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

Abstract

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.

Figure 1. Central Pathways

CVOs (AP, OVLT, SFO) have a weakened blood-brain barrier due to their fenestrated capillaries. These CNS sites may be important for sensing circulating hormones and/or plasma osmolality [14]. The CVOs send projections to integrative and regulatory sites, such as the MnPO (involved in salt appetite and regulating drinking behavior) and the PVN (critical for defense of homeostasis; e.g., [19]). These pathways continue onto pre-sympathetic areas, such as the RVLM and the IML of the spinal cord [35], which allows for modulation of sympathetic activity and arterial pressure [95, 96].

Figure 2. Model of the Role of BSPs in the Pathogenesis of DOCA-Salt Hypertension

We propose that benzamil-sensitive ion channels play a role in DOCA-salt hypertension. Following DOCA treatment and subsequent increased MR activity, there would be an increased localization of BSPs to the membrane of HSD2 expressing cells. We believe that this increase in membrane-localized BSPs is likely to occur at sodium-sensitive sites, such as the OVLT, because BSPs seem to play a role both in chronic models such as DOCA-salt hypertension, as well as in acute sensitivity to CSF sodium levels. Membrane localization of BSPs would increase neural activity in the presence of high sodium levels; this in turn, could modulate activity at downstream sites, and thereby regulate SNA and increases in arterial pressure.

Figure 3. Acute Pressor Response to ICV Saline is Blocked by Benzamil Hydrochloride

Pentobarbital anesthetized rats (n=6) were instrumented for direct measurement of mean arterial pressure (MAP) via a femoral arterial catheter. MAP was measured by connecting the catheter to a volume displacement pressure transducer coupled to a Grass polygraph. Under stereotaxic guidance, the tip of a 23-gauge stainless steel cannula was placed in the lateral cerebral ventricle. After obtaining baseline MAP for 5 minutes, 50 µL of 1.5% saline was injected over 30 seconds. MAP was recorded for an additional 20 minutes post injection. 20µL of 20nM benzamil hydrochloride (e.g., 0.4 picomoles) was then delivered ICV into the lateral ventricle. A second injection of saline was administered 5 minutes after the benzamil injection, and MAP recorded for 20 minutes. Arrow indicates time of ICV hypertonic saline injection.