Marie-Laure Grichois

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.

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 ± 2.4μmol P1/mg/hr; p < 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.

Furosemide and Bumetamide-Sensitive Na+ Fluxes in Erythrocytes from Genetically Hypertensive Rats (SHR)

Na+ and K+ fluxes from erythrocytes of SHR and WKY control rats have been studied. Fluxes were measured in both fresh and sodium-loaded (after exposure to ouabain or PCMBS) erythrocytes. Pump activity appears higher and furosemide or bumetamide sensitive Na+ efflux lower in SHR compared to WKY suggesting a similar abnormality in SHR genetic hypertension as in human hypertension.

Cell Membrane Changes After in Vivo Acute Na+ Load in Normotensive and Spontaneously Hypertensive Rats

Our previous observation of a greater increase in erythrocyte Na+ in SHR than in WKY after an acute Na+ load may result either from a genetic membrane property or from a specific plasma influence. In order to elucidate this question, membrane characteristics were compared with or without an acute Na+ load. Na+ transport was measured in Ringer and in plasma on Na+ enriched and K+ depleted red cells. Platelet microviscosity was measured as an index of membrane structural changes. After acute Na+ load a similar reduction of net Na+ extrusion and of K+ influx was observed in both strains. This indicates an inhibition of the Na+,K+-pump. Platelet microviscosity was similarly increased in SHR and WKY. Thus an acute Na+ load induced alterations of membrane properties in both SHR and WKY. The higher erythrocyte Na+ content in SHR stems rather from their intrinsic membrane properties than from a plasma factor.

Modulation of Na+ transport systems in Wistar rat erythrocytes by excess dietary Na+ intake

Plasma Na+, erythrocyte Na+ content and the activity of Na+ transport systems of red cells were measured in Wistar rat fed a normal or high Na+ diet. Net Na+ and K+ fluxes of erythrocytes were also measured in the presence of plasma of rats fed with excess Na+.Na+−K+ cotransport and passive Na+ permeability were increased. Erythrocyte Na+ content was increased after 2 months but not after 8 days of high Na+ intake. No significant difference in plasma Na+ and pump activity could be detected after such a diet. No factor acting in Na+ extrusion was found to be present in plasma of salt loaded rats. These results indicate that Na+ intake may modulate Na+ transport systems, namely passive permeability and Na+−K+ cotransport and that increased Na+ erythrocyte content is not a causative factor.

Effects of bradykinin on short-term variability in blood pressure and heart rate in rats: a spectral study.

Using a spectral procedure, we studied the effects of two treatment regimens of bradykinin (BK) on the blood pressure (BP) and heart rate (HR) variabilities in conscious Wistar rats. We performed a second series of experiments with hydralazine, at doses equihypotensive to those used in BK treatments, to discriminate between a specific effect of the peptide and those induced by vasodilation. We assessed the activity of the sympathetic nervous system (SNS), using the responses to atenolol and prazosin. First, at a subhypotensive treatment regimen, BK (5 μg/min) increased low-frequency (LF, 0.02–0.2 Hz) and mid-frequency (MF, 0.2–0.6 Hz) frequency components of BP variability and also activated the SNS. Lesser enhancements of LF and MF areas were induced by hydralazine (0.15 mg/kg). Second, high-dose treatment regimens of BK (100 μg/min) and hydralazine (2 mg/kg), which markedly decreased BP, did not change the areas of the LF and MF components of BP variability, whereas overactivity of the SNS was still assessed with the adrenergic blockers. On the other hand, high-dose BK induced a sixfold increase in the amplitude of the highfrequency (HF, respiratory) component of BP. The effect of bradykinin on HF domain was associated with an increase in the depth of respiration in a group of anesthetized rats. Hoe 140 (60 μg/kg), a B-2 BK-receptor antagonist, abolished both the effects of BK on HF fluctuations in BP and the effects on breathing pattern. Our results demonstrate that BK induced different effects on LF and MF fluctuations in BP depending on the treatment regimen, whereas the SNS was activated by the two selected treatment regimens. Therefore, the MF component of BP variability should be considered only as a marker of the activity of the SNS when the BP level was not affected. Furthermore, we characterized an amplifying effect of BK on the HF domain of BP variability partly mediated by an increase in the depth of respiration.

Plasma catecholamines in conscious rats: influence of sodium, stress and heredity.

Plasma catecholamines are increased in sodium-loaded rats under both resting and stress conditions. Under stress, Na+ resistant rats have lower plasma catecholamines than salt-sensitive ones.

Irreversible Changes in Rat Erythrocyte Na+ Transport Systems with Progesterone and Estradiol Administration

In order to test whether alterations in Na+ transport systems occurring in women when the hormonal status is disturbed such as in pregnancy, under contraceptive or estradiol therapy are a direct result of hormonal action on these transport systems, Na+,K+-pump Na+,K+ outward cotransport and passive Na+ permeability were measured in erythrocytes of ovariectomized rats receiving estradiol or progesterone. No significant changes in Na+,K+-pump were observed in either conditions. Conversely, progesterone was found to decrease Na+, K+-cotransport and estradiol to increase this system, whereas both steroids increased passive Na+ permeability.

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.

Erythrocyte Na + Handling in Rat Genetic Hypertension

Recent investigations have described various alterations of cation fluxes in the erythrocyte membrane of essential hypertensives (for review, see [1]). Transmembrane pathways in the red cells of genetically hypertensive rats are also altered [2]. This paper summarizes our previous findings on Na+ transport systems in SHR and SbH and reports some new data on the effect of chronic and acute Na+ loads on cellular Na+ handling.