Atrial natriuretic peptide binding properties of purified rat glomerular membranes

125I-ANP (3-[125I] iodotyrosyl28) binding studies with purified rat glomerular membranes indicate two types of physiologically relevant hormonal receptors, Types I and II, Kd approximately 5 pM and approximately 2.5 nM, respectively. All preparations were essentially free of capsular and tubular contamination. Binding data indicated that Type I receptors were three times more concentrated than Type II receptors in purified membrane fractions. When purified membranes were cross-linked with 125I-rANP, using disuccinimidyl suberate and separated by SDS-PAGE, approximately 75- and approximately 140-kDa proteins were specifically labeled in a ratio of approximately 3:1, respectively. Thus, in purified renal glomerular membranes, Type I receptors with molecular weight of approximately 75-kDa appeared to predominate and would be detectably saturated at circulating ANP concentrations as low as 15 pg/ml. These findings could account for the exquisite sensitivity of natriuretic response to ANP.

Hypertension and obesity in rats with ventromedial-hypothalamic lesions and low salt intake

The arterial hypertension that frequently coexists with ventromedial hypothalamic (VMH) lesion-induced obesity has been obtained in rats chronically sustained on diets of standard-to-high salt content. Since VMH lesions also compromise the renal circulation and enhance ingestion of salt-loaded diets, the resulting haemodynamic adjustments, including hypertension, have been interpreted as possibly being sodium-dependent. This notion implies that chronically restricting dietary salt would ameliorate the hypertension, whereas the persistence of hypertension under such a regiment would suggest the potential contribution of non-sodium-related factors. This study assessed whether chronic maintenance of a low-salt diet would buffer the hypertensive and haemodynamic consequences of obesity, accompanying electrolytic VMH lesions in male Wistar-Kyoto (WKY) rats, compared with sham-treated controls. Despite the low-salt diet, the results showed that VMH obesity was accompanied by hypertension, elevated plasma norepinephrine, and redistribution of blood flow away from the brain, kidney and splanchnic organs. The compatibility of these results with earlier studies permitting salt ingestion, is difficult to reconcile with the notion that VMH-hypertension is sodium-dependent. Instead, it suggests that the VMH obesity-hypertension syndrome may provide a useful model for clarifying the autonomic consequences of either augmented carbohydrate ingestion of hyper-insulinaemia.

Pathophysiological considerations in left ventricular hypertrophy

Considerable evidence is available to relate development of left ventricular hypertrophy to the maintenance and progression of hypertensive vascular disease and its associated hemodynamic changes that induce an increased left ventricular afterload. However, when critical investigators question in depth the variety of other nonhemodynamic factors that may also be associated with the development (or regression) of left ventricular hypertrophy, they are faced with three disquieting conclusions. First, knowledge of those mechanisms that account for the development and maintenance (or, for that matter, even regression) of left ventricular hypertrophy in hypertension is incomplete. Second, little is known as to how the physical hemodynamic and mechanical factors associated with the pathogenesis of hypertension are translated into the biochemical events associated with the development of cardiac muscle hypertrophy. Finally, to understand these important factors more clearly, the investigator (clinical or experimental) must design studies more precisely to control for the additional complicating, nonhemodynamic factors.

Future directions in the use of echocardiography

The heart may play an active, passive, or incidental role in the pathogenesis of hypertension. Echocardiography probably contributes little to understanding of active mechanisms, although it may provide important information relative to structural and functional adaptive changes associated with development of left ventricular hypertrophy. Moreover, because other clinical conditions frequently coexist with hypertensive heart disease, echocardiography may provide another dimension in the assessment of obesity, coronary heart disease, mitral valve prolapse, idiopathic hypertrophic subaortic stenosis, and asymmetric septal hypertrophy in the overall problem. Critical in this understanding are the subtle changes that occur in the individual patient, reflecting the natural history of the disease or response to its treatment. Since technical problems preclude echocardiographic evaluation in all patients with hypertension, particular care must be exercised in making epidemiologic generalizations.

Potential mechanisms explaining the risk of left ventricular hypertrophy

Major risk factors have been identified that enhance the chances of cardiovascular morbidity and mortality. These include such modifiable factors as hypertension, hyperlipidemia, obesity, diabetes mellitus, smoking and hyperuricemia. Other factors that also increase risk are not modifiable and include advancing age, male gender and black race. The development of left ventricular (LV) hypertrophy imposes another significant risk for increased morbidity and mortality. Development of LV hypertrophy may be produced by hemodynamic as well as nonhemodynamic mechanisms. Included in the latter group are some of the same factors that in and of themselves participate in the production of increased LV mass (i.e., aging, gender and race, obesity, coronary disease, diabetes and the underlying mechanisms that subserve the hypertensive disease). This article discusses the concept, drawn from clinical and experimental studies, that demonstrate that the additional increased risk of LV hypertrophy may be ascribed to loss of reserve cardiac function, accelerated atherosclerosis, development of abnormal cardiac rhythm secondary to ischemia, fibrosis or drug-induced hypokalemia, inherent predisposition to ventricular dysrhythmias and sudden death, risks directly or coincidentally related to associated diseases or perhaps even the paradoxical risk of beneficial antihypertensive therapy.

Is reversal of cardiac hypertrophy a desirable goal of antihypertensive therapy?

Over the past several years, a growing body of information has confirmed and extended our initial concept that nonhemodynamic as well as hemodynamic factors are responsible for the development of left ventricular hypertrophy in hypertension. We reported the dissociation of these factors in the regression of left ventricular mass and hypertrophy with antihypertensive therapy. Several lines of clinical and experimental studies have been pursued to determine whether cardiac performance, myocardial contractility, and reserve are normal with regression of ventricular mass with treatment. Too few studies have been conducted in vivo, and in the conscious state, and at pretreatment pressures to conclude at this time that normal cardiac function and performance is restored or maintained. Until such data are available, we must conclude that although left ventricular hypertrophy confers a risk, in and of itself, to cardiovascular morbidity and mortality, we do not know whether pharmacologic reversal of cardiac hypertrophy is a desirable therapeutic goal. Several years ago we were convinced that a new and impressive body of information emanating mostly from our laboratories strongly indicated a new concept previously unexpressed by others. We suggested that increasing ventricular mass in hypertension (i.e., left ventricular hypertrophy), although dependent in part on arterial pressure and other hemodynamic factors, was also dependent on participation of a number of "nonhemodynamic" mechanisms. Our early findings, supported by associated reports, suggested that in addition to left ventricular afterload, factors including other pressor mechanisms (e.g., adrenergic function and norepinephrine levels, humoral substances, the renopressor system), aging, race, gender, coexisting diseases, pharmacologic agents, and others may also participate.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide decreases circulatory capacitance in areflexic rats

The short-term hemodynamic response to atrial natriuretic peptide appears to be partly mediated by decreased venous return, which could result from increased circulatory capacitance or decreased blood volume. To determine if rat atrial natriuretic peptide 99-126 (0.5 microgram/kg/min IV for 30-70 minutes) dilated capacitance vessels or decreased blood volume, mean circulatory filling pressure (measured during brief circulatory arrest by inflating an intraatrial balloon) and blood volume (51Cr-erythrocytes) were measured in anesthetized rats. Mean circulatory filling pressure, central venous pressure, and blood volume decreased by 0.4 mm Hg, 0.5 mm Hg, and 3.4 ml/kg, respectively. To determine the total circulatory pressure-volume relationship without influence from autonomic reflexes, mean circulatory filling pressure and blood volume were measured in spinal-cord-transected rats before and immediately after infusing or withdrawing 5 ml blood. Atrial natriuretic peptide decreased mean circulatory filling pressure, central venous pressure, and blood volume by 0.9 mm Hg, 1.7 mm Hg, and 8.0 ml/kg, respectively, and displaced the pressure-volume relationship toward the pressure axis by decreasing extrapolated unstressed volume. Similar results were obtained in spinal-cord-transected rats that had initial vascular tone restored to a greater level by norepinephrine infusion. In anephric rats, atrial natriuretic peptide decreased central venous pressure by 0.3 mm Hg and blood volume by 1.6 ml/kg. The results indicate that short-term infusion of atrial natriuretic peptide reduced circulatory capacitance in rats and suggest that this reduction resulted from diminished blood volume due to urinary fluid loss followed by passive vascular recoil and active venoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)

Atriopeptin III does not alter cardiac performance in rats

The effects of atriopeptin III (APIII) on systemic haemodynamics were examined in 12 anaesthetized rats. Five minutes following intravenous injection (i.v.) of 10 micrograms/kg APIII, cardiac output CO, measured by electromagnetic flowmetry, stroke volume and mean arterial pressure (MAP) decreased by 14, 13 and 8% (P less than 0.05), respectively, and total peripheral resistance (TPR) increased by 10% (P less than 0.05). Heart rate (HR) and left ventricular end-diastolic pressure (LVEDP) did not change. In order to examine cardiac performance, whole blood was infused into three groups of 12 rats each receiving either no injection, APIII (10 micrograms/kg i.v.) or APIII (10 micrograms/kg i.v.) plus a continuous infusion of phenylephrine to increase MAP to pre-injection levels. Cardiac performance curves did not differ among the three groups. These data indicate that the immediate decreases in MAP and CO produced in rats by a maximum natriuretic bolus dose of APIII were not mediated by a negative myocardial inotropic effect.

Hemodynamic effects of atrial natriuretic hormone

The atrial natriuretic hormone (ANH) alters cardiovascular function independent of changes in body fluid volume. Most investigators agree that ANH decreases mean arterial pressure (MAP). However, although some investigators have observed a decrease in total peripheral resistance in association with the decrease in MAP, a more frequent observation has been decreased cardiac output (CO). The mechanism whereby ANH decreases CO is unknown, but does not appear to be the result of direct myocardial depression, reductions in intravascular or cardiopulmonary volumes, or venodilation. Alterations in skeletal muscle and splanchnic blood flow have been reported by some but not all investigators. Although increases in renal blood flow have been reported, they are transitory and have not been consistently observed by all researchers. The cardiovascular effects of ANH appear to be influenced not only by the dose, but also by the cardiovascular control mechanisms that operate at the time of ANH administration. Non-renin-dependent hypertensive models exhibit a decrease in MAP associated with decreased CO, whereas in renin-dependent animals this hypotension is associated with a decrease in total peripheral resistance.

Dehydration attenuates the acute natriuretic response to rat atriopeptin III (rAPIII)

The acute natriuretic response to atrial peptides (AP) is highly variable in anesthetized rats, and some rats are unresponsive. To determine if this response to AP was affected by dehydration, we measured hematocrit, plasma volume, and natriuresis (delta UNaV) after intravenous injection of 3 micrograms/kg of rat atriopeptin III (rAPIII) in anesthetized rats deprived of water for 0, 12, 20, 29, 44, and 68 hours. Data were compared with those from rats receiving 1.5 mg/kg furosemide (FU) after 0 and 68 hours without water. There were 10- and 3-fold decreases in delta UNaV following rAPIII and FU injection after 20 and 68 hours without water, respectively. Hematocrit increased and plasma and total blood volumes decreased after 12 hours of dehydration. Plasma volumes and delta UNaV were correlated (r = 0.64, p less than 0.05; r = 0.75, p less than 0.001) in the combined groups receiving rAPIII (n = 30) and FU (n = 10), respectively. These results demonstrate that a relatively short period of water deprivation (WD) and the resulting hemoconcentration in rats decreased their acute natriuretic response to diuretics. Thus, differences in water intake may account for some of the large variation in delta UNaV after exogenous administration of rAPIII.

The effect of prolonged administration of CGS 10078B on systemic and regional haemodynamics in normotensive and spontaneously hypertensive rats

The effects of 3 weeks treatment with CGS 10078B (30 mg/kg orally) on systemic and regional haemodynamics and cardiac mass were studied in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. The significant decrease in mean arterial pressure (MAP) (174 +/- 3 versus 156 +/- 4 mmHg, P less than 0.002) in SHR was associated with a significantly slower heart rate. No significant alteration in systemic haemodynamics was observed in WKY rats. The reduced MAP in SHR was related to the preserved blood flow to the vital organs, and therefore reduced renal and cerebrovascular resistances. Left ventricular mass index was reduced in both rat strains of treated animals. Therefore, the reduced MAP and heart rate in the SHR without haemodynamic changes in the WKY indicates that CGS 10078B was an effective antihypertensive agent that decreased cardiac mass in rats through mechanisms that may be dissociated from their haemodynamic effects.

Baroreflex control of heart rate after immediate and prolonged pressure reduction with urapidil in conscious normotensive and spontaneously hypertensive rats

Baroreflex control of heart rate was studied in conscious normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats before and after acute and prolonged arterial pressure reduction with urapidil, a central and peripheral inhibitor of adrenergic function. While immediate treatment (1 mg/kg i.v.) caused significant reduction in mean arterial pressure (MAP) in both strains, prolonged treatment (20 mg/kg daily by gastric tube for 3 weeks) reduced MAP in the SH rats only. Following an i.v. bolus injection of norepinephrine (10 micrograms/kg) before each mode of treatment, and after immediate and prolonged treatment with urapidil, similar increases in MAP were observed in both strains, but only prolonged MAP reduction caused a significantly increased sensitivity of the reflex control of heart rate in SH rats (P less than 0.01). It is concluded that prolonged, but not immediate, reduction in MAP with urapidil was associated with a significant increase in the baroreflex sensitivity in the hypertensive strain. This may be related to the central effects of the drug.

Pathophysiology of hypertension in the elderly

Combined systolic and diastolic arterial hypertension and isolated systolic hypertension in the elderly are proven risk factors for stroke, sudden death, coronary artery disease, and congestive heart failure. Because hemodynamics, vascular and cardiac adaptations, fluid volume, and endocrine functions are distinctly altered in the elderly hypertensive patient compared with a younger patient, antihypertensive treatment should be individualized, and an unsophisticated regimen, such as a stepped-care approach, is too rigid to be as beneficial for elderly hypertensive patients as for young hypertensive patients.

Left ventricular hypertrophy as a risk factor

Left ventricular hypertrophy is an adaptive structural cardiac response to the afterload imposed by the hypertensive (and other pressure overload) diseases. As such, it maintains a stable cardiac performance until further adaptation is no longer possible, and then cardiac failure supervenes. Offsetting this "beneficial" effect is a distinct risk demonstrated epidemiologically by a greater number of cardiovascular morbid and mortal events, more severe ischemic disease, cardiac dysrhythmias, and sudden death. The precise mechanisms are just beginning to be elucidated. Other new and intriguing areas related to ventricular hypertrophy include its regression with certain forms of pharmacologic therapy and not others. This problem is also being investigated further at this time.

Renal involvement follows cardiac enlargement in essential hypertension

To assess the relationship between early clinically detectable involvement of hypertensive vascular disease in heart and kidneys, we obtained systemic and renal hemodynamic and M-mode echocardiographic measurements in 65 patients with essential hypertension. The results indicate that patients with and without left ventricular hypertrophy had similar renal hemodynamic findings. In contrast, patients with altered renal hemodynamic measurements (ie, reduced renal distribution of cardiac output and, therefore, absolute renal blood flow with increased renal vascular resistance) and increased serum uric acid levels also had increased left ventricular posterior and septal wall thicknesses and mass index. Moreover, these data also demonstrated that in patients with altered renal hemodynamics, the lower the renal distribution of cardiac output and the higher the serum uric acid levels, the greater were the indexes of cardiac enlargement. These results demonstrated that the pathophysiological and hemodynamic effects of essential hypertension in the heart precede those in the kidneys.

Immediate and short-term hemodynamic effects of diltiazem in patients with hypertension

The immediate effects of intravenous diltiazem effects and short-term (4 weeks) of the oral drug on systemic and regional hemodynamics, cardiac structure, and humoral responses were evaluated by previously reported methods in nine patients with mild-to-moderate essential hypertension and in one patient with primary aldosteronism. Diltiazem was first administered in three intravenous doses of 0.06, 0.06, and 0.12 mg/kg, respectively; patients were then treated for 4 weeks with daily doses ranging from 240 to 360 mg (average 300 mg). Intravenous diltiazem immediately reduced mean arterial pressure (from 115 +/- 3 to 96 +/- 3 mm Hg; p less than .01) through a fall in total peripheral resistance index (from 37 +/- 3 to 23 +/- 2 U/m2; p less than .01) that was associated with an increase in heart rate (from 66 +/- 2 to 77 +/- 3 beats/min; p less than .01) and cardiac index (from 3.3 +/- 0.3 to 4.3 +/- 0.4 liters/min/m2; p less than .01). These changes were not associated with changes in plasma levels of catecholamines or aldosterone or in plasma renin activity. After 4 weeks the significant decrease in mean arterial pressure persisted (104 +/- 3 mm Hg; p less than .01) and there were still no changes in the humoral substances or plasma volume. Renal blood flow index increased (from 368 +/- 52 to 462 +/- 57 ml/min/m2; p less than .01) and renal vascular resistance index decreased (from 0.37 +/- 0.06 to 0.26 +/- 0.04 U/m2; p less than .01), while splanchnic hemodynamics did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical assessment of the patient with borderline hypertension

Borderline hypertension occurs in approximately 15% of adults. While these individuals are at increased risk for developing sustained hypertension, roughly three-fourths do not progress to that level. In order to facilitate the clinical assessment, we have reviewed the known pathophysiology of the borderline hypertensive state. The recommended clinical evaluation including history, physical and laboratory examinations is directed at identifying those borderline hypertensives at highest risk for future hypertension and cardiovascular complications. Therapeutic interventions are directed at the high risk subgroup.

Hemodynamic effects of calcium entry-blocking agents in normal and hypertensive rats and man

To be considered ideal for the treatment of hypertension, a class of drugs should produce a maintained reduction of arterial pressure primarily through a fall in total peripheral resistance that is at least uniformly distributed through the organ circulations of the target organs of hypertensive vascular disease. Such drugs should be useful as monotherapeutic agents, should not expand the extracellular fluid compartment, should not reflexively stimulate the heart and vessels, should reverse the structural changes in the target organs without compromising function and, of course, should be relatively free of adverse effects. As a heterogeneous class of agents, the calcium entry-blocking drugs fulfill most of these criteria, although preliminary studies suggest that their effects on organ vascular resistances and on structural cardiovascular changes are highly variable. Thus, further vigorous investigation of these new agents is highly recommended.

Disparate cardiac effects of afterload reduction in hypertension

Cardiac performance, as judged by preload, afterload, and myocardial contractility, was evaluated by non-invasive M-mode echocardiography before and after acute oral administration of calcium entry blockers (nitrendipine and verapamil), angiotensin converting enzyme (ACE) inhibitors (captopril and lisinopril), and a dopamine receptor agonist (fenoldopam) in patients with mild to moderate essential hypertension. Left ventricular end diastolic volume was taken as an index of preload, end systolic stress as an index of afterload, and the ratio of systolic pressure to end systolic volume (SBP:ESV), ejection fraction (EF), and mean velocity of circumferential fibre shortening (Vcf) as indices of contractility. Reductions of afterload and mean arterial pressure were achieved with all antihypertensive agents involved (mean percentage change +/- s.e.m.: 15 +/- 2% and 10 +/- 1%, respectively, P < 0.05) but the afterload reduction with lisinopril was greatest (21 +/- 9%; P < 0.01). The dopamine receptor agonist fenoldopam decreased by preload 24% (P < 0.05) and increased all of the three parameters of myocardial contractility (SBP:SV 66%; EF 17%, Vcf 19%; P < 0.01). In contrast, no effect on these parameters was observed with either calcium entry blocker or either ACE inhibitor.

Acute pressure increase and intrarenal hemodynamics in conscious WKY and SHR rats

Spontaneously hypertensive rats (SHR) develop proteinuria and glomerular lesions during the first year of their life. To determine whether an intrarenal hemodynamic abnormality might participate in the pathogenesis of these lesions, normotensive Wistar-Kyoto (WKY) and SHR rats were subjected to acute pressure increase with phenylephrine infusion (2.5 and 5 micrograms X kg-1 X min-1), and renal blood flow (RBF), afferent and efferent arteriolar resistances, glomerular filtration rate (GFR), and glomerular capillary hydrostatic pressure were measured or calculated. The results indicate that the two strains responded differently to the pressure rise. Thus, although the increment in renal perfusion pressure and afferent arteriolar resistance increased similarly in the two strains, efferent resistance increased only in the SHR (7.2 +/- 1.6 vs. 9.0 +/- 2.1 units, P less than 0.02) but decreased in the WKY (5.3 +/- 0.8 vs. 4.0 +/- 0.5 units, P less than 0.05). This was associated with a decreased RBF of SHR (8.31 +/- 0.71 vs. 7.22 +/- 0.57 ml X min-1 X g kidney-1, P less than 0.05) but stable RBF in WKY (7.36 +/- 0.55 vs. 7.79 +/- 0.51 ml X min-1 X g kidney-1); GFR remained unchanged in both strains. Calculated glomerular hydrostatic pressure, however, increased in the SHR (43.6 +/- 3.0 vs. 48.5 +/- 2.2 mmHg, P less than 0.05) but decreased in the WKY (33.2 +/- 2.5 vs. 28.8 +/- 1.7 mmHg, P less than 0.01). The observed higher base-line glomerular pressure in the SHR and a greater rise with phenylephrine infusion may participate in the pathogenesis of the SHR glomerular lesions and proteinuria.(ABSTRACT TRUNCATED AT 250 WORDS)