Are hypertensive effects of aldosterone, angiotensin, vasopressin, and norepinephrine chronically additive?

Enhanced resistance to permeability transition in interfibrillar cardiac mitochondria in dogs: effects of aging and long-term aldosterone infusion

Functional differences between subsarcolemmal and interfibrillar cardiac mitochondria (SSM and IFM) have been observed with aging and pathological conditions in rodents. Results are contradictory, and there is little information from large animal models. We assessed the respiratory function and resistance to mitochondrial permeability transition (MPT) in SSM and IFM from healthy young (1 yr) and old (8 yr) female beagles and in old beagles with hypertension and left ventricular (LV) wall thickening induced by 16 wk of aldosterone infusion. MPT was assessed in SSM and IFM by Ca(2+) retention and swelling. Healthy young and old beagles had similar mitochondrial structure, respiratory function, and Ca(2+)-induced MPT within SSM and IFM subpopulations. On the other hand, oxidative capacity and resistance to Ca(2+)-induced MPT were significantly greater in IFM compared with SSM in all groups. Old beagles treated with aldosterone had greater LV wall thickness and worse diastolic filling but normal LV chamber volume and systolic function. Treatment with aldosterone did not alter mitochondrial respiratory function but accelerated Ca(2+)-induced MPT in SSM, but not IFM, compared with healthy old and young beagles. In conclusion, in a large animal model, oxidative capacity and resistance to MPT were greater in IFM than in SSM. Furthermore, aldosterone infusion increased susceptibility to MPT in SSM, but not IFM. Together this suggests that SSM are less resilient to acute stress than IFM in the healthy heart and are more susceptible to the development of pathology with chronic stress.

Recent advances in the understanding of water metabolism in heart failure

Hyponatremia is common in advanced heart failure and relates to the severity of the disease. Non-osmotic arginine vasopressin (AVP) release and biosynthesis have been shown to be increased during chronic cardiac failure (CHF) and baroreceptors pathways have been demonstrated to play a major role in this non-osmotic stimulation of AVP. Decreased cardiac output unloads the baroreceptors and activates the sympathetic nervous system, thus stimulating AVP through a separate pathway which overrides the osmotic pathway. Besides sympathetic nervous system activation, neurohumoral peptides, such as angiotensin II, endothelins, natriuretic peptides and prostaglandins, could also participate in the non-osmotic AVP activation. The vasoconstrictor effect of AVP has been supported by the decrease systemic vascular resistance during the administration of V1 receptor AVP antagonist in CHF patients. Administration of V2 receptor AVP antagonists corrects the hyponatremia and has been demonstrated to improve survival in animal models of heart failure. Preliminary data in humans with CHF also demonstrate urinary dilution and correction of hyponatremia with orally active non-peptide V2 receptor antagonists. Finally, upregulation of the AVP-regulated water channels, aquaporin-2 (AQP2), located in the collecting duct cells has been shown in experimental heart failure. This AQP2 upregulation can be entirely suppressed by V2 receptor AVP antagonists paralleling the correction of the hyponatremia. Thus, non-osmotic release of AVP in CHF upregulates AQP2 water channels, enhances water reabsorption and causes hyponatremia. The V1, and perhaps the V2, receptor activation may also diminish cardiac function.

Endocrinological abnormalities in angiotensinogen-gene knockout mice: studies of hormonal responses to dietary salt loading

OBJECTIVE

Physiological roles of the renin-angiotensin system in maintaining blood pressure and sodium-water balance in angiotensinogen gene-knockout mice were evaluated with special reference to endogenous pressor substances.

METHODS

Angiotensinogen-gene knockout mice and control mice were fed a 0.3 or 4% NaCl diet for 2 weeks. Systolic blood pressure and urinary excretions of electrolytes, creatinine, aldosterone, adrenaline, noradrenaline, dopamine and vasopressin were measured.

RESULTS

About 60% of our angiotensinogen-gene knockout mice did not survive until weaning. These mice presented with hypotension and polyuria. Urinary excretion of aldosterone from such mice was significantly lower (not detected) than that from control mice (2.0+/-0.3 pg/mg creatinine). In contrast, urinary excretion of vasopressin from angiotensinogen-gene knockout mice (0.7+/-0.1 ng/mg creatinine) was greater than that from control mice (0.3+/-0.1 ng/mg creatinine), and those of adrenaline and of noradrenaline were similar for knockout and control mice. After salt loading (a 4% NaCl diet), angiotensinogen-gene knockout mice exhibited a significant increase in systolic blood pressure (from 68.3+/-2.9 to 95.9+/-5.9 mmHg), significant decreases in urinary excretions of adrenaline (from 65+/-8 to 40+/-7 pg/mg creatinine) and noradrenaline (from 467+/-48 to 281+/-41 pg/mg creatinine) and no change in excretion of vasopressin compared with such mice fed a 0.3% NaCl diet

CONCLUSION

The present results with angiotensinogen-gene knockout mice confirm that the renin-angiotensin system plays fundamental roles in maintaining the blood pressure and sodium-water balance. Because the vasopressin and catecholaminergic systems may be altered by lack of angiotensin in angiotensinogen-gene knockout mice, these systems perhaps are not able to restore blood pressure and sodium-water depletion to normal levels in these mice.

Paradoxical hypertension after repair of coarctation of the aorta: a review of its causes

Correction of a coarctation of the aorta, an apparent simple cause of hypertension, paradoxically can provoke two hypertensive responses, one of which is potentially fatal. The first, limited to the first 24 hours, occurs in nearly one half of the patients. This is likely due to the high set of the carotid baroreceptors. The second, which may be associated with abdominal pain and, in some, with necrosis of the small bowel as a result of severe arteritis confined to arteries arising from the aorta below the coarctation, develops in about one half of the first responders. Norepinephrine excretion greatly increases for several days, whereas angiotensin levels are elevated for 3 to 4 days. The hypertension responds to beta-blockers, to arterial smooth muscle relaxants, and to angiotensin converting enzymes. A theory is advanced to explain the second response. It is the adaptation gone awry that ensures adequate flow to exercising muscles below the coarctation, above and beyond that delivered by increasing the systolic pressure. It could be a regionally controlled mechanism similar to the rationing of blood flow in diving mammals.

Role of vasopressin in regulation of sodium excretion

It has been proposed that arginine vasopressin (AVP) contributes to the regulation of renal sodium excretion by direct intrarenal actions, by neural-hormonal interactions, and via secondary effects of fluid volume retention. The present studies were designed to determine the extent to which the natriuretic effects of AVP are secondary to volume expansion. Three groups of dogs were studied: the first was infused with AVP for 2 weeks in amounts that increased plasma levels from 3 to 15 pg/mL, while water intake was maintained constant by intravenous (iv) water infusion. The second group received the same amount of AVP and was permitted to drink ad libitum. The third group was infused with the same amount of AVP, while total body weight and volume were maintained at a constant level by use of an electronically servo-controlled water infusion system. The results showed a large increase in total body weight (+1.5 kg) and arterial pressure (mean arterial pressure (MAP); +40 mm Hg) in dogs receiving a fixed water intake. This was accompanied by a continuing natriuresis over a 2-week period and severe hyponatremia (115 mEq/L). Dogs allowed ad libitum drinking retained much less fluid (+0.5 kg). MAP was not significantly elevated, and natriuresis did not occur in this group, but hyponatremia was observed (130 mEq/L), and plasma renin activity (PRA) was suppressed. Servo-controlled dogs exhibited no change in MAP, plasma sodium, or PRA, and only a small (-15 mEq) natriuresis occurred on day 1 of AVP infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin and arterial pressure regulation. Special lecture

Data from conscious rats, dogs, and humans show that plasma arginine vasopressin (AVP) begins to exert vasoconstrictor activity at concentrations in the same range as those associated with maximum antidiuretic activity. Minimum pressor responses are observed with elevated plasma AVP, due in part to decreases of cardiac output and in part to withdrawal of sympathetic neural tone to various regions of the systemic circulation. These responses appear to some extent to be species-dependent. In conscious dogs, but not in rats, the fall of cardiac output is mediated by AVP stimulation of baroreceptor reflex pathways. Studies in rats indicate that AVP inhibits the sympathetic nervous system by direct action on the central nervous system. No evidence was found for inhibition at peripheral sites such as autonomic ganglia or vascular smooth muscle receptors. Also, AVP plays an important role in the regulation of arterial pressure with blood loss by direct vasoconstriction and by AVP enhancement of the strength of the baroreceptor reflex responses. The role of AVP in the long-term control of arterial pressure and in hypertension remains controversial, but plasma AVP is elevated in many experimental and human forms of hypertension. The link between plasma AVP and hypertension remains unclear because long-term elevation of AVP alone cannot sustain volume expansion or hypertension, and excess AVP does not enhance hypertension produced by sodium-retaining hormones or other vasoconstrictor agents. It appears that AVP plays mainly a permissive role by its fluid-retaining effects in most forms of hypertension. It is also possible that it acts as a central nervous system neural transmitter and modifies autonomic pathways in some forms of hypertension.