Baroreflex responsiveness in borderline hypertensives: a study with neostigmine

Arterial baroreceptor reflex counteracts long-term blood pressure increase in the rat model of renovascular hypertension

INTRODUCTION

The present study tested the hypothesis that long-term effects of baroreceptor activation might contribute to the prevention of persistent arterial blood pressure (BP) increase in the rat model of renovascular hypertension (HTN).

METHODS

Repetitive arterial baroreflex (BR) testing was performed in normo- and hypertensive rats. The relationship between initial arterial BR sensitivity and severity of subsequently induced two-kidney one-clip (2K1C) renovascular HTN was studied in Wistar rats. Additionally, the time course of changes in systolic BP (SBP) and cardiac beat-to-beat (RR) interval was studied for 8 weeks after the induction of 2K1C renovascular HTN in the rats with and without sinoaortic denervation (SAD). In a separate experimental series, cervical sympathetic nerve activity (cSNA) was assessed in controls, 2K1C rats, WKY rats, and SHR.

RESULTS

The inverse correlation between arterial BR sensitivity and BP was observed in the hypertensive rats during repetitive arterial BR testing. The animals with greater initial arterial BR sensitivity developed lower BP values after renal artery clipping than those with lower initial arterial BR sensitivity. BP elevation during the first 8 weeks of renal artery clipping in 2K1C rats was associated with decreased sensitivity of arterial BR. Although SAD itself resulted only in greater BP variability but not in persistent BP rise, the subsequent renal artery clipping invariably resulted in the development of sustained HTN. The time to onset of HTN was found to be shorter in the rats with SAD than in those with intact baroreceptors. cSNA was significantly greater in the 2K1C rats than in controls.

CONCLUSIONS

Arterial BR appears to be an important mechanism of long-term regulation of BP, and is believed to be involved in the prevention of BP rise in the rat model of renovascular HTN.

Different etiologies of intrauterine growth restriction and different consistencies in the occurrence of abnormal local nonstationarity of fetal heart rate

Fetuses with intrauterine growth restriction (IUGR) can be programmed in utero to develop hypertension in adult life. The etiology of IUGR in human fetuses is not uniform. The present study demonstrated that different etiologies of growth restriction, idiopathic cause, and pregnancy-induced hypertension, produce different consistencies in the occurrence of an abnormal local very-short-term nonstationarity of heart rate during intrauterine period. Whether the consistent abnormality that was found in the growth-restricted fetuses associated with pregnancy-induced hypertension is linked to the different risk of later hypertension requires future studies.

A Consistent Abnormality in the Average Local Smoothness of Fetal Heart Rate in Growth-Restricted Fetuses Affected by Severe Pre-Eclampsia

An abnormality in cardiovascular regulation during the prenatal period has been suggested to be the pathophysiological link between fetal growth restriction and adult hypertension. The purpose of this study was to determine how consistently abnormal the local smoothness of the very-short-term heart rate is in growth-restricted fetuses associated with severe pre-eclamptic pregnancy. Multifractal Hurst analysis on the structure function of heart rate was performed in control fetuses (n =150), in fetuses affected by severe pre-eclampsia and not showing growth restriction (n =66) and in fetuses affected by severe pre-eclampsia and showing growth restriction (n =58). The very-short-term (< or =15 heart beats) generalized Hurst exponents of the order of -5 to 5 in three groups were compared. Each exponent quantifies an average local heart rate smoothness at 15-successive-heart rate sites, which were specified by the magnitude of the heart rate variation within the sites determined by and positively correlated with the order of the exponent. This means that the fetal heart rates within the sites of q > or =2 have a large fetal heart rate (FHR) variation, and those within the sites of q < or =-2 have a small FHR variation. In the fetuses affected by severe pre-eclampsia and not showing growth restriction, only values of the exponents of the order > or =2 were abnormally lower. In the fetuses affected by severe pre-eclampsia and showing growth restriction, the values of the exponents of all orders were abnormally lower. In conclusion, the local smoothness of heart rate is consistently abnormal regardless of the magnitude of heart rate variation within a very-short-term period in growth-restricted fetuses affected by severe pre-eclampsia.

Baroreflex stabilization of the double product

We investigated whether the baroreflex control of heart rate (HR) stabilizes the product of arterial pressure (P(A)) and HR, called the double product (DP), an indirect indicator of left ventricular oxygen consumption. During pharmacological increases and decreases of P(A) in conscious rabbits, the mean (+/-SE) rate of change of the DP with respect to P(A) (dDP/dP(A)) was -88 +/- 36 and -20 +/- 36 DP units/mmHg, respectively. Regression analysis of all peak responses obtained in individual rats produced a dDP/dP(A) value of 15 +/- 16 DP units/mmHg. These estimates were significantly less than the dDP/dP(A) value predicted if HR were constant (184 +/- 7 DP units/mmHg) and were not significantly different from zero. We also compared values of baroreflex sensitivity (BRS) from the literature with those calculated to provide ideal stabilization of the DP. BRS values were significantly correlated with the calculated ideal values (R = 0.95; n = 14). BRS averaged 128 +/- 24% of the ideal value in all species and 148 +/- 28% in mammals and birds. Our results suggest that stabilization of the DP is a common consequence of the baroreflex control of heart rate.

Salt-induced plasticity in cardiopulmonary baroreceptor reflexes in salt-resistant hypertensive patients

To investigate the effects of salt loading on cardiopulmonary and arterial baroreceptor reflexes, 34 hypertensive patients underwent two 4-day periods with different dietary sodium intakes (70 and 370 meq/day). The patients were classified as salt-sensitive or salt-resistant depending on whether the mean arterial pressure value obtained on day 4 of high salt intake did or did not increase by 8% or more. In 22 patients cardiopulmonary and carotid baroreceptor reflexes were assessed during each dietary period by measuring the reflex responses to the application of -10 mm Hg lower body negative pressure and of +60 mm Hg increase in neck tissue pressure. Salt-resistant patients (n = 16) retained less sodium than salt-sensitive patients (n = 6) and showed a reduction in plasma norepinephrine and forearm vascular resistance during high sodium intake, whereas the salt-sensitive patients did not. During low sodium diet, no significant differences could be detected in the reflex responses to cardiopulmonary and carotid baroreceptor unloading between the two groups. High salt diet, however, potentiated the gain of cardiopulmonary baroreceptor reflex, which was expressed as the increase in plasma norepinephrine or forearm vascular resistance per millimeter of mercury decrease in pulmonary capillary wedge pressure, only in the salt-resistant hypertensive patients. In addition, the atrial natriuretic factor response to changes in pulmonary capillary wedge pressure was significantly enhanced by high salt intake only in the salt-resistant hypertensive patients. The reflex responses to carotid baroreceptor unloading were unaffected by salt loading in either group. In the remaining 12 patients, the hemodynamic effects of graded lower body negative pressure (-5, -10, -15 mm Hg) and neck tissue positive pressure (+30, +45, +60 mm Hg) were tested for both diets. Again, high salt intake significantly potentiated the cardiopulmonary baroreceptor reflex gain, expressed as the slope of the linear correlation between the changes in forearm vascular resistance (mm Hg/ml/min/100 g) and pulmonary capillary wedge pressure (mm Hg), in salt-resistant (from 3.8 +/- 0.9 to 7.2 +/- 1.0, p less than 0.05) but not in salt-sensitive patients (from 4.2 +/- 0.9 to 3.2 +/- 0.6, NS). In conclusion, the present study demonstrates that high salt diet potentiates cardiopulmonary baroreceptor reflexes and enhances atrial natriuretic factor response in salt-resistant but not in salt-sensitive hypertensive patients. The salt-induced plasticity of cardiopulmonary baroreceptor reflexes may exert a protective effect against the development of salt-induced hypertension by augmenting the reflex vasodilatory response to volume expansion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of acute and chronic beta-adrenoceptor blockade on baroreflex sensitivity in humans

To determine whether beta-adrenoceptor blockade lowers blood pressure by potentiating arterial baroreflex sensitivity (BRS), we compared the effect of acute i.v. and chronic oral beta-blockade on the BRS (phenylephrine technique) of 51 subjects with essential hypertension. Subjects were randomly assigned in a double-blind protocol to one of atenolol, metoprolol, pindolol or propranolol. There was an increase in BRS, unrelated to changes in heart rate, after both acute and chronic beta-blockade. This effect was most evident in younger and less hypertensive subjects. Decreases in blood pressure after 5-months' treatment were unrelated to increases in BRS, indicating that the hypotensive action of these drugs is not dependent upon augmented baroreflex control of heart rate. Only propranolol, of the 4 beta-blockers, increased BRS significantly after acute and chronic treatment. The acute effect of propranolol was significantly different from that of i.v. metoprolol (P less than 0.008) but the effect of long-term treatment with propanolol was not significantly different from that of the other 3 beta-blockers. We conclude that the impaired reflex regulation of heart rate can be improved in younger and mild-to-moderate hypertensive patients by beta-adrenoceptor blockade. Further studies, involving larger numbers and perhaps fewer drugs are needed to determine the relative importance of lipophilicity and beta 1- or beta 2-receptor selectivity in mediating the increase in baroreflex sensitivity seen with treatment.

Impaired control of vasopressin release in hypertensive subjects with cardiac hypertrophy

The effects of graded lower body negative pressure (-10 and -40 mm Hg) on vascular resistance and plasma vasopressin, norepinephrine, and renin activity were assessed in seven hypertensive subjects with left ventricular hypertrophy and seven sex-matched and age-matched normotensive subjects. In both groups increasing levels of lower body negative pressure induced a progressive decrease in right atrial pressure and an increase in vascular resistance. In normal subjects plasma vasopressin, norepinephrine, and renin activity were progressively raised, whereas only the higher level of stimulation increased plasma renin activity, norepinephrine, and vasopressin in hypertensive subjects. Propranolol induced a significant increase in plasma vasopressin in normal subjects (from 1.3 +/- 0.1 to 2.0 +/- 0.1 pg/ml; p less than 0.05) but not in hypertensive subjects. In this latter condition -10 mm Hg lower body negative pressure failed to increase plasma vasopressin, norepinephrine, and renin activity in normal subjects. Propranolol abolished the change in plasma renin activity in both groups, reduced the increase in vascular resistance induced by -40 mm Hg lower body negative pressure in normotensive subjects, but did not modify the rise in vasopressin elicited by this stimulus in normal subjects or the humoral and hemodynamic reflex responses evoked in hypertensive subjects. These results suggest that cardiopulmonary receptors are involved in the control of vasopressin release in normal subjects, whereas in hypertensive subjects with left ventricular hypertrophy, this control is altered because of an impaired function of cardiopulmonary receptors.

Impaired responsiveness of the ventricular sensory receptor in hypertensive patients with left ventricular hypertrophy

We studied the control of forearm vascular resistance (FVR) by cardiopulmonary receptors in seven patients with hypertension and left ventricular hypertrophy (LVH) and in seven normotensive control subjects. Increasing levels of lower body negative pressure (LBNP) (-10 and -40 mm Hg) induced a progressive decrease in central venous pressure (CVP) and an increase in FVR. The changes in these two variables were correlated both in normal subjects and patients with hypertension (slope for normal subjects = -29.9, for patients with hypertension = -40.3, NS). After propranolol, there was a significant reduction in the increase in FVR induced by -40 mm Hg LBNP in normal subjects (+107 +/- 5 vs +129 +/- 15 mm Hg/ml/sec, p less than .05) but not in patients with hypertension. Consequently, the slope of the delta CVP/delta FVR regression was reduced in normal subjects (-20.6, p less than .01) but not in patients with hypertension. In another seven normal subjects and seven patients with hypertension and LVH we assessed the effects of -10 and -40 mm Hg LBNP on left ventricular filling pressure (LVFP). LBNP induced similar changes in CVP, LVFP, and total peripheral resistance both in normal subjects and in patients with hypertension. Propranolol failed to modify the effects of LBNP on CVP and LVFP in both groups and reduced the response of total peripheral resistance to -40 mm Hg LBNP only in normal subjects. Propranolol did not reduce the response of FVR to the cold pressor test and sustained handgrip or the arterial baroreflex response to the injection of phenylephrine and increased neck tissue pressure. Thus, hypertension-induced LVH seems to be associated with a selective impairment of the left ventricular sensory receptors.

Carotid sinus reflex control of coronary blood flow in human subjects

Systemic and coronary hemodynamics were assessed before and during a reduction in carotid transmural pressure. This reduction was induced by means of a pneumatic neck chamber in 15 normal subjects and 15 hypertensive patients with a normal coronary arteriogram. A reduced baroreflex responsiveness was demonstrated in hypertensive patients as compared with normal subjects by evaluating both the reflex bradycardia evoked by intravenous administration of phenylephrine and the reflex increase in blood pressure during carotid sinus hypotension. In normal subjects, the reduction in carotid transmural pressure induced a significant increase in mean blood pressure, total peripheral resistance, cardiac output, heart rate, coronary vascular resistance, coronary blood flow assessed by the continuous thermodilution method and myocardial oxygen consumption. In hypertensive patients, the same stimulus significantly increased mean blood pressure, cardiac output, heart rate and coronary blood flow while no significant change was detected in coronary vascular resistance and myocardial oxygen consumption. The increase in mean blood pressure, total peripheral resistance and cardiac output was significantly higher in normal subjects than in hypertensive patients. These results suggest that in normal subjects carotid sinus hypotension evokes reflex coronary vasoconstriction, whereas this response is blunted in hypertensive patients with reduced baroreflex sensitivity.