y(1)-receptor antagonist; 100 microg kg(-1)), prazosin (alpha(1)-receptor antagonist; 20 microg kg(-1)), and combined blockade. In males, vascular conductance increased 1.1 +/- 0.3 microl min(-1) mmHg(-1) above baseline with BIBP3226, and 2.4 +/- 0.4 microl min(-1) mmHg(-1) above baseline with prazosin (both P < 0.05). The increase in vascular conductance during combined blockade (5.1 +/- 0.7 microl min(-1) mmHg(-1)) was greater than the sum of the independent BIBP3226 and prazosin responses (P < 0.05). In females, basal hindlimb vascular conductance was unaffected by Y(1)-receptor blockade. However, alpha(1)-receptor blockade resulted in a 3.5 +/- 0.6 microl min(-1) mmHg(-1) increase in vascular conductance above baseline, which was not different than the combined blockade condition. Males had greater skeletal muscle neuropeptide Y concentration (P < 0.05; ELISA) than females. Furthermore, compared with females, male skeletal muscle contained greater Y(1)-receptor expression (P < 0.05; Western blot). It was concluded that, under baseline conditions, agonist and receptor-based mechanisms for Y(1)-receptor dependent control of vascular conductance in skeletal muscle was greater in male versus female rats.

Y1- and alpha1-receptor control of basal hindlimb vascular tone

The role of endogenous Y(1)-receptor activation on skeletal muscle vasculature under baseline conditions is currently debated and no in vivo studies have been performed to address this issue. Therefore, this study was designed to address the effect of Y(1)-receptor and/or alpha(1)-adrenoceptor antagonism on basal hindlimb vascular conductance in male Sprague-Dawley rats in vivo. Left hindlimb vascular conductance, carotid artery mean arterial pressure, and heart rate were measured during low volume infusion of N(2)-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-d-arginine amide (BIBP3226; 100 microg/kg), prazosin (20 microg/kg), and combined blockade to the left hindlimb. Vascular conductance increased 1.5 +/- 0.5 microl.min(-1).mmHg(-1) with BIBP3226 infusion, 1.7 +/- 0.5 microl.min(-1).mmHg(-1) with prazosin infusion, and 4.8 +/- 1.0 microl.min(-1).mmHg(-1) with combined blockade (P < 0.05). Interestingly, systolic vascular conductance increased in all three conditions, but diastolic vascular conductance only increased in the two conditions where BIBP3226 was present. These data indicate that Y(1)-receptor activation plays an important role in the regulation of vascular conductance in the resting rat hindlimb. Furthermore, this effect was of the same magnitude as the alpha(1)-adrenoceptor contribution. The differential flow profiles following alpha(1) blockade with and without Y(1)-receptor blockade supports local differences in receptor distribution.

Effects of alpha 1-receptor blockade on the hemodynamic responses to exercise in young hypertensives

The purpose of this study was to determine if alpha 1-adrenergic receptor blockade alters the hemodynamic response to exercise in young (less than 25 yr) male borderline hypertensives differently than in young normotensives. Five hypertensive (HTN, MAP greater than 105 mmHg) and 7 normotensive (NTN, MAP less than 95 mmHg) college-age males underwent two 30 min bouts of cycle ergometry exercise at 50% VO2pk in a warm (25 degrees C, 50% rh) environment; one following alpha 1-receptor blockade with prazosin (PRAZ) and the other following placebo administration (PLAC). During resting PLAC and compared to NTN, HTN exhibited an elevated cardiac index (CI, p = .002), similar HR and elevated total peripheral resistance index (TPRI, p = .015). During resting PRAZ, CI and TPRI were similar but HR was higher (p = .013) in HTN than NTN. While reduced during PRAZ, resting MAP was higher in HTN than NTN (p = .007) for both trials. With exercise and PLAC, CI was higher (p = .029) while HR and TPRI were similar for HTN compared to NTN. With PRAZ, the exercise CI, TPRI and HR responses were similar for both groups. Exercise MAP was blunted in both groups with PRAZ. While not differing significantly between groups for each treatment, MAP was stable for NTN while it declined after 10 min of exercise in HTN. The elevated CI seen in exercising HTN with PLAC was removed with PRAZ; the exercise response was otherwise unaltered by alpha 1-blockade. Consequently, these data suggest that young male hypertensives have an elevated blood pressure due to an elevated CI incompletely offset by a reduced TPRI. While alpha 1-blockade lowers MAP by lowering CI, the MAP response to exercise remains unaltered.

Arterial hemodynamics in human hypertension. Effects of adrenergic blockade

BACKGROUND

Resistance, pulse wave velocity, and wave reflections have been shown to be increased in patients with essential hypertension compared with normotensive controls. These alterations are completely normalized by nitroprusside infusion but exacerbated during beta-adrenergic blockade, suggesting an enhanced smooth muscle tone that is in part modulated by adrenergically mediated vasodilation. The present study was performed to examine the extent to which this apparently enhanced smooth muscle tone is a result of alpha-adrenergically mediated vasoconstriction.

METHODS AND RESULTS

Age-matched normotensive and hypertensive Chinese subjects were instrumented with catheter-tipped micromanometers and an electromagnetic flow velocity sensor positioned in the ascending aorta. Aortic impedance and wave reflection properties were obtained from Fourier analysis of the pressure and flow signals during baseline conditions, after beta-blockade with propranolol (0.15 mg/kg i.v.), and after alpha-blockade with intravenous phentolamine (range, 15-80 mg) that was sufficient to either normalize blood pressure or produce a pressure that could not be further lowered. Compared with normotensives, in the baseline state, hypertensives had elevated resistance (1,962 versus 1,268 dyne.sec/cm5, p less than 0.001), total power (1,893 versus 1,568 mW, p less than 0.08), reflected pressure wave component (25.6 versus 13.5 mm Hg, p less than 0.001), ratio of reflected to forward wave (0.65 versus 0.42, p less than 0.001), and pulse wave velocity as determined from the frequency of the first zero-crossing of impedance phase angle (4.6 versus 3.5 Hz, p less than 0.03). During combined alpha- and beta-adrenergic blockade, blood pressure decreased into the normal range (from 162/103 to 131/87 mm Hg) but was still somewhat higher than that in the normotensive subjects. Resistance (1,914 dyne.sec/cm5, p less than 0.03), reflected wave (19.5 mm Hg, p less than 0.01), and ratio of reflected to forward wave (0.61, p less than 0.001) were, however, persistently elevated above normal values.

CONCLUSIONS

alpha-Adrenergically mediated vasoconstriction cannot account for all of the hemodynamic alterations seen in essential hypertension.

Brain and the regulation of blood pressure: a hemodynamic perspective

The central nervous system subserves the homeostasis of the circulation and is organized as a negative feedback system. The following properties of such a feedback system are of interest: a) the setting; b) the range of the regulation; and c) the nature of the feedback (regulated) variable. In this review we show that in hypertension blood pressure is set at a higher level, but regulation of blood pressure in hypertension is normal. The central nervous system is involved in maintaining a higher set point in early human hypertension as well as in many forms of experimental hypertension. Results from trials of antihypertensive drugs suggest that setting of the baseline level of blood pressure and regulation of blood pressure variability are independent properties and are probably regulated by different areas of the central nervous system. It is, therefore, unlikely that research on blood pressure "reactivity" will elucidate the pathophysiology of the central resetting of baseline blood pressure level in hypertension. We present evidence that in subserving the circulation, the central nervous system regulates and senses blood pressure and not flow. Pathophysiologic implications of this concept are discussed particularly in regards to the apparent decrease of sympathetic tone during the evolution of hypertension.

The effect of prazosin on skin microcirculation as assessed by laser Doppler flowmetry

1. Laser Doppler flowmetry was used in six normal volunteers to record changes in fingertip skin blood flow after the administration of prazosin to block postsynaptic alpha 1-adrenoceptors. 2. Prazosin (0.5 mg orally) did not alter systolic or diastolic blood pressure or heart rate. 3. Prazosin did significantly increase basal skin blood flow 2 h after its administration but this effect was no longer evident after contralateral hand warming. Prazosin markedly reduced the skin vasoconstrictor response to deep inspiration and to contralateral hand cooling. 4. This study suggests that postsynaptic alpha 1-adrenoceptors are involved in maintaining skin vasoconstrictor tone at rest and are also involved in the rapid skin vasoconstriction seen in response to a deep inspiration and to contralateral hand cooling.

Dose-titration study of alfuzosin, a new alpha 1-adrenoceptor blocker, in essential hypertension

An open, dose-titration study of alfuzosin, a new selective post-synaptic alpha 1-adrenoceptor antagonist with additional direct vasodilator properties has been performed. After a 3-week run-in placebo period, 12 patients with essential hypertension received alfuzosin 5 mg oral b.d., and then the dose was doubled every week, up to a maximum of 20 mg q.i.d. if the supine diastolic blood pressure was greater than 90 mm Hg. The study lasted for 4 weeks. Supine blood pressure (SBP) decreased from 160/102 (Day 0) to 148/89 mm Hg and upright blood pressure (UBP) from 151/102 (Day 0) to 137/84 mm Hg. Alfuzosin did not cause any significant change in supine or upright heart rate. In addition, after the first dose of alfuzosin, supine and upright blood pressure and heart rate (SHR and UHR) were measured every 30 min for 5 h. The fall in blood pressure was significant after 90 min and it lasted up to the 5th hour; the maximum effect was observed after 3 h: SBP decreased from 159/103 (time 0) to 137/84 mm Hg and UBP from 150/102 (time 0) to 123/79 mm Hg. SHR was increased from 72 (time 0) to 81 beats/min at the 5th hour and UHR from 87 to 101 beats/min at the 4th hour. A weak but significant correlation was observed between the hypotensive effect 12 h after drug intake and the plasma concentration of the drug at that time. A 10% decrease in supine diastolic blood pressure was found at a drug plasma concentration higher than 7 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise stress test in young hypertensive patients. Response to vasodilators (prazosin) vs. beta-blocker (atenolol) agents

Dynamic response of arterial blood pressure during exercise has been studied in 40 normotensive young subjects and 20 mild hypertensive young patients (20-40 years of age). Hypertensive patients were treated with atenolol (beta blocker) and prazosin (vasodilator). Both groups underwent maximal exercise stress test. A double-blind nonrandomized study was practiced in hypertensive patients with placebo, prazosin (3 mg/12 h), and atenolol (100 mg/24 h). Heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP), and exercise duration (ED) were analyzed. All parameters remained stable in both groups. The hypertensive patients showed an increase in maximum SBP more than 230 mmHg during the placebo phase. This same group showed a significant increase in HR at rest two hours after administration of prazosin. Atenolol produced a significant reduction in HR both during rest and exercise. Both drugs produced a significant decrease in SBP and DBP (at rest and exercise). We conclude that exercise test is a noninvasive procedure that could distinguish mild arterial hypertension. The dynamic changes of arterial blood pressure can be controlled with prazosin (3 mg/12 h) or 1 daily intake of 100 mg atenolol.

Antihypertensive drugs and baroreceptor reflex control of heart rate and blood pressure

The arterial baroreceptor reflex (BR; aortic and carotid sinus BR) and the cardiopulmonary BR are the most important reflexes acting as buffer systems for the maintenance of arterial pressure around a fixed physiologic value. They act as permanent inhibitory systems on the central cardiovascular structures and they can be either activated or deactivated by using selective techniques. During chronic hypertension there are structural alterations of the peripheral and/or central components of the BR that become "reset", with a shift in the function curve relating BR activity to blood pressure (BP) in the direction of higher pressure values. As a consequence of the hypertension-induced resetting phenomenon, both the threshold pressure and sensitivity of BR are disturbed. However, if BR resetting during hypertension clearly decreases the sensitivity of BR control of heart rate (HR), BR control of peripheral resistance and arterial pressure as a whole is preserved and even increased when hypertension develops. Thus, this apparent discrepancy between BR control of HR and BP during hypertension demonstrates that evaluation of an antihypertensive therapy on reflex control of HR alone cannot predict what will happen to BR control of the whole cardiovascular system. Regarding BR control of HR, in contrast to the classical arteriolar vasodilators such as hydralazine and its derivatives, the majority of the modern antihypertensive drugs do not evoke reflex tachycardia in response to lowering of BP in normotensive or hypertensive subjects. Although the intrinsic pharmacologic mechanisms of action of these drugs on BR may be quite different (e.g., alpha 1-or beta-adrenoreceptor blocking agents, converting enzyme inhibitors, certain calcium-channel blockers of the phenyldihydropyridine group, and so on), they all shift in a parallel manner the set-point of the BR response curve towards lower pressures, with no change in HR or R-R interval. Generally, this resetting phenomenon occurs after several weeks or months of antihypertensive therapy, but it can also occur acutely (e.g. after IV injection) after administration of drugs such as prazosin or ketanserin. Finally, antihypertensive agents such as clonidine and methyldopa can simultaneously reset the BR and increase its sensitivity, thus leading to almost complete restoration of control of HR response despite the concomitant decrease in BP. Regarding BR control of blood pressure, only captopril and especially celiprolol (a beta 1-adrenoreceptor blocking drug with vasodilating properties) are able to restore almost normal BR control of arterial pressure.

Ketanserin and the arterial baroreceptor reflex in normotensive subjects

The effects of oral ketanserin 40 and 120 mg on the responses to baroreflex activation and deactivation by phenylephrine and nitroglycerin, respectively, were investigated in normotensive subjects. Plasma catecholamine levels were measured at the same times. Two hours after the administration of ketanserin, and regardless of its effect on arterial pressure (no change after 40 mg, decrease after 120 mg), there was no alteration either in resting heart rate or baroreflex sensitivity during baroreceptor activation or deactivation. The lack of reflex tachycardia in response to the drug-induced hypotension may be related to the alpha 1-adrenoceptor blockade-mediated sympathoinhibitory effect of ketanserin, which leaves unaffected both plasma catecholamines and the normal reactivity of the sympathetic system.

Alpha 1-adrenergic blockade and cardiovascular pressor responses in essential hypertension

The effects of selective alpha 1-adrenergic blockade with terazosin on blood pressure and cardiovascular pressor responsiveness were assessed in 17 subjects with mild to moderate essential hypertension (mean age, 48 +/- 2 [SEM] years). As compared with a 2-week placebo period, 8 weeks of terazosin treatment (mean dose, 10.5 +/- 1.7 mg/day) caused a fall of supine (from 153/103 +/- 3/2 to 143/96 +/- 4/2 mm Hg; p less than 0.025) and upright (from 145/106 +/- 4/2 to 131/94 +/- 5/3 mm Hg; p less than 0.01) arterial pressure; a marked blunting of cardiovascular pressor responsiveness to norepinephrine, as judged from the pressor dose (from 73 +/- 9 to 2156 +/- 496 ng/kg/min; p less than 0.02) and from the rightward shift (p less than 0.01) of the plasma concentration-blood pressure response curve; and a slight increase in plasma norepinephrine concentration (from 37.7 +/- 3.3 to 52.2 +/- 7.8 ng/dl; p less than 0.01). Heart rate, body weight, exchangeable sodium, blood volume, and norepinephrine plasma clearance; plasma epinephrine, renin, angiotensin II, and aldosterone levels; the relationships between angiotensin II-induced increases in arterial pressure or plasma aldosterone and the concomitant increments of plasma angiotensin II; and heart rate responsiveness to isoproterenol did not change significantly after terazosin treatment. These findings suggest that the fall of arterial pressure induced by selective alpha 1-adrenergic blockade in subjects with essential hypertension is associated with, and probably explained by, inhibition of alpha 1-mediated, noradrenergic-dependent vasoconstriction. alpha 1-Adrenergic receptor antagonism did not modify body sodium concentration, the adrenomedullary component of the sympathetic nervous system, angiotensin II levels, or beta-adrenergic dependent mechanisms.

Acute systemic and regional hemodynamic effects of alpha 1-adrenoceptor blockade in conscious spontaneously hypertensive rats

The acute hemodynamic effect of the selective alpha1-antagonist, 2-[4-(n-butyryl)-homopiperazine-1-y1]-4-amino-6,7-dimethoxy-quinazoline (bunazosin; E-643), was studied by the microsphere technique in 62 conscious spontaneously hypertensive rats. We found that a large fall in total peripheral resistance with a moderate increase in cardiac output accounted for the moderate decrease in arterial pressure; the heart rate did not change; the skeletal muscle, brain, heart, and gastrointestinal tract sensitively dilated, and kidney less sensitively dilated, their resistance vessels, with little change in their blood flow.

Haemodynamic and hormonal effects of prazosin on head-up tilt in essential hypertensive patients: comparison with those of propranolol

To evaluate the haemodynamic and hormonal effects of prazosin, head-up tilt was performed in 10 essential hypertensive patients, and these effects of prazosin on the tilt were compared with those of propranolol. The tilts were performed in control phase and the last days of treatment for two weeks with propranolol (90 mg/day) or prazosin (3-6 mg/day). Each drug significantly lowered the mean blood pressure at rest, and also suppressed its rise on the tilt. Heart rates were significantly increased by the tilt in the control phase, in the propranolol phase and in the prazosin phase. Cardiac index was significantly reduced by the tilt from 2.66 (s.e.m. = 0.22) 1/min per m2 to 2.08 (s.e.m. = 0.20) in the propranolol phase. However, there were not significant changes in other phases. Total peripheral resistance indices were significantly increased by the tilt in all three phases. Plasma renin activity and plasma aldosterone were significantly increased by the tilt from 2.14 (s.e.m. = 0.47) ng/ml per h to 2.46 (s.e.m. = 0.54) and from 50.6 (s.e.m. = 12.9) pg/ml to 74.9 (s.e.m. = 14.9) respectively, in the control phase. And they were also significantly increased from 1.06 (s.e.m. = 0.29) to 1.65 (s.e.m. = 0.45) and from 41.4 (s.e.m. = 16.3) to 54.0 (s.e.m. = 17.4) in the prazosin phase. There were no significant increases during the administration of propranolol. We observed that prazosin did not alter heart rate and cardiac index, but suppressed the renin-angiotensin system at rest. It is suggested that prazosin did not influence haemodynamic and hormonal responses to the tilt.

Hemodynamic and neuroendocrine responses to acute and chronic alpha-adrenergic blockade with prazosin and phenoxybenzamine

We investigated the relevance of the selective alpha 1-adrenergic receptor blockade produced by prazosin to its blood pressure-lowering efficacy in man. The hemodynamic and neuroendocrine responses to the acute and chronic oral administration of prazosin and phenoxybenzamine were compared in a randomized, double-blind, placebo-controlled, crossover study of 11 patients with essential hypertension. These responses were also evaluated during lower body negative pressure and dynamic bicycle exercise, which produce potent but diversified activation of the sympathetic nervous system. In the acute studies, arterial blood pressure decreased to similar levels with prazosin or phenoxybenzamine; however, hemodynamic and neuroendocrine responses differed both before and during sympathetic nervous system activation. Prazosin lowered arterial blood pressure by reducing total peripheral resistance (p less than 0.05). In contrast, phenoxybenzamine produced a modest reduction in cardiac output (8%, p less than 0.05) with little change in total peripheral resistance, forearm vascular resistance or forearm blood flow. Additionally, plasma norepinephrine concentration and heart rate rose to significantly higher levels with prazosin (p less than 0.02) than with phenoxybenzamine, a difference that was most evident with lower body negative pressure or dynamic exercise. Baroreceptor control of arterial pressure homeostasis was preserved with both agents, except during marked degrees of cardiovascular stress. With chronic therapy, the circulatory responses adapted to the alpha-adrenergic antagonists, and both drugs produced similar hemodynamic and neuroendocrine profiles. The differences with acute administration may be the result of a more rapid onset of action and a more marked degree of alpha-adrenergic blockage with prazosin than with phenoxybenzamine therapy, rather than to any difference in their alpha 1- and alpha 2-adrenergic receptor blocking properties. Moreover, the findings of the present study suggest that the prejunctional alpha 2-receptor, autoinhibitory to sympathetic neuronal norepinephrine release, is of no functional significance in patients with essential hypertension.

Effect of indapamide on the baroreceptor reflex in essential hypertension

The effect of chronic treatment with indapamide on blood pressure (BP), baroreceptor sensitivity (BRS) and vascular reactivity (VR) were investigated in 10 patients with essential hypertension. After 3 months of therapy with indapamide 2.5 mg/d the mean arterial pressure (MAP) had decreased from 135 +/- 6 to 112 +/- 2 mmHg (p less than 0.001); the heart rate (HR) had not changed, VR had decreased from 6.1 +/- 1.2 to 4.8 +/- 1.8 (pg . min . kg)-1 (p less than 0.05), and BRS had increased from 8.3 +/- 3.7 to 12.2 +/- 5.3 ms/mmHg (p less than 0.005), with a leftshift of the relationship between BP and heart period. An inverse correlation was found between the pre-treatment systolic blood pressure and the change in baroreceptor sensitivity after indapamide (r = 0.59; p less than 0.05). In conclusion, chronic treatment with indapamide enhances BRS and resets the reflex. The resetting may account for the lack of tachycardia at rest observed after treatment with indapamide. The mechanism by which indapamide interferes with the baroreceptor reflex requires further investigations.

Within patient comparison of prazosin and UK-33,274. A new alpha-adrenoceptor-antagonist

Eleven patients with uncomplicated essential hypertension received increasing single daily doses of UK-33,274, a new alpha-1 adrenoceptor antagonist, and prazosin for 4 days, in a open cross-over study. Doses were increased until a satisfactory blood pressure response was obtained. Average doses reached were 4.5 mg for UK-33,274 and 2.4 mg for prazosin. The maximum effect of the two drugs on standing blood pressure was similar, but prazosin was more effective in the supine position. Both drugs had a greater effect on standing than on supine blood pressure. UK-33,274 caused a consistent increase in heart rate while prazosin did not. Whereas there was no clear difference between the two compounds in the duration of the reduction in blood pressure the onset of action was more gradual for UK-33,274. The incidence of side-effects was similar for both drugs. The data suggest that UK-33,274 is less effective than prazosin in reducing blood pressure.

Modification of arterial baroreflexes by captopril in essential hypertension

Captopril lowers blood pressure without increasing heart rate and plasma norepinephrine, which suggests that this drug may potentiate arterial baroreflexes. In eight subjects with untreated essential hypertension, blood pressure was monitored intraarterially and the effects of baroreceptor stimulation or deactivation were assessed by measuring (1) the slopes of the relations between increase or reduction in systolic pressure (intravenous phenylephrine or nitroglycerin) and the resulting lengthening or shortening in R-R interval, and (2) the increase or decrease in mean arterial pressure induced by increasing and decreasing carotid transmural pressure (neck chamber). The measurements were made before and after a hypotensive oral dose of captopril (50 mg). Before captopril, the slopes of the R-R interval changes with increase and reduction in systolic pressure were 8 and 4 ms/mm Hg, respectively. The slopes of the mean arterial pressure changes with increase and reduction in carotid transmural pressure were 0.51 and 0.40 mm Hg, respectively. After captopril, the responses to baroreceptor stimulation were unaltered but those to baroreceptor deactivation were augmented. The pressor and heart rate responses to hand-grip and cold exposure were unchanged by captopril. Administration of captopril is accompanied by a baroreflex potentiation which involves the lower portion of the stimulus-response curve of the reflex. This phenomenon (which may originate at the afferent baroreceptor fibers or centrally) may avoid a reduction in the tonic baroreflex influence during captopril-induced hypotension, thus contributing to the hemodynamic effects of the drug.

Prazosin depression of baroreflex function in hypertensive man

Prazosin is a post synaptic alpha adrenergic blocker effective in hypertension, whose hypotensive effect is unaccompanied by reflex tachycardia or hyperreninemia, nor by other evidence of increased sympathetic activity. We studied the baroreceptor reflex arc as a potential mediator of these effects. Twenty-two essential hypertensive men were treated with prazosin alone versus placebo, and experienced a blood pressure fall (from 114.8 +/- 3.6 down to 101.1 +/- 2.5 mmHg, p less than 0.005) unaccompanied by any change in heart rate, plasma renin activity, or several other indices of sympathetic nervous system activity (plasma dopamine-beta-hydroxylase activity; urinary excretion of free catecholamines and vanillyl mandelic acid; all p less than 0.1). Concomitant with the blood pressure fall, there was a significant depression of baroreflex arc sensitivity, from 11.4 +/- 2.0 ms/mmHg down to 6.6 +/- 1.9 ms/mmHg (p less than 0.05), without an associated change in cardiac vagal inhibition (291.2 +/- 46.2 versus 300.3 +/- 19.2 ms, p greater than 0.1). Baroreflex arc sensitivity depression may in part explain the lack of reflex sympathetic outflow noted during prazosin treatment of hypertension.