Role of cardiopulmonary mechanoreceptors in ADH release in normal humans

Hyponatremia accompanying volatile hypertension caused by baroreflex failure after neck surgery: case report and literature review

: The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the most common cause of euvolemic hyponatremia, and many disorders have been associated with it. Baroreflex failure is a rare disorder characterized by extreme blood pressure (BP) fluctuations, most frequently caused by neck or head trauma and irradiation. We report a case of a 48-year-old patient referred to our department for asymptomatic hyponatremia and volatile hypertension. His past medical history included nasopharyngeal carcinoma treated with surgery and bilateral neck radiation. Following the diagnostic algorithm for hyponatremia, the diagnosis of SIADH was made. Ambulatory BP monitoring revealed highly variable BP; extensive autonomic nervous system function testing suggested baroreflex-cardiovagal failure. We propose the hypothesis that not only labile hypertension because of baroreflex failure but also hyponatremia can develop as a late consequence of neck trauma and irradiation.

Age-dependent impairment of the erythropoietin response to reduced central venous pressure in HFpEF patients

Despite growing research interest in the pathophysiology of heart failure with preserved ejection fraction (HFpEF), it remains unknown whether central hemodynamic alterations inherently present in this condition do affect blood pressure and blood volume (BV) regulation. The present study sought to determine hemodynamic and endocrine responses to prolonged orthostatic stress in HFpEF patients. Central venous pressure (CVP) assessed via the internal jugular vein (IJV) aspect ratio with ultrasonography, arterial pressure and heart rate were determined at supine rest and during 2 hours of moderate (25-30°) head-up tilt (HUT) in 18 stable HFpEF patients (71.2 ± 7.3 years), 14 elderly (EC), and 10 young (YC) healthy controls. Parallel endocrine measurements comprised main BV-regulating hormones: pro-atrial natriuretic peptide, copeptin, aldosterone, and erythropoietin (EPO). At supine rest, the IJV aspect ratio was higher (>30%) in HFpEF patients compared with EC and YC, while mean arterial pressure was elevated in HFpEF patients (98.0 ± 13.1 mm Hg) and EC (95.6 ± 8.3 mm Hg) versus YC (87.3 ± 5.0 mm Hg) (P < 0.05). HUT increased heart rate (+10%) and reduced the IJV aspect ratio (-52%), with similar hemodynamic effects in all groups (P for interaction ≥ 0.322). The analysis of endocrine responses to HUT revealed a group×time interaction for circulating EPO, which was increased in YC (+10%) but remained unaltered in HFpEF patients and EC. The EPO response to a given reduction in CVP is similarly impaired in HFpEF patients and elderly controls, suggesting an age-dependent dissociation of EPO production from hemodynamic regulation in the HFpEF condition.

Reduction in central venous pressure enhances erythropoietin synthesis: role of volume-regulating hormones

AIMS

Erythropoiesis is a tightly controlled biological event, but its regulation under non-hypoxic conditions, however, remains unresolved. We examined whether acute changes in central venous blood pressure (CVP) elicited by whole-body tilting affect erythropoietin (EPO) concentration according to volume-regulating hormones.

METHODS

Plasma EPO, angiotensin II (ANGII), aldosterone, pro-atrial natriuretic peptide (proANP) and copeptin concentrations were measured at supine rest and up to 3 h during 30° head-up (HUT) and head-down tilt (HDT) in ten healthy male volunteers. Plasma albumin concentration was used to correct for changes in plasma volume and CVP was estimated through the internal jugular vein (IJV) aspect ratio with ultrasonography.

RESULTS

From supine rest, the IJV aspect ratio was decreased and increased throughout HUT and HDT respectively. Plasma EPO concentration increased during HUT (13%; P = 0.001, P for linear component = 0.017), independent of changes in albumin concentration. Moreover, ANGII and copeptin concentrations increased during HUT, while proANP decreased. The increase in EPO concentration during HUT disappeared when adjusted for changes in copeptin. During HDT, EPO, ANGII and copeptin concentrations remained unaffected while proANP increased. In regression analyses, EPO was positively associated with copeptin (β = 0.55; 95% CI = 0.18, 0.93; P = 0.004) irrespective of changes in other hormones and albumin concentration.

CONCLUSION

Reduction in CVP prompts an increase in plasma EPO concentration independent of hemoconcentration and hence suggests CVP per se as an acute regulator of EPO synthesis. This effect may be explained by changes in volume-regulating hormones.

Arterial stiffness is strongly and negatively associated with the total volume of red blood cells

BACKGROUND

Erythropoiesis is partly regulated through classic feedback pathways that govern blood volume (BV) as sensed by veno-atrial but also arterial stretch receptors. Hence, the total volume of red blood cells (RBCV) could be associated with arterial stiffness (AS), although such hypothesis has not yet been tested. Therefore, we sought to investigate the association of AS with hematological variables including RBCV.

METHODS

Fourteen healthy physically active individuals volunteered for the study (age=23±2). RBCV, plasma volume (PV), and BV were calculated from measures of hematocrit and total hemoglobin mass (Hbmass) determined by CO-rebreathing. Carotid compliance with ultrasonography and carotid-ankle pulse wave velocity (PWV) were determined at rest and immediately after a maximal exercise test. The rationale for assessment of AS after exercise derives from the potential marked role of AS in the regulation of erythropoiesis in the setting of reduced central venous pressure.

RESULTS

At rest, carotid compliance was positively associated with Hbmass, RBCV, BV, but not PV, with coefficients of determination (R(2)) ranging from 0.39 to 0.57. Following exercise, closer positive associations were observed between carotid compliance and Hbmass, RBCV, or BV. Moreover, carotid-ankle PWV was negatively associated with all hematological variables after exercise except for PV, with R(2) ranging from 0.49 to 0.75. Similar results were observed when adjusted by body weight.

CONCLUSIONS

AS is strongly and inversely associated with RBCV in healthy individuals. These findings suggest that AS may adversely intercede in the regulation of erythropoiesis through the alteration of mechanisms that control BV.

Neurohormonal interactions on the renal oxygen delivery and consumption in haemorrhagic shock-induced acute kidney injury

Haemorrhagic shock is a common cause of acute kidney injury (AKI), which is a major risk factor for developing chronic kidney disease. The mechanism is superficially straightforward. An arterial pressure below the kidney's autoregulatory region leads to a direct reduction in filtration pressure and perfusion, which in turn cause renal failure with reduced glomerular filtration rate and AKI because of hypoxia. However, the kidney's situation is further worsened by the hormonal and neural reactions to reduced perfusion pressure. There are three major systems working to maintain arterial pressure in shock: sympathetic signalling, the renin-angiotensin system and vasopressin. These work to retain electrolytes and water and to increase peripheral resistance and cardiac output. In the kidney, the increased electrolyte reabsorption consumes oxygen. At the same time, at the signalling level seen in shock, all of these hormones reduce renal perfusion and thereby oxygen delivery. This creates an exaggerated hypoxic situation that is liable to worsen the AKI. The present review will examine this mechanistic background and identify a number of areas that require further studies. At this time, the ideal treatment of haemorrhagic shock appears to be slow fluid resuscitation, possibly with hyperosmolar sodium, low chloride and no artificial colloids. From the standpoint of the kidney, renin-angiotensin system inhibitors appear fruitful for further study.

[Regulation of blood volume during weightlessness simulation of long duration]

To study the effects of microgravity on the mechanisms involved in the regulation of body hydrous status, total body water (TBW), plasma volume (PV), and its main regulating hormones (plasma renin, aldosterone, atrial natriuretic peptide (ANP), anti-diuretic hormone (ADH)) were determined, by isotopic dilution, Dill and Costill's formula, and radio-immunologic dosages, in 9 male subjects submitted to a 90-d head-down bed rest (HDBR). ADH was determined in 24 h urinary collection as well as osmolality, sodium, and potassium. Body mass decreased (-2.8 +/- 0.8 kg) as well as TBW(-7.2% +/- 0.9%, i.e., -2.6 +/- 0.7 kg) and PV (-4.7% +/- 1.8%). Renin and aldosterone were enhanced (+109.0% +/- 15.4% and +87.2% +/- 38.9%, respectively). Simultaneously, we observed a decrease in ANP (-33.2% +/- 20.4%). Other variables, including ADH, were not affected by HDBR. Body mass and TBW decrease (and consequently lean body mass) are associated with muscle atrophy. Renin, aldostrerone, and ANP modifications are well explained by the decrease in PV, which was not enough to induce ADH changes. It suggests that in man, the main regulatory factor for ADH secretion is osmolality, when PV is modestly and progressively decreased without arterial pressure modification, which was the case in the present protocol.

Single-unit activity of paraventricular nucleus neurons in response to intero- and exteroceptive stressors in conscious, freely moving rats

Extracellular recordings of 114 neurons in the hypothalamic paraventricular nucleus (PVN) of conscious, freely moving male rats were performed using a movable electrode system. Single-unit activities were examined for their spontaneous firing patterns and responses to intero- and exteroceptive stressors, including disturbance in arterial blood pressure, water deprivation, air-jet stimulation, and systemic administration of cholecystokinin-8 (CCK). PVN neurons were assigned to one of two groups on the basis of their spontaneous firing patterns: phasic (n=29) and non-phasic (n=85). Intravenous (i.v.) administration of phenylephrine (8 microg/kg) resulted in the inhibition of a greater percentage of phasic-type (88.9%; 24/27) than non-phasic-type neurons (14.9%; 11/74). Most phasic-type neurons showed excitation in response to i.v. administration of sodium nitroprusside (20 microg/kg, 66.7%; 18/27) and water deprivation (15 h, 77.8%; 7/9) when compared to non-phasic-type neurons. Conversely, a greater number of non-phasic-type neurons showed excitation in response to air-jet stimulation (5 l/min, 10 s, 29.0%; 20/69) and to i.v. administration of CCK (5 microg/kg, 24.5%; 11/45) when compared to phasic-type neurons. However, most non-phasic-type neurons that demonstrated excitation in response to i.v. administration of CCK (88.9%; 8/9) did not respond to air-jet stimulation. The present study indicated that phasically firing neurons recorded from the PVN in conscious, freely moving rats are putative vasopressin-secreting neurons on the basis of their responses to intero- and exteroceptive stressors. These data contribute to our understanding of local neural mechanisms within the PVN that are responsible for stress responses in conscious rats.

Effect of heat stress on muscle sympathetic nerve activity in humans

To elucidate the effect of heat stress on the sympathetic nervous system, we evaluated changes in muscle sympathetic nerve activity (MSNA), plasma arginine vasopressin (AVP), tympanic temperature, skin blood flow, cardiac output, mean blood pressure, and heart rate in 9 subjects in response to acute heat stress induced by raising the ambient temperature from 29 to 34 degrees C and then to 40 degrees C. With the heat exposure, MSNA was significantly increased with a significant increase in tympanic temperature. Skin blood flow and heart rate were also significantly increased, while mean blood pressure tended to decline and cardiac output tended to increase. The combination of the increased MSNA and skin blood flow may have caused the redistribution of the circulatory blood volume from the muscles to the skin, facilitating convection heat loss. The increases in MSNA counteracted the lowered blood pressure during heat exposure. Thus, the increased MSNA may play an important role both in thermoregulation and in the maintenance of blood pressure against heat stress.

Antidiuretic hormone and atrial natriuretic peptide during lower body negative or positive pressure in hypertensive patients with and without left ventricular hypertrophy

Aim of the study was to evaluate the effect of cardiopulmonary receptors activation and deactivation on antidiuretic hormone (ADH) and atrial natriuretic peptide (ANP) incretion in hypertensive and normotensive subjects. Twenty-one male subjects, 7 normotensives and 14 mild hypertensives, 7 without and 7 with left ventricular hypertrophy (LVH) were admitted to the study. Each subject underwent selective loading and unloading of cardiopulmonary receptors, by application of a positive (LBPP) or negative (LBNP) pressure to the lower body. Blood samples were taken for measurement of ANP, ADH, PRA, immunoreactive renin, aldosterone, noradrenaline and adrenaline. ADH plasma concentration increased during cardiopulmonary receptors inhibition, but this increase became statistically significant (p less than 0.05) at a step of LBNP (-40 mm Hg), in which an involvement of the sinoaortic receptors cannot be excluded. ANP plasma levels increased progressively during LBPP (p less than 0.05 at least). These changes were significantly reduced in hypertensive patients with LVH.

Volume homeostasis and osmoregulation in human pregnancy

This chapter reviews alterations in volume and sodium homeostasis and osmoregulation during human pregnancy. Pregnant women undergo extracellular and plasma volume increases of 50-70%, and these changes accompany marked cumulative sodium retention shared by both mother and fetus. Pregnancy alters several factors with opposing effects on renal salt handling; however, mechanisms by which gestational sodium accumulation and volume expansion are achieved remain obscure. Furthermore, despite substantial increases in absolute blood volume, considerable uncertainty exists as to how this volume is sensed, particularly in late pregnancy when a rapid increase in volume is associated with decreases in peripheral resistance and blood pressure. Attempts to assess 'effective' intravascular volume suggest that pregnant women sense their volume as normal. Osmoregulation is also changed. Body tonicity and the osmotic thresholds for AVP release and thirst decrease by about 10 mosm/kg. The mechanisms responsible for the osmoregulatory changes are obscure. Haemodynamic stimuli such as decrements in blood pressure and of 'effective circulating volume' do not seem to account for them. Of the many increments in hormone levels known to accompany gestation, only hCG has so far been implicated in these changes. Pregnant women experience three- to fourfold increments in AVP disposal rates between early and mid pregnancy; this may be caused by the striking rise in circulating cystine-aminopeptidase (vasopressinase) which also occurs during this period. The increments in MCR may be one reason why the hormonal response to a given osmotic stimulus appears to decrease in late pregnancy. All these alterations permit speculation on the manner in which the decrease in Posm occurs and is maintained within narrow limits. Lowering the osmotic threshold to drink stimulates a rise in water intake and dilution of body fluids. Since AVP release is not suppressed at the usual level of hypotonicity, AVP continues to circulate at levels sufficient to permit water retention. Posm continues to decline until it decreases below the new osmotic thirst threshold, when a new steady state is established. At this point water turnover, too, resembles that in the non-pregnant state. The change in MCR and the marked increment in plasma vasopressinase may explain certain observations regarding disordered water metabolism during late pregnancy. These are the transient DI syndromes due either to subclinical hypothalamic disease or to a disorder peculiar to pregnancy which is AVP-resistant but dDAVP-responsive; the latter analogue resists degradation by vasopressinase.

Interaction between renin and the autonomic nervous system in hypertension

The abnormal distribution of plasma renin values described in established essential hypertension are also found in patients with very early, borderline hypertension. In established hypertension, renin values have been used to draw inferences about the pathophysiology of blood pressure elevation. Within this concept, the low-renin state is considered a volume-dependent (volume expanded) form of hypertension. The high-renin state is viewed as high-resistance hypertension caused by a renin-dependent vasoconstriction. However, the pathophysiology of high- and low-renin borderline hypertension does not follow the prediction from the volume-vasoconstriction theory. The high-renin state is often associated with an increase in cardiac output and normal values of vascular resistance. Even when the cardiac output is normal and the total peripheral resistance is elevated in high renin, the vasoconstriction is not renin-angiotensin dependent. The high-renin borderline and mild hypertension is a state of generalized, increased, sympathetic drive to the heart, blood vessels, and kidneys. After the influence of the autonomic nervous system is removed by pharmacologic blockade, blood pressure in patients with high-renin values becomes normal. To the contrary, pharmacologic antagonization of angiotensin II with a converting enzyme inhibitor does not lead to normal blood pressure values in patients with high-renin. Patients with borderline hypertension with low renin have normal plasma and blood volume values. However, because of decreased compliance of the peripheral capacitance space, the blood volume is shifted from the peripheral to the central (cardiopulmonary) portion of the circulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in vasopressin regulation in Alzheimer's disease

A decreased concentration of vasopressin (AVP) in the plasma of patients with Alzheimer's disease has been shown recently and suggests damage to hypothalamic neurosecretory cells. To verify this, osmolar and hypotension (sodium nitroprusside) stimulations on AVP release were applied. The effect of metoclopramide, a powerful stimulator of AVP, was also assessed. Patients with Alzheimer's disease released AVP normally after hypotension. However, AVP response to osmotic stimulation was altered in eight out of 10 patients, owing to low osmoreceptor sensitivity and/or high threshold. Metoclopramide increased AVP in controls but not in patients. Normal AVP response to hypotension in patients with Alzheimer's disease makes it unlikely that there is a significant anatomical loss or damage of hypothalamic neurosecretory cells. Alterations in osmoreceptor function and AVP unresponsiveness to metoclopramide point to damage in the control of AVP release in Alzheimer's disease.

Impaired control of vasopressin release in hypertensive subjects with cardiac hypertrophy

The effects of graded lower body negative pressure (-10 and -40 mm Hg) on vascular resistance and plasma vasopressin, norepinephrine, and renin activity were assessed in seven hypertensive subjects with left ventricular hypertrophy and seven sex-matched and age-matched normotensive subjects. In both groups increasing levels of lower body negative pressure induced a progressive decrease in right atrial pressure and an increase in vascular resistance. In normal subjects plasma vasopressin, norepinephrine, and renin activity were progressively raised, whereas only the higher level of stimulation increased plasma renin activity, norepinephrine, and vasopressin in hypertensive subjects. Propranolol induced a significant increase in plasma vasopressin in normal subjects (from 1.3 +/- 0.1 to 2.0 +/- 0.1 pg/ml; p less than 0.05) but not in hypertensive subjects. In this latter condition -10 mm Hg lower body negative pressure failed to increase plasma vasopressin, norepinephrine, and renin activity in normal subjects. Propranolol abolished the change in plasma renin activity in both groups, reduced the increase in vascular resistance induced by -40 mm Hg lower body negative pressure in normotensive subjects, but did not modify the rise in vasopressin elicited by this stimulus in normal subjects or the humoral and hemodynamic reflex responses evoked in hypertensive subjects. These results suggest that cardiopulmonary receptors are involved in the control of vasopressin release in normal subjects, whereas in hypertensive subjects with left ventricular hypertrophy, this control is altered because of an impaired function of cardiopulmonary receptors.

Hormonal responses to cardiac tamponade: inhibition of release of atrial natriuretic factor despite elevation of atrial pressures

Atrial distension, rather than change in intra-atrial pressure, has been suggested as a principal mediator of release of atrial natriuretic factor (ANF). During cardiac tamponade, atrial pressures rise whereas transmural pressures and atrial stretch may not be affected. The roles of atrial pressure and atrial distension were investigated in six open-chest dogs subjected to cardiac tamponade and rapid volume expansion as disparate means of affecting intra-atrial pressures and atrial stretch. Hemodynamic measurements, immunoreactive ANF (ir-ANF), plasma renin activity, antidiuretic hormone, epinephrine, and norepinephrine were monitored before, during, and after three interventions: (1) tamponade, (2) rapid volume loading followed by tamponade, and (3) volume loading during tamponade. Volume expansion increased right atrial pressure and caused a significant rise in ir-ANF. Elevations of right atrial pressure caused by tamponade were comparable to those induced by volume infusion, but an increase in ir-ANF was not elicited during tamponade, and the ir-ANF response to volume loading was abolished when performed during tamponade. The relation between the change in ANF concentration and change in right atrial pressure were highly significant in the absence of tamponade, when atrial stretch was freely responsive to volume expansion (r = .73, p less than .0001), but not when stretch was inhibited (r = -.16, p = NS). These observations underscore the importance of considering the modulating effects of atrial compliance, transmural pressure, and atrial stretch on the relation between atrial pressures and ANF release.

Hormonal responses and blood pressure maintenance in normal and hypertensive subjects during acute blood loss

Blood pressure (BP) and plasma indices of three major pressure control systems--plasma norepinephrine and epinephrine, plasma renin activity (PRA), and plasma arginine vasopressin--were measured simultaneously in 12 normal and 15 mildly essential hypertensive subjects before and after removal of 480 ml of blood by phlebotomy, to determine if there were differences in the compensatory response to acute blood loss. Responses to postural stress (change from supine to sitting position) following phlebotomy were also compared in a second group of subjects. Before phlebotomy, supine plasma hormone levels did not differ in the two groups. After phlebotomy, both groups exhibited only slight decreases (5 mm Hg) in systolic BP and a transient rise in heart rate. Only plasma norepinephrine increased significantly in both groups (35% above control in normal and 43% in hypertensive subjects). Similar results were obtained in a second group of normal and hypertensive subjects, who were also subjected to a 10-minute postural challenge after phlebotomy. After 10 minutes in a sitting position, BP in these subjects remained unchanged but heart rate and plasma norepinephrine increased further to levels almost twice that produced by phlebotomy alone. Plasma epinephrine levels and PRA also increased with this additional stress, but plasma vasopressin remained unchanged. Changes in BP, heart rate, plasma norepinephrine and epinephrine, and PRA did not differ significantly between the two groups. These data indicate that hypertensive subjects are as capable as normal subjects of maintaining BP when subjected to standard phlebotomy, the sympathetic nervous system appears to be the predominant pressor mechanism activated following an acute, nonhypotensive blood loss in both groups of subjects,(ABSTRACT TRUNCATED AT 250 WORDS)