The adrenergic system and the release and pressor action of vasopressin

Benefits and side effects of blood pressure lowering treatment: what was wrong with doxazosin in the ALLHAT?

The lowering of high blood pressure is supposed to protect target organs from hypertensive damage. The Antihypertensive and Lipid Lowering Treatment to Prevent Heart Attack Trial was designed to compare the cardioprotective properties of three antihypertensives from different classes (lisinopril, amlodipine and doxazosin) with chlorthalidone. Despite effective blood pressure lowering and a favorable metabolic profile, the doxazosin arm of the trial had a significantly higher relative risk of cardiovascular disease and heart failure compared with the chlorthalidone arm. This article speculates on possible causes for this unexpected result and suggests that the culprit may be accentuation of the vascular effects of vasopressin, which are maximized under alpha-adrenergic blockade. These findings may have implications for the large number of older men who receive monotherapy with alpha-blockers for treatment of prostatic symptoms.

N(N)-nicotinic blockade as an acute human model of autonomic failure

Pure autonomic failure has been conceptualized as deficient sympathetic and parasympathetic innervation. Several recent observations in chronic autonomic failure, however, cannot be explained simply by loss of autonomic innervation, at least according to our current understanding. To simulate acute autonomic failure, we blocked N(N)-nicotinic receptors with intravenous trimethaphan (6+/-0.4 mg/min) in 7 healthy subjects (4 men, 3 women, aged 32+/-3 years, 68+/-4 kg, 171+/-5 cm). N(N)-Nicotinic receptor blockade resulted in near-complete interruption of sympathetic and parasympathetic efferents as indicated by a battery of autonomic function tests. With trimethaphan, small postural changes from the horizontal were associated with significant blood pressure changes without compensatory changes in heart rate. Gastrointestinal motility, pupillary function, saliva production, and tearing were profoundly suppressed with trimethaphan. Plasma norepinephrine level decreased from 1.1+/-0.12 nmol/L (180+/-20 pg/mL) at baseline to 0.23+/-0.05 nmol/L (39+/-8 pg/mL) with trimethaphan (P<.001). There was a more than 16-fold increase in plasma vasopressin (P<.01) and no change in plasma renin activity. We conclude that blockade of N(N)-cholinergic receptors is useful to simulate the hemodynamic alterations of acute autonomic failure in humans. The loss of function with acute N(N)-cholinergic blockade is more complete than in most cases of chronic autonomic failure. This difference may be exploited to elucidate the contributions of acute denervation and chronic adaptation to the pathophysiology of autonomic failure. N(N)-Cholinergic blockade may also be applied to study human cardiovascular physiology and pharmacology in the absence of confounding baroreflexes.

Pressor hormones in elderly hypertensive persons. Racial differences

The purpose of this study was to examine pressor hormones and platelet alpha 2-adrenergic receptors in elderly unmedicated free-living subjects. Eighty-seven subjects, 70 +/- 5 years old (mean +/- SD), hypertensive or normotensive (blood pressure less than 160/90 mm Hg) were recruited for measurement of blood levels of norepinephrine, epinephrine, and vasopressin, as well as density and affinity of alpha 2-adrenergic receptors from platelet membranes, assessed by maximal binding (Bmax) and dissociation constant (Kd) of rauwolscine. They were separated into white hypertensive (n = 22) or normotensive (n = 41), and black hypertensive (n = 11) or normotensive (n = 13) groups, with similar age distribution throughout and similar blood pressure levels in the hypertensive and normotensive groups. Vasopressin was higher in the black hypertensive than white hypertensive group (1.5 +/- 1.0 vs. 0.7 +/- 0.5 pg/ml, respectively, p less than 0.005), whereas epinephrine correlated inversely with diastolic blood pressure (r = -0.7, p less than 0.02, in the black hypertensive group). Kd was higher in the black normotensive group than in the other groups (1.6 +/- 0.6 vs. 1.0 +/- 0.2, 1.1 +/- 0.3, or 1.0 +/- 0.3 nM in the white normotensive, black hypertensive, or white hypertensive group, respectively, p less than 0.002). Bmax was no different among groups but was significantly correlated with vasopressin levels for the whole group (r = 0.4, p less than 0.0004) although no such correlation existed within the black hypertensive group. The data suggest that various vasoconstrictor systems participate to different extents in the mechanisms generating and sustaining hypertension in elderly white and black subjects.

The role of vasopressin in blood pressure maintenance in diabetic orthostatic hypotension

The purpose of these studies was to assess the role of vasopressin in maintaining supine and upright blood pressures in hypertensive diabetic subjects. Patients with (n = 6) or without (n = 10) evidence of autonomic insufficiency had blood pressure and heart rate monitored before and after receiving an intravenous injection of 0.5 mg of a V1 vasopressin inhibitor. None of the patients had supine changes in blood pressure or heart rate. However, upon assuming the erect position, the six patients with preexisting orthostatic hypotension had an average blood pressure fall of 44 mm Hg after vasopressin inhibition (as opposed to 20 mm Hg before), accompanied by a modest rise in heart rate of 20 beats/min. Those without autonomic dysfunction were separated into two subgroups. Four developed an average fall in orthostatic blood pressure of 18 mm Hg after vasopressin inhibition, whereas the remaining six had no change. There were no distinguishing hormonal characteristics (vasopressin, renin, and catecholamine levels) between the groups, but in the patients with autonomic dysfunction, the renin level failed to rise when upright. We conclude that vasopressin plays an important role in preventing or minimizing orthostatic hypotension in diabetic patients. Its pressor contribution is crucial in those with autonomic insufficiency and impaired renin and sympathetic responses, in whom the pressor effectiveness of vasopressin is greatly enhanced.