Baroreflex sensitivity changes during the reversal of Goldblatt two-kidney one-clip hypertension in rats

Reduced spontaneous baroreceptor sensitivity in patients with renovascular hypertension

OBJECTIVE

Sympathetic nerve activity is increased in hypertensive patients with renal artery stenosis. Less is known about cardiac vagal function in these patients before and after renal angioplasty. The aim of the present study was to investigate cardiac baroreceptor reflex sensitivity together with total body noradrenaline (NA) spillover in hypertensive patients with renal artery stenosis before, and in some patients, 1 year after renal angioplasty.

MATERIAL AND METHODS

Spontaneous baroreceptor reflex sensitivity and total body noradrenaline (NA) spillover were measured in patients with renovascular hypertension before intervention (n = 18), patients being cured/improved 1 year after renal angioplasty (n = 5) and age-matched healthy subjects (n = 25).

RESULTS

Hypertensive patients with renal artery stenosis had higher total body NA spillover (4630 +/- 619 versus 3132 +/- 210 pmol/min, P < 0.05) and reduced cardiac baroreceptor reflex sensitivity (6.1 +/- 1.0 versus 10.7 +/- 1.0 ms/mmHg, P < 0.01) compared with healthy subjects. Similar results were obtained (before intervention) in a subgroup of patients (n = 9) with renovascular hypertension defined as cured/improved 1 year following renal angioplasty. Baroreceptor reflex sensitivity improved after renal angioplasty in a subset of patients showing good blood pressure control 1 year after intervention (6.4 +/- 0.7 to 9.4 +/- 1.7 ms/mmHg, P < 0.05).

CONCLUSIONS

Patients with renovascular hypertension showed reduced cardiac baroreceptor reflex sensitivity and increased noradrenergic activity, which to some extent was reversed 1 year following successful renal angioplasty.

Cardiac baroreflexes and hypertension

1. The role of cardiac reflexes in baroreflex control mechanisms and the changes that occur in chronic hypertension is reviewed. The rapid resetting properties of the arterial baroreceptors ensures its role in short-term rather than long-term control of blood pressure. 2. In hypertensive humans and animals, the baroreceptor-heart rate reflex has diminished sensitivity due mainly to reduced maximum capacity of the cardiac vagal component rather than a change to the sympathetic. 3. The development of this vagal deficit in spontaneously hypertensive rats (SHR) coincides with the onset of cardiac hypertrophy rather than vascular hypertrophy. A combination of chronic perindopril treatment regimens in young and older SHR showed that the vagal deficit was better correlated with the degree of cardiac hypertrophy than with other variables such as blood pressure, hypertension or indices of vascular hypertrophy. Similar results have been shown for renovascular hypertension in rats. 4. The bradycardia produced by electrical stimulation of the vagus in urethane anaesthetized young SHR was enhanced compared to Wistar-Kyoto rats (WKY) while responses observed in adult SHR and WKY were similar, suggesting that the vagal deficit in hypertensive rats is not due to a defect in the efferent arm of the baroreflex. 5. The association of the vagal deficit with cardiac hypertrophy, but not with the vagal efferent pathways, suggests an important role for cardiac afferents in hypertension in mediating the baroreflex deficit. 6. A diminished baroreflex and also a reduced heart rate variability is an independent risk factor for sudden death following myocardial infarction. Thus, antihypertensive therapy, which more effectively reduces cardiac hypertrophy, should have a desirable effect on baroreceptor reflexes and therefore in reducing blood pressure variability.

Vagal and sympathetic components of the heart rate range and gain of the baroreceptor-heart rate reflex in conscious rats

Previous studies in conscious rats have examined the relationship between mean arterial pressure (MAP) and heart rate (HR) only during relatively small or unidirectional changes in blood pressure. We have now examined this relationship more fully in conscious Sprague-Dawley rats using graded bolus i.v. doses of phenylephrine and nitroprusside to alter MAP over a range of 60-160 mm Hg with a view to determining the contribution made by the vagus and sympathetic (after atenolol or methylatropine, respectively). In 25 rats the relationship of HR to MAP followed a sigmoidal rather than a linear function (P less than 0.001) with clear upper and lower HR plateaus. The HR range estimated from a logistic equation was 217 +/- 7 b/min while the average gain between the inflection points was 4.1 +/- 0.2 b/min/mm Hg (1.8 times greater than from a linear fit). The vagus makes a greater contribution to the HR range than the sympathetic (61 vs 39%, respectively) while the converse is the case for the gain (46% vagus, 63% sympathetic). In the presence of both blocking drugs, changes to HR were less than 7% of control. These results suggest that the baroreceptor-HR reflex in the conscious rat is best characterised by a sigmoid curve with approximately equal contributions from both the cardiac vagus and the sympathetic nerves.