Effect of carbidopa on the excretion of sodium, dopamine, and ouabain-like substance in the rat

Salt, Na+,K+-ATPase and hypertension

Chronic hypertension is characterized by a persistent increase in vascular tone. Sodium-rich diets promote hypertension; however, the underlying molecular mechanisms are not fully understood. Variations in the sodium content of the diet, through hormonal mediators such as dopamine and angiotensin II, modulate renal tubule Na(+),K(+)-ATPase activity. Stimulation of Na(+),K(+)-ATPase activity increases sodium transport across the renal proximal tubule epithelia, promoting Na(+) retention, whereas inhibited Na(+),K(+)-ATPase activity decreases sodium transport, and thereby natriuresis. Diets rich in sodium also enhance the release of adrenal endogenous ouabain-like compounds (OLC), which inhibit Na(+),K(+)-ATPase activity, resulting in increased intracellular Na(+) and Ca(2+) concentrations in vascular smooth muscle cells, thus increasing the vascular tone, with a corresponding increase in blood pressure. The mechanisms by which these homeostatic processes are integrated in response to salt intake are complex and not completely elucidated. However, recent scientific findings provide new insights that may offer additional avenues to further explore molecular mechanisms related to normal physiology and pathophysiology of various forms of hypertension (i.e. salt-induced). Consequently, new strategies for the development of improved therapeutics and medical management of hypertension are anticipated.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

Endogenous digitalis: pathophysiologic roles and therapeutic applications

Endogenous digitalis-like factors, also called cardiotonic steroids, have been thought for nearly half a century to have important roles in health and disease. The endogenous cardiotonic steroids ouabain and marinobufagenin have been identified in humans, and an effector mechanism has been delineated by which these hormones signal through the sodium/potassium-transporting ATPase. These findings have increased interest in this field substantially. Although cardiotonic steroids were first considered important in the regulation of renal sodium transport and arterial pressure, subsequent work has implicated these hormones in the control of cell growth, apoptosis and fibrosis, among other processes. This Review focuses on the role of endogenous cardiotonic steroids in the pathophysiology of essential hypertension, congestive heart failure, end-stage renal disease and pre-eclampsia. We also discuss potential therapeutic strategies that have emerged as a result of the increased understanding of the regulation and actions of cardiotonic steroids.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Ouabain-like factor

Accumulated evidence has suggested that several sodium pump inhibitors, similar to cardiotonic steroids, are present in the human body. Ouabain-like factor, the most appealing candidate, has been found to be increased with high sodium intake and hypervolaemia, and in essential hypertension, mineralocorticoid hypertension, and pregnancy-induced hypertension. Furthermore, blocking the action of ouabain-like factor with digibind or a novel anti-ouabain agent lowers blood pressure in several models of hypertension. Several important questions remain, however, before it can be concluded that ouabain-like factor is indeed involved in the regulation of sodium homeostasis and blood pressure.