Platelet cytosolic proton and free calcium concentrations in essential hypertension

Alterations in the metabolism of intracellular messengers, such as calcium and cyclic adenosine 5'-phosphate (cAMP), have been reported in essential hypertension. Since intracellular pH (pHi) participates in the control of fundamental cell functions, we looked for changes in platelet cytosolic H+ concentration [( H+]i) in hypertension and investigated whether or not its impaired metabolism is linked to the calcium handling abnormalities. The fluorescent pH indicator BCECF has been used to evaluate intracellular H+ concentration in platelets, unstimulated ex vivo, from normotensive (n = 20) and hypertensive patients (n = 20). Cytosolic [H+] was 20% lower in hypertensive than in normotensive subjects (49.5 +/- 3.4 and 61.8 +/- 2.2 nmol/l cells, respectively, P less than 0.005; mean pHi values were 7.21 and 7.33, respectively). Platelet cytosolic H+ and free Ca2+ concentrations ([Ca2+]i) were determined in parallel in 15 normotensive and 15 hypertensive patients. [Ca2+]i was found to be 19% higher (P less than 0.01), and [H+]i 22% lower (P less than 0.02), in the hypertensive patients compared with the normotensive subjects. Platelet pHi and [Ca2+]i were increased simultaneously in some hypertensive patients. These results are compatible with the hypothesis of an in vivo activation of platelets in hypertension. If a similar alkalinization exists in smooth muscle cells, it may participate in cell proliferation and in an enhanced sensitivity to agonists, two parameters thought to be involved in blood pressure elevation.

Body fluid variations and endogenous digitalis-like compounds during chronic NaCl loading in Wistar rats

Circulating digitalis-like compounds have been proposed to be raised in volume expanded hypertension and to participate in Na+ homeostasis. We have investigated the temporal relationships between the activity of these circulating digitalis-like compounds, blood pressure and body fluid volume variations during a chronic NaCl load in the Wistar rat. Characteristics of salt-loaded rats were compared to those of weight-matched controls. At one week, when extracellular fluid volume (ECFV) was elevated, the capacity of plasma extracts to inhibit the Na+K+ATPase activity begun to rise. At two weeks, ECFV remained elevated, and plasma volume, blood pressure and the activity of plasma digitalis-like compounds increased. After 13 weeks, the continuous rise in plasma digitalis-like activity and in blood pressure was accompanied by the return of body fluid volumes towards control values. These changes in plasma digitalis-like activity, body fluid volumes, and systolic blood pressure during a high NaCl diet are compatible with the proposed role of circulating digitalis-like compounds as natriuretic and hypertensive factors.

Plasma levels of digitalis-like substance in Nigerians with essential hypertension

A plasma Na+-K+ ATPase inhibitor, which is estimated by a technique in which it competes with ouabain for binding on red cells, was measured in three groups of individuals: (a) normotensive subjects without a family history of hypertension, (b) normotensive subjects with a family history of hypertension, (c) untreated essential hypertensive subjects. The mean value of the inhibitor in group (b) subjects was significantly higher than the mean value in group (a). The mean value in group (c) subjects was also significantly higher than in group (a) subjects. However, the means of the values in groups (b) and (c) were not significantly different. There was a significant positive correlation between the levels of the inhibitor and the urinary Na+ excretion in all subjects. However, there was no correlation between the inhibitor levels and mean arterial pressure. The relevance of these results to the pathophysiology of hypertension in the black African subject is discussed.

Correlations between plasma levels of an endogenous digitalis-like substance and haemodynamic parameters measured during cardiac catheterization

It has been postulated that one or more plasma digitalis-like compounds may play an important role in body fluid regulation and in essential hypertension, although very little is known about their possible role in general haemodynamics. We therefore measured plasma inhibition of human kidney Na+,K+-ATPase and plasma cross-reactivity with digoxin antibodies in 11 normotensive cardiopathic subjects admitted to our clinic for heart catheterization. Possible correlations with haemodynamic parameters were studied. Plasma digoxin-like activity correlated directly with left atrial pressure and with pulmonary circulation data. The ability of the plasma to inhibit Na+,K+-ATPase showed an inverse correlation with cardiac output and cardiac index. No correlations were found with any of the other parameters measured, notably systemic resistance, blood pressure and natriuresis. These findings suggest the presence of more than one substance sharing chemical properties with digitalis: (1) a substance cross-reacting with digoxin antibodies and dependent on pulmonary vascular congestion; and (2) a substance capable of inhibiting the Na+-K+ pump and present in large amounts in heart diseases with a reduced cardiac index.

Platelet 5-HT content and uptake in essential hypertension: role of endogenous digitalis-like factors and plasma cholesterol

A decrease in platelet 5-HT content linked to partial inhibition of 5-HT uptake has been described in essential hypertension. Transport of 5-HT through platelet membrane is dependent upon transmembranal Na+ and K+ gradients. It is inhibited by Na+, K+-ATPase inhibitors such as ouabain and endogenous digitalis-like compounds isolated from hemodiafiltrate. The activity of such compounds in plasma extracts, measured by inhibition of Na+,K+-ATPase or ouabain binding to human erythrocytes, and platelet 5-HT content were determined in parallel in essential hypertensive patients. Significant negative correlations were observed between these parameters in men, suggesting that high levels of digitalis-like compounds can affect platelet 5-HT content. In addition, in essential hypertensive patients, total plasma cholesterol was inversely related to both platelet 5-HT content (n = 15, r = -0.594, P less than 0.02) and maximal velocity of 5-HT uptake (n = 15, r = -0.717, P less than 0.003). In normotensive control subjects, no variation of platelet 5-HT content with cholesterol was observed. This suggests that the platelet membranes of essential hypertensive patients are more sensitive to increases in plasma cholesterol than those of normotensive subjects.

Fluorescence measurements of free Ca2+ concentration in human erythrocytes using the Ca2+-indicator fura-2

We report here the use of the fluorescent Ca2+-chelator fura-2 to directly measure free Ca2+ concentration within intact human erythrocytes and the influence of viscosity on the fluorescence of this probe. The bright fluorescence of fura-2 has permitted the use of low concentrations of indicator and cells, thus minimizing the screening effect and the intrinsic fluorescence of haemoglobin. Erythrocytes (10(8) cells/ml) were loaded with 0.5 microM fura-2AM then diluted at 10(7) cells per ml for measurements. The extracellular signal was suppressed by addition of manganese ions just before recording spectra. Under these conditions, a blood sample of 100 microliter was sufficient for analysis. To study the influence of viscosity on fura-2 fluorescence, gelatin and polyvinylpyrrolidone at various concentrations were added to a physiological buffer to perform fura-2-Ca fluorescence standard curves. Fluorescence intensities and the apparent affinity constant for Ca2+ were modified by viscosity. When intra-erythrocytic viscosity was simulated with 21 g/l polyvinylpyrrolidone to obtain a mean viscosity of 14 mPa.s similar to that observed in human erythrocytes, the mean value of free Ca2+ concentration measured in erythrocytes from healthy subjects was 78 +/- 16 nM (mean +/- S.D., n = 29).

Endogenous digitalislike circulating substances in spontaneously hypertensive rats

Circulating digitalislike compounds have been proposed to be involved in some Na+-dependent types of experimental hypertension and in human essential hypertension. The level of circulating Na+-K+ pump inhibitor(s) was investigated in the spontaneously hypertensive rat of the Okamoto strain (SHR), its normotensive control, Wistar-Kyoto rat (WKY), and the regular Wistar rat using the following criteria: the ability of whole plasma to inhibit the total active Na+ efflux from Wistar rat erythrocytes and to cross-react with digoxin antibodies and the ability of plasma extracts to inhibit Na+,K+-adenosine triphosphatase (ATPase) activity of membranes from rat kidney. SHR plasma inhibited the net Na+ efflux from Wistar erythrocytes by up to 27% compared with WKY or Wistar plasma. For a given number of cells, the inhibition increased with the amount of available plasma. Cross-reactivity with digoxin antibodies was twice as high in SHR as in WKY or Wistar plasma. It was already enhanced in 3- to 4-week-old rats. Plasma extracts from SHR significantly inhibited Na+,K+-ATPase activity when compared with WKY extracts (75.6 +/- 2.6 vs 89.3 +/- 2.4 mumol Pi/mg/hr; p less than 0.01) but did not differ from Wistar plasma extracts. These results strongly suggest that circulating digitalislike compound(s) are present in elevated amounts in SHR as early as 3 to 4 weeks of age, but their exact participation in blood pressure elevation or maintenance remains to be clarified.

[Endogenous compounds of the digitalis type in essential and arterial hypertension]

Endogenous digitalis-like compounds are present in biological fluids and in some tissues. Little is known, so far, about their chemical nature and the tissues where they originate. Plasma and urinary levels of these compounds are elevated in essential and experimental arterial hypertension, and this rise may contribute to the genesis and maintenance of an abnormally high blood pressure. This is explained by an increase in intracellular calcium concentration resulting from inhibition of the sodium/potassium pump and increase of intracellular sodium. Concerning the chemical nature of these endogenous digitalis-like compounds two hypotheses have been put forward: peptides or steroids. Precise identification of these compounds and a knowledge of their mechanism of action on the sodium/potassium pump will probably lead to the development of new drugs in the field of cardiovascular and renal diseases.

[Variations in the plasma concentrations of atrial natriuretic factor and endogenous digitalis compounds during acute volume expansion in the conscious rat]

Atrial natriuretic peptide (ANP) and Na+ pump inhibitor (digitalis-like substance, DLS) have both been proposed to participate in body sodium and water homeostasis. Plasma levels and ANP and DLS have been reported to be increased in physiological or pathological states characterized by volume expansion. In order to investigate possible mutual relationships, their concentrations were measured in parallel during acute volume expansion by injection of 25 ml/kg isotonic NaCl (A) or blood (B) in the conscious rat. ANP was measured by radioimmunoassay and DLS by inhibition of renal Na+, K+-ATPase activity and digoxin-like immunoreactivity (DLI). Five minutes after injection, plasma ANP increased to reach 700 pg/ml (A, n = 21) or 1,500 pg/ml (B, n = 5) but the ability of plasma extracts to inhibit the renal Na+, K+-ATPase activity was unchanged (16.6 +/- 2.5 vs 16.9 +/- 2.0 p. 100, A, n = 8 and 6). Digoxin-like immunoreactivity was slightly lowered after NaCl injection from (74.4 +/- 6.2 to 65.4 +/- 5.1 pg/ml, n = 21) and unchanged after blood injection (79.0 +/- 3.4 vs 81.2 +/- 5.0 pg/ml, n = 5). Plasma ANP concentrations then decreased and had returned to preinjection values before 30 (A) or 90 (B) minutes, whereas the capacity of plasma to inhibit the Na+, K+-ATPase tended to increase (25.9 +/- 4.7 p. 100 at 3 hours after injection, n = 12 compared to 17.6 +/- 1.6 p. 100, n = 20) and DLI remains stable.(ABSTRACT TRUNCATED AT 250 WORDS)

Red blood cell ionized calcium concentration in spontaneous hypertension: modulation in vivo by the calcium antagonist PN 200.110

1. Altered calcium regulation has been observed in experimental and human hypertension. In this study erythrocyte (RBC) intracellular calcium concentration ([Ca2+]i) was compared in conscious spontaneously hypertensive rats (SHR) and their normotensive controls (WKY) at rest and after injection of the dihydropyridine calcium antagonist PN 200.110. 2. Resting [Ca2+]i was similar in SHR and WKY. 3. PN 200.110 administration induced a rapid decrease in blood pressure in SHR and WKY. Five minutes after the injection no change in [Ca2+]i was observed; at 1 h [Ca2+]i was significantly decreased in SHR, but not in WKY. 4. These results suggest that the mutual adaptation of the rate of calcium influx through calcium channels and the activity of the calcium extruding pump differ between WKY and SHR.

Antihypertensive effect of canrenone in a model where endogenous ouabain-like factors are present

The effect of canrenone, an antialdosterone and partial ouabain-agonist drug, was studied in rats that developed volume expansion and hypertension after renal mass reduction and excess Na+ intake (RRM-salt). The RRM-salt was characterized by: (1) increased endogenous "digitalis-like" compounds in plasma [cross reactivity with digoxin-antibodies (57.5 +/- 5.0 vs. 42.1 +/- 3.8 pg/ml, p less than 0.02); inhibition of kidney Na+, K+-ATPase activity (135 +/- 5 vs. 154 +/- 5 mumol/mg/h, p less than 0.01); and inhibition of Na+ extrusion from normal erythrocytes (5.96 +/- 0.40 vs. 7.68 +/- 0.34 mmol/L cells/h, p less than 0.01)]; (2) reduced Na+, K+-pump activity (7.34 +/- 0.29 vs. 10.88 +/- 0.41 mmol/L cells/h, p less than 0.001) and increased Na+ content (4.66 +/- .08 vs. 4.16 +/- 0.11 mmol/L cells, p less than 0.01) in erythrocytes; and (3) low plasma renin activity (2.1 +/- 0.9 vs. 12.6 +/- 1.6 ng/ml/h). Ninety minutes after the administration to RRM-salt of a single oral dose of 60 mg/kg of canrenone, the systolic blood pressure decreased by 36 +/- 4 mm Hg (mean +/- SEM). Chronic canrenone administration (60 mg/kg/day) resulted in a marked antihypertensive effect associated to a correction of volume expansion, a decrease in endogenous "digitalis-like" compounds, and a partial recovery of Na+, K+-pump activity and Na+ content in erythrocytes. Our results suggest that the antihypertensive effect in RRM-salt rats results, at least in part, from antagonism with endogenous "digitalis-like" compounds.

Platelet cytosolic free Ca2+ concentration and plasma cholesterol in untreated hypertensives

It has been proposed that plasma cholesterol, a major risk factor for atherosclerosis, may modify cellular Ca2+. In particular, higher membrane cholesterol contents induce higher Ca2+ influx and decrease the activity of the Ca2+ pumps. Cellular Ca2+ may also control the number of accessible low density lipoprotein receptors. We investigated the question of whether plasma cholesterol influences cellular Ca2+ metabolism, by analysing the cytosolic free Ca2+ concentration [( Ca2+]i) in unstimulated platelets from 61 untreated hypertensive patients [systolic and diastolic arterial pressure: 157 +/- 3/95 +/- 2 mmHg (mean +/- s.e.m.), respectively, age 41.8 +/- 1.7 years, body mass index 24.7 +/- 0.6 kg/m2]. The subjects' plasma total cholesterol (5.5 +/- 0.5 mmol/l) and platelet [Ca2+]i concentration (228 +/- 7 nmol/l) were positively correlated (r = 0.375, P less than 0.003). This correlation persisted at constant age, arterial pressure or body mass index. Platelet [Ca2+]i tended to increase with plasma low-density lipoprotein concentration (n = 21, P = 0.01), and to decrease with the ratio of high-density lipoprotein cholesterol to total cholesterol (n = 21, P = 0.08). The observation that in normocholesterolaemic hypertensive patients [Ca2+]i concentration in unstimulated platelets was correlated with plasma cholesterol suggests that cell activation may be modulated by membrane fluidity or that cholesterol metabolism is influenced by cell Ca2+.

The role of antihypertensive drugs in counteracting adverse influence on large arteries

Studies in humans have shown that chronic elevation of blood pressure induces early alterations in the large arteries, consisting mainly of increased diameter, increased pulse wave velocity, and decreased arterial compliance. Acute reduction of blood pressure, per se, does not necessarily correct these alterations homogeneously. For instance, cadralazine reduced brachial artery diameter and urapidil decreased pulse wave velocity, without accompanying changes in forearm arterial compliance. In contrast, acebutolol, isosorbide dinitrate, nicardipine, and nitrendipine, for the same acute decrease in blood pressure, improved forearm arterial compliance by a concomitant decrease in pulse wave velocity and increase in arterial diameter. To determine whether or not these vascular effects persist with long-term acebutolol therapy, nine hypertensive patients were treated for a period of 3 months. Forearm hemodynamic and cytosolic free calcium concentrations in platelets were simultaneously assessed before and after treatment. Compared with placebo baseline values, chronic acebutolol therapy significantly decreased mean arterial pressure (p less than 0.01), pulse wave velocity (p less than 0.01), and platelet free calcium concentration (p less than 0.05); forearm arterial compliance was increased (p less than 0.01), but brachial artery diameter did not change. Platelet calcium concentration correlated closely with pulse wave velocity even at constant mean arterial pressure. These findings suggest a relaxant effect of acebutolol on the smooth musculature of large arteries, which is independent of changes in blood pressure and arterial diameter, and possibly mediated by changes in cytosolic calcium levels.

[Endogenous digitalis compounds and adrenal gland activity]

Endogenous inhibitors of the Na+,K+-pump have been postulated to participate in Na+ and water homeostasis. When present in high amounts, they have been proposed to induce a rise in blood pressure. There is no general agreement concerning their chemical structure and their possible tissue origin has not been well defined. In an attempt to identify a tissue source, (i) the digitalis-like contents of various organs (cross reactivity with digoxin antibodies and inhibition of ouabain binding to the Na+ pump) were determined and (ii) the circulating digitalis-like activity was analyzed under various conditions known to alter the activity of the putative source organs. High levels of digitalis-like compounds were present in the adrenal and pituitary glands (4.1 +/- 0.1, n = 15 and 3.7 +/- 0.4, n = 13 ng digoxin equivalents/g) or 328 +/- 66, n = 6 and 460 +/- 76, n = 6 ng ouabain equivalents/g) respectively. Much lower levels were found in heart, kidney, brain and hypothalamus. Plasma digitalis-like activity was measured in 5 groups of rats: control, ACTH treated, hypophysectomised, and in DOCA-salt treated animals with and without bilateral adrenalectomy. Changes in the plasma digitalis-like activity measured by the crossreactivity with anti-digoxin antibodies or by the inhibition of ouabain binding followed the same pattern. When compared to the control values, the plasma digitalis-like activity was partially decreased after hypophysectomy, markedly decreased after adrenalectomy and enhanced in ACTH-treated animals. These results suggest that the pituitary-adrenal axis plays a major role in the control of the circulating "digitalis-like" activity.

Platelet cytosolic free Ca2+ concentration and serotonin (5-HT) content in essential hypertension

Cytosolic free [Ca2+] and serotonin (5-HT) content were measured in platelets from 22 untreated hypertensive patients and 16 normotensive subjects. In hypertensive patients, cytosolic free [Ca2+] was significantly higher (239 +/- 13 nmol/l versus 186 +/- 7 nmol/l, n = 22 and 16, P less than 0.01) and 5-HT content was significantly lower than those measured in cells from control subjects (3.22 +/- 0.26 versus 4.99 +/- 0.38 X 10(-7) mol/10(11) cells, n = 22 and 16, P less than 0.001). These two parameters were closely correlated (r = -0.565, n = 38, P less than 0.001). These two concomitant changes in platelet characteristics might result from a common cause, such as cell membrane alterations. As ritanserine, a specific 5-HT2 receptor antagonist, did not modify the cytosolic free [Ca2+], a higher stimulation of 5-HT2 receptors is not likely to be responsible for the enhanced free Ca2+ levels.

Evidence of an endogenous digitalis-like factor in the plasma of patients with acromegaly

Evidence suggests that plasma-volume expansion leads to the release of a digitalis-like factor, which is thought to act on the renal tubular cells and cause natriuresis. We postulated that this factor might be present in patients with acromegaly (in whom plasma volume is elevated) and might return to normal levels when the disease was treated successfully. We measured the ability of plasma extracts from patients with acromegaly to inhibit the binding of ouabain to the sodium pump in normal red cells and to inhibit the enzymatic activity (sodium-potassium-ATPase) of the sodium pump in membrane preparations from normal kidneys. In 21 patients with active acromegaly, the mean (+/- SE) level of ouabain-binding inhibition (1.56 +/- 0.38) was higher (P less than 0.01) than that in either 11 successfully treated patients (0.18 +/- 0.05) or in 27 normal controls (0.19 +/- 0.03). The inhibition of sodium-potassium-ATPase activity by plasma was also greater in patients with active acromegaly (38.1 +/- 6.8 percent) than in successfully treated patients (18.4 +/- 5.6 percent, P less than 0.05) or controls (21.1 +/- 2.7 percent, P less than 0.05). Significant correlations were found between plasma volume and ouabain-binding inhibition in 23 patients (r = 0.72, P less than 0.01) and sodium-potassium-ATPase inhibition in 19 patients (r = 0.62, P less than 0.01). Pituitary adenomectomy decreased plasma volume and the inhibition by plasma of ouabain binding. We conclude that an endogenous digitalis-like factor is present in the plasma of patients with chronic volume expansion due to acromegaly. These results are consistent with the hypothesis that this natriuretic factor may have a physiologic role in water and sodium homeostasis.

Changes in platelet free Ca2+ concentration after chronic digoxin treatment

Cell Na+ and Ca2+ concentrations control each other by various mechanisms. In excitable cells from various origins, Ca2+ extrusion from the cell and its entry are dependent for a large part on the activity of the Na+, Ca2+-countertransport system. Cytosolic free Ca2+ concentration is also controlled by the Na+-H+ exchange activity. To analyze the changes in cytosolic Ca2+ concentration accompanying the reduction of the membrane Na+ gradient, cytosolic free Ca2+ concentration ([Ca2+]i) was measured by fluorescent dyes in platelets and erythrocytes from healthy subjects, before and during digoxin treatment (0.25 mg/day for 6 days). [Ca2+]i was increased in platelets from 169 +/- 30 to 321 +/- 61 nmol/l (n = 7, P less than 0.02) and unchanged in erythrocytes (121 +/- 6 and 104 +/- 7 nmol/l). This increase in platelet [Ca2+]i was not accompanied by a change in serotonin content (5.43 +/- 0.67 vs 5.49 +/- 0.61 10(-7) mol per 10(11) cells) and could not be reproduced by in vitro addition of 10(-4) mol/l ouabain (198 +/- 33 vs 186 +/- 73 nmol/l). The enhanced [Ca2+]i in platelets is thus not a short-term consequence of a reduced membrane Na+ gradient, but reflects either the overload of intracellular Ca2+ stores or an enhanced in vivo stimulation by hormones or neurotransmitters.

Relationship between platelet cytosolic free calcium concentration and plasma renin activity

1. Plasma renin activity (PRA) and platelet cytosolic free calcium concentration ([Ca2+]i) were simultaneously determined in 18 untreated essential hypertensive subjects and 17 normotensive controls. A significant positive correlation was found between [Ca2+]i and PRA (slope = 42 nmol/l/ng/ml/h) in these 35 subjects. 2. Two determinations more than one week apart in nine subjects confirmed the parallel fluctuations of [Ca2+]i and PRA. A strict sodium restriction produced a progressive PRA elevation associated with a parallel rise in [Ca2+]i in one subject. 3. These results are consistent with the hypothesis that angiotensin II causes a concentration-dependent calcium mobilization.

Endogenous digitalis-like compounds in essential and experimental hypertension

The hypothesis that endogenous digitalis-like compounds might participate in body sodium and water homeostasis have led us to investigate the presence in plasma of compounds interacting with digoxin antibodies in man and rats. The apparent levels of digoxin-equivalents in plasma of control subjects (n = 21) and patients with essential hypertension (n = 48) or end-stage renal failure (n = 13) were 24.7 +/- 3.2, 34.4 +/- 4.4 and 98.7 +/- 17.4 pg/ml, p less than 0.05 and p less than 0.01 respectively. Positive correlations were observed between systolic and diastolic blood pressure and the apparent immunoreactivity of plasma. No relationship was found with the renal Na+ excretion or the plasma renin activity. The apparent digoxin-like immunoreactivity of the plasma was correlated with its ability to inhibit ouabain binding to the erythrocyte Na+ pump and to reduce the renal Na+,K+-ATPase activity. In rats with experimental hypertension, the plasma cross-reactivity with antidigoxin antibodies was also enhanced when compared to control rats (71.6 +/- 10.2 pg/ml, n = 12 and 57.3 +/- 5.0 pg/ml, n = 33 in Na+ loaded rats and in rats with reduced renal mass respectively compared to 43.4 +/- 3.7 pg/ml, n = 36, p less than 0.05). In spontaneously hypertensive rats (SHR), the apparent levels of digoxin- equivalents were higher than that of age-matched WKY normotensive rats. This increase was already present in prehypertensive SHR (3 week-old) (105.8 +/- 12.4 vs 40.0 +/- 6.5 pg/ml, n = 9 and 8, p less than 0.001) and persisted after hypertension has developed (134 +/- 12.6 vs 85 +/- 7.9 pg/ml, n = 7 and 8, p less than 0.005 in 30 week-old rats). The apparent affinity of the erythrocyte Na+,K+ cotransport for intracellular Na+ and the maximal rate of the Na+ pump were correlated with the plasma digoxin-like levels. These results confirm the presence in plasma of compounds possessing some of the functional and structural properties of cardioactive steroids, associated with a rise in blood pressure.

Chemical and clinical studies of endogenous digitalis-like factor in hypertension

Endogenous digitalis-like factor (endalin) was investigated by measuring the ability of rat and human plasma and urine to inhibit [3H]ouabain-specific binding, digoxin-antidigoxin antibodies interaction, and renal Na+, K+-ATPase activity. Endalin was detected in plasma (and urine) of one third of 112 patients with sustained and moderate hypertension (Na+ intake = 110 mmol/l). Endalin tended to be increased in the more pronounced hypertensives. No correlation with any other clinical and biological parameter could be detected. An activity to inhibit Na+, K+-ATPase was also detected in the rat after acute and chronic Na+ loading, in reduced renal mass-type hypertension and in SHRs as compared to WKY rats. Comparison of the plasma and urine inhibitory effects in the different tests revealed some chemical heterogeneity. However, a compound possessing the biochemical and pharmacological characteristics of digitaline was extracted from human urine. Chromatographic and spectral analysis of about 1,000 liters revealed a compound with apparent chemical homogeneity, molecular weight around 500, devoid of peptidic bound and of aliphatic structure.

Structural and functional alterations of the cell membrane in the prehypertensive rat of the Okamoto Aoki strain

Plasma membrane properties of 3 to 4-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were investigated in both excitable cells, i.e. cardiomyocytes and platelets, and non-excitable cells, i.e. erythrocytes. Cardiac sarcolemma from SHR exhibited: lowered capacity of high-affinity Ca2+-binding sites; higher Ca2+ pump activity; higher Na+K+-ATPase activity due to increased density of Na+ pump units; suppression of the above three effects by the Ca2+-calmodulin complex, and increased Na+Ca2+ exchange. No difference in platelet cytosolic free Ca2+ concentration was observed between SHR and WKY. In both substrains, erythrocyte intracellular Na+ content was similar in spite of reduced Na+ and K+ net fluxes. Isolated membranes from SHR erythrocytes were also characterized by: lowered phosphoinositide turnover; decreased ATP-dependent Ca2+-transport, and lowered capacity of high affinity Ca2+-binding sites. Structural alterations detected by fluorescence polarization of diphenylhexatriene were observed in SHR cardiac sarcolemma, erythrocyte and brain synaptosomal membranes. Membrane organization and activity of transport systems controlling the intracellular Na+ and Ca2+ level were thus already modified in the prehypertensive animals whereas the resulting intracellular ion contents were still unaltered.

Increased platelet cytosolic free calcium concentration in essential hypertension

Cytosolic free calcium concentration [Ca2+] was studied in platelets of hypertensive patients with the use of the fluorescent indicator Quin 2/AM. Cytosolic free Ca2+ was significantly higher in platelets of hypertensive patients than in those of normotensive subjects (241 +/- 9 versus 192 +/- 7 nmol/l, n = 58 and 57, respectively P less than 0.001). When all 115 subjects were included, there was a significant correlation between cytosolic free Ca2+ and systolic or diastolic blood pressure (r = 0.262, P less than 0.0025 and r = 0.251, P less than 0.0025, respectively). Intracellular Quin 2 concentration was measured to evaluate the formaldehyde production (a product of Quin 2/AM hydrolysis which has been described as reducing the adenosine triphosphate (ATP) production). The Quin 2 concentrations in platelets of the two groups of subjects were observed to be similar (0.41 +/- 0.03 versus 0.38 +/- 0.03 mmol/l, n = 8 and 7 for hypertensives and normotensives, respectively). The effects of prostaglandin E1 (PGE1), an adenylate cyclase stimulator, on cytosolic free Ca2+ were studied. The presence of 10(-7) mol/l PGE1 lowered the Ca2+ in platelets of hypertensive patients only, suppressing the difference between the two groups.

High sodium diet and circulating digitalis-like compound in the rat

The effects of a high salt diet (8% NaCl) on blood pressure and intra-erythrocytic Na+ content were studied in Wistar rats. The ability of the plasma to inhibit the renal Na+,K+-ATPase activity and to cross-react with digoxin antibodies was also investigated. After 1 week, neither systolic blood pressure nor intra-erythrocytic Na+ content were modified, but plasma extracts slightly inhibited renal Na+,K+-ATPase (70.9 +/- 1.7 versus 76.3 +/- 2.1 mumol Pi/mg per h, P = 0.05). After 2 weeks, the plasma inhibitory activity, systolic blood pressure and intra-erythrocytic Na+ content were higher than corresponding values in control animals (65.5 +/- 1.6 versus 79.1 +/- 2.8 mol Pi/mg per h, P less than 0.001; 132 +/- 2 versus 114 +/- 4 mmHg, P less than 0.001, and 4.95 +/- 0.32 versus 3.81 +/- 0.36 mmol/l cells, P less than 0.05, respectively). After 3 months, the plasma digoxin-like immunoreactivity and its ability to inhibit the Na+ pump were elevated (68.7 +/- 7.9 versus 48.2 +/- 5.4 pg/ml, P less than 0.02; 57.8 +/- 1.8 versus 72.9 +/- 1.8 mumol Pi/mg per h, P less than 0.001, respectively) whereas intra-erythrocytic Na+ content had returned to control levels. The results demonstrated that this high salt intake led to simultaneous increases in systolic blood pressure and in the activity of a digitalis-like compound present in plasma. The inhibition of Na+,K+-ATPase was correlated with systolic blood pressure and digoxin-like immunoreactivity (r = 0.569, n = 76, P less than 0.001 and r = 0.414, n = 34, P less than 0.02, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma digitalislike activity in essential hypertension or end-stage renal disease

Plasma extracts from 119 subjects showed a digitalislike activity, as evidenced by the ability of these extracts to inhibit ouabain binding to the Na+-K+ pump. High levels of the digitalislike compound were found in 18 of 54 untreated hypertensive subjects, 7 of 21 normotensive subjects with a family history of hypertension, and 10 of 14 patients with end-stage renal failure. Dialysis significantly reduced the activity of this compound. These results suggest 1) that endogenous digitalislike factor is not directly linked to hypertension but rather is related to sodium balance and 2) that it neither originates nor is activated by renal tissue, as it was present in four of six anephric patients.