Influence of sodium intake on the cardiovascular and renal effects of brain mineralocorticoid receptor blockade in normotensive rats

Extracellular Na(+) levels regulate formation and activity of the NaX/alpha1-Na(+)/K(+)-ATPase complex in neuronal cells

MnPO neurons play a critical role in hydromineral homeostasis regulation by acting as sensors of extracellular sodium concentration ([Na⁺]_out). The mechanism underlying Na⁺-sensing involves Na⁺-flow through the Na_X channel, directly regulated by the Na⁺/K⁺-ATPase α1-isoform which controls Na⁺-influx by modulating channel permeability. Together, these two partners form a complex involved in the regulation of intracellular sodium ([Na⁺]_in). Here we aim to determine whether environmental changes in Na⁺ could actively modulate the Na_X/Na⁺/K⁺-ATPase complex activity. We investigated the complex activity using patch-clamp recordings from rat MnPO neurons and Neuro2a cells. When the rats were fed with a high-salt-diet, or the [Na⁺] in the culture medium was increased, the activity of the complex was up-regulated. In contrast, drop in environmental [Na⁺] decreased the activity of the complex. Interestingly under hypernatremic condition, the colocalization rate and protein level of both partners were up-regulated. Under hyponatremic condition, only Na_X protein expression was increased and the level of Na_X/Na⁺/K⁺-ATPase remained unaltered. This unbalance between Na_X and Na⁺/K⁺-ATPase pump proportion would induce a bigger portion of Na⁺/K⁺-ATPase-control-free Na_X channel. Thus, we suggest that hypernatremic environment increases Na_X/Na⁺/K⁺-ATPase α1-isoform activity by increasing the number of both partners and their colocalization rate, whereas hyponatremic environment down-regulates complex activity via a decrease in the relative number of Na_X channels controlled by the pump.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Aldosterone and mineralocorticoid receptors in the cardiovascular system

Aldosterone is currently thought to exert its physiologic effects by activating epithelial mineralocorticoid receptors, and its pathologic effects on the cardiovascular system via mineralocorticoid receptors in the heart and blood vessels. Recent studies have extended this understanding to include a reevaluation of the roles of aldosterone and mineralocorticoid receptor activation in blood pressure control; the rapid, nongenomic effects of aldosterone; the role of cortisol as a mineralocorticoid receptor agonist under conditions of redox change/tissue damage/reactive oxygen species generation; the growing consensus that primary aldosteronism accounts for approximately 10% of all essential hypertension; recent new insights into the cardioprotective role of spironolactone; and the development of third- and fourth-generation mineralocorticoid receptor antagonists for use in cardiovascular and other inflammatory disease. These findings on aldosterone action and mineralocorticoid receptor blockade are analyzed in the context of the prevention and treatment of cardiovascular disease.

Mechanisms of hypertension in the cardiometabolic syndrome

Arterial hypertension is often part of a constellation of anthropometric and metabolic abnormalities that occur simultaneously to a higher degree than would be expected by chance alone, supporting the existence of a discrete disorder, the so-called metabolic syndrome. It is the result of interactions among a large number of interconnected mechanisms, which eventually lead to both an increase in cardiovascular and renal risk, and the development of diabetes. Mechanisms involved in the metabolic syndrome are obesity, insulin resistance, and a constellation of independent factors, which include molecules of hepatic, vascular, and immunologic origin with pro-inflammatory properties. At each of these key points are interactions of demographics, lifestyle, genetic factors, and environmental fetal programming. Superimposing upon these are infections or chronic exposure or both to certain drugs that can also make their contribution. Skeletal muscle and the liver, not adipose tissue, are the two key insulin-response tissues involved in maintaining glucose balance, although abnormal insulin action in the adipocytes also plays a role in development of the syndrome. Factors commonly associated with and partly dependent on obesity, insulin resistance, such as overactivity of the sympathetic, stimulation of the renin-angiotensin-aldosterone systems, abnormal renal sodium handling, endothelial dysfunction, and large vessels' alterations, may play a key role in the blood pressure elevation of the syndrome.

Aldosterone in the brain

Pharmacological and physiological phenomena suggest that cells somewhere inside the central nervous system are responsive to aldosterone. Here, we present the fundamental physiological limitations for aldosterone action in the brain, including its limited blood-brain barrier penetration and its substantial competition from glucocorticoids. Recently, a small group of neurons with unusual sensitivity to circulating aldosterone were identified in the nucleus of the solitary tract. We review the discovery and characterization of these neurons, which express the enzyme 11beta-hydroxysteroid dehydrogenase type 2, and consider alternative proposals regarding sites and mechanisms for mineralocorticoid action within the brain.

Minireview: aldosterone and the cardiovascular system: genomic and nongenomic effects

There is clear evidence for rapid nongenomic effects of aldosterone in the cardiovascular system in addition to its well characterized effects of unidirectional transepithelial sodium transport. Many of these effects are mediated by the classical mineralocorticoid receptors, although others may be exerted independently. Given that mineralocorticoid receptors are largely constitutively occupied but not activated by physiological glucocorticoids, effects of aldosterone administered in vitro or in vivo may or may not equate with true physiological mineralocorticoid roles. In many systems (e.g. blood pressure regulation and cardiac fibrosis), the time course of effects is such that it is not possible, and perhaps not important, to distinguish between rapid nongenomic and classical genomic effects in the context of homeostatic physiology.

Abnormal renovascular parathyroid hormone-1 receptor in hypertension: Primary defect or secondary to angiotensin ii type 1 receptor activation?

We previously reported that PTHrP-induced renal vasodilation is impaired in mature spontaneously hypertensive rats (SHR) through down-regulation of the type 1 PTH/PTHrP receptor (PTH1R), a feature that contributes to the high renal vascular resistance in SHR. Here we asked whether this defect represents a prime determinant in genetic hypertension or whether it is secondary to angiotensin II (Ang II) and/or the mechanical forces exerted on the vascular wall. We found that the treatment of SHR with established hypertension by the Ang II type 1 receptor antagonist, losartan, reversed the down-regulation of PTH1R expression in intrarenal small arteries and restored PTHrP-induced vasodilation in ex vivo perfused kidneys. In contrast, the PTH1R deregulation was not found in intrarenal arteries isolated from prehypertensive SHR. Moreover, this defect, which is not seen in extrarenal vessels (aorta, mesenteric arteries) from mature SHR appeared kidney specific in accordance with the acknowledged enrichment of interstitial Ang II in this organ and its enhancement in SHR. In deoxycorticosterone-acetate-salt rats, an Ang II-independent model of hypertension, renovascular PTH1R expression and related vasodilation were not altered. In SHR-derived renovascular smooth muscle cells (RvSMCs), the PTH1R was spontaneously down-regulated and its transcript destabilized, compared with Wistar RvSMCs, both effects being antagonized by losartan. Exogenous Ang II elicited down-regulation of PTH1R mRNA in RvSMCs from Wistar rats. Together, these data demonstrate that Ang II acts via the Ang II type 1 receptor to destabilize PTH1R mRNA in the renal vessel in the SHR model of genetic hypertension. This process is kidney specific and independent from blood pressure increase.