Biochemical evaluation of sympathetic nerve tone in essential hypertension

A role for benzamil-sensitive proteins of the central nervous system in the pathogenesis of salt-dependent hypertension

1. Although increasing evidence suggests that salt-sensitive hypertension is a disorder of the central nervous system (CNS), little is known about the critical proteins (e.g. ion channels or exchangers) that play a role in the pathogenesis of the disease. 2. Central pathways involved in the regulation of arterial pressure have been investigated. In addition, systems such as the renin-angiotensin-aldosterone axis, initially characterized in the periphery, are present in the CNS and seem to play a role in the regulation of arterial pressure. 3. Central administration of amiloride, or its analogue benzamil hydrochloride, has been shown to attenuate several forms of salt-sensitive hypertension. In addition, intracerebroventricular (i.c.v.) benzamil effectively blocks pressor responses to acute osmotic stimuli, such as i.c.v. hypertonic saline. Amiloride or its analogues have been shown to interact with the brain renin-angiotensin-aldosterone system (RAAS) and to effect the expression of endogenous ouabain-like compounds. Alterations of brain RAAS function and/or endobain expression could play a role in the interaction between amiloride compounds and arterial pressure. Peripheral treatments with benzamil, even at higher doses than those given centrally, have little or no effect on arterial pressure. These data provide strong evidence that benzamil-sensitive proteins (BSPs) of the CNS play a role in cardiovascular responsiveness to sodium. 4. Mineralocorticoids have been linked to human hypertension; many patients with essential hypertension respond well to pharmacological agents antagonizing the mineralocorticoid receptor and certain genetic forms of hypertension are caused by chronically elevated levels of aldosterone. The deoxycorticosterone acetate (DOCA)-salt model of hypertension is a benzamil-sensitive model that incorporates several factors implicated in the aetiology of human disease, including mineralocorticoid action and increased dietary sodium. The DOCA-salt model is ideal for investigating the role of BSPs in the pathogenesis of hypertension, because mineralocorticoid action has been shown to modulate the activity of at least one benzamil-sensitive protein, namely the epithelial sodium channel. 5. Characterizing the BSPs involved in the pathogenesis of hypertension may provide a novel clinical target. Further studies are necessary to determine which BSPs are involved and where, in the nervous system, they are located.

Plasma catecholamines and essential hypertension. An analytical review

Of 78 comparative studies of plasma catecholamines in patients with essential hypertension and in normotensive controls, most reported higher catecholamine levels in the hypertensives, although only about 40% of the studies were positive (reporting statistically significant hypertensive-normotensive differences). Although there was dramatic variability in catecholamine values within and across studies, virtually all studies of norepinephrine in young, consistently hypertensive patients were positive. The likelihood that a study was positive with respect to norepinephrine was independent of the likelihood with respect to epinephrine, so that total catecholamine values, or else the sum of norepinephrine plus epinephrine, differentiated hypertensives from normotensives to a greater extent than levels of either substance alone. The preponderance of literature on the subject supports the hypothesis that increased plasma catecholamine concentrations occur in some patients with essential hypertension. Elevated plasma norepinephrine in relatively young, established hypertensive patients is consistent with a pathophysiologic role for increased sympathetic neural activity in this subgroup.

A sensitive radioenzymatic assay for adrenaline and noradrenaline in plasma

1 An existing radioenzymatic assay for plasma catecholamines using catechol-o-methyl transferase and [3H]-S-adenosyl-methionine has been modified resulting in a more sensitive assay for the measurement of plasma adrenaline and noradrenaline. 2 The lower limit of sensitivity for this method is 25 pg for adrenaline and 30 pg for noradrenaline/ml of plasma. 3 Resting supine (60 min) plasma adrenaline concentration was (mean +/- s.d.) 124 +/- 76 pg/ml(n=11) in males and 130 +/- 71 pg/ml (n=7) in females; plasma noradrenaline concentrations were respectively 444 +/- 129 pg/ml and 550 +/- 87 pg/ml. 4 The changes in plasma catecholamine concentrations in response to 40 degrees head-up tilt have been determined in a group of healthy normal subjects and have been shown to be related to changes in blood pressure and heart rate.