Abnormal Ca2+ handling and increased Mg2+ permeability in platelets of hypertensive rats

Electrolytes and pH changes in pre-eclamptic rats

BACKGROUND

Intracellular free calcium [Ca2+]i and magnesium [Mg2+]i ions play major roles in the mechanism of vascular smooth muscle (VSM) contraction. Although essential hypertension and abnormal intracellular homeostasis of these ions have long been recognized as major icons in the pathogenesis of pre-eclampsia, the underlying mechanism(s) remain poorly understood.

METHODS

Alterations of vascular smooth muscle and platelet intracellular cations [Ca2+]i, [Mg2+]i and [H+]i relative to plasma concentrations of these ions in nitric oxide synthase (NOS) blockade-induced models of pre-eclampsia have been evaluated in the present study.

RESULTS

Pregnant rats injected with the NOS inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) developed a significantly elevated arterial blood pressure, proteinuria and other clinical parameters characteristic of pre-eclampsia compared to age-matched pregnant and non-pregnant rat controls that received the L-NAME vehicle only. Plasma total calcium concentration was significantly lower in pre-eclamptic rat models compared to normal pregnant rats (10.29+/-0.08 vs 10.67+/-0.18 mg/dl, p<0.05). A significant increase in plasma calcium was observed in pregnant controls compared to non-pregnant rats (10.67+/-0.18 vs 10.14+/-0.09 mg/dl, p<0.01). Plasma Ca2+ levels in pre-eclamptic rats were consistently lower than those of pregnant controls (5.69+/-0.09 vs 5.98+/-0.06 mg/dl, p<0.05). Resting levels of [Ca2+]i was significantly higher in pre-eclamptic rats than in pregnant controls. (351+/-45.2 vs 196+/-23.2 nmol/l, p<0.01). Blood pH was significantly increased in pre-eclamptic rats as compared to pregnant controls (7.16+/-0.02 vs 7.05+/-0.03, p<0.05). There was no significant difference in plasma and intracellular magnesium concentrations between the three rat groups.

CONCLUSIONS

These findings suggest that a significantly decreased plasma level of Ca2+ coupled with a concomitant increase in VSM [Ca2+]i concentrations and an altered blood pH are associated with pre-eclampsia in the pregnant rat. Routine monitoring of serum pH, Ca2+ and Mg2+ especially in the late third trimester, may have potential in the early detection of patients at risk for pre-eclampsia, and monitoring the progress of diverse therapeutic regimens during clinical management.

Effect of amlodipine and cilazapril treatment on platelet Ca2+ handling in spontaneously hypertensive rats

Abnormal Ca2+ handling and enhanced aggregation response have been reported in platelets from spontaneously hypertensive rats (SHR) and patients with essential hypertension, and thought to be involved in the progression of target organ damage of hypertension. It is important to examine whether antihypertensive therapy can improve the abnormal platelet response in hypertension. We investigated the effect of antihypertensive treatment such as amlodipine and cilazapril on Ca2+ handling and aggregation response in SHR platelets. Four-week-old male SHR were divided into three groups. Each group was treated with amiodipine (A: 10 mg/kg/day), cilazapril (C: 10 mg/kg/day) or vehicle (V) for 8 weeks by gavage. At 12-week-old, platelet [Ca2+]i was measured with fura-2 in each group of SHR and age-matched Wistar-Kyoto rats (WKY) as normal control. Systolic blood pressure in amlodipine and cilazapril treated groups were similar with WKY and significantly lower than vehicle treated group (A: 124 +/- 9, C: 126 +/- 9, WKY: 122 +/- 10 and V: 180 +/- 9 mmHg, respectively). The basal [Ca2+]i in the three groups of SHR were similar and higher than WKY (A: 47 +/- 1.7, C: 47 +/- 1.2, V: 48 +/- 3.9 and WKY: 40 +/- 4.0 nmol/l, respectively). There were no significant differences in thrombin (0.1 U/ml)-stimulated [Ca2+]i rise in the presence or absence of extracellular Ca2+ among the three groups of SHR and these were higher than WKY. Intracellular Ca2+ discharge capacity, assessed by the ionomycinstimulation was similar in the all groups. Thrombin-induced maximum platelet aggregation responses in the three groups of SHR were similar and higher than WKY. The antihypertensive treatment of Ca2+ antagonist or ACE inhibitor gave no change in intraplatelet Ca2+ metabolism in SHR. These results support the hypothesis that an abnormal Ca2+ handling in SHR platelet is genetically determined and not improved by hypotensive therapy.

Exchangeable magnesium pool masses in spontaneously hypertensive rats

We explored magnesium (Mg) metabolism by determination of exchangeable Mg pool masses and Mg kinetic parameters using stable Mg isotopes in spontaneously hypertensive rats (SHRs). Classical intracellular and extracellular Mg status biomarkers were also measured. Male SHRs and their male Wistar Kyoto (WKY) controls were fed a semipurified diet containing Mg 550 mg /kg for 2 weeks. Each rat received then an intravenous injection of 1.37 mg (25)Mg. The plasma (25)Mg disappearance curve over the next 7 days was used to measure the mass and fractional transport rate of 3 rapidly exchanging Mg metabolic pools, M1, M2, and M3. In the SHRs, plasma and erythrocyte Mg levels and urinary Mg excretion were not modified compared with their control WKYs, but tibia Mg level was significantly lower in the SHRs. Pool M3, the deep tissue pool, was significantly lower in SHRs compared with WKYs, but pools M1 and M2, the extracellular Mg pools, were statistically similar. The fractional transport rate of Mg from M1 to M2 and from M2 to M1 in the SHRs was higher than in the controls. The half-life of M1 was significantly decreased in SHRs compared with WKYs. In conclusion, this work demonstrates a decrease in intracellular Mg stores in SHRs compared with WKYs and disturbance of Mg exchanges in extracellular Mg, confirming a Mg metabolism disturbance in spontaneously hypertensive rats. Further work is now needed to elucidate the origin of the Mg depletion in SHRs and to explore Mg pools in hypertensive patients.

Comparison of inhibitory effects of Y-27632, a Rho kinase inhibitor, in strips of small and large mesenteric arteries from spontaneously hypertensive and normotensive Wistar-Kyoto rats

Since Y-27632, a specific inhibitor of Rho kinase, decreases the blood pressure in spontaneously hypertensive rats (SHR), it is suggested that Rho kinase is involved in the pathophysiology of hypertension. However, the effects of Y-27632 on isolated resistance arteries have never been determined. This study aimed to examine the possible role of the Rho/Rho kinase pathway during arterial contraction in isolated resistance arteries from SHR. The profile of arterial relaxant effects of Y-27632 was compared in endothelium-denuded strips of small and large mesenteric arteries from 13-week-old SHR and normotensive Wistar-Kyoto rats (WKY). The addition of 10(-6) mol/l norepinephrine (NE) to the strips of small arteries caused an initial peak followed by a tonic contraction in both strains. There was no difference between the two strains in either the initial peak or the tonic contraction. The addition of Y-27632 (0.3-3 micromol/l) to the tonic contraction of these strips caused a concentration-dependent relaxation in both strains. The relaxation was greater in SHR than in WKY. Similar results were observed in strips of large arteries. The relaxant effects of Y-27632 were greater in the large artery than in the small artery. Y-27632 also induced a concentration-dependent relaxation in strips precontracted with 65.9 mmol/l K+ depolarization. In both arteries, this relaxation was greater in SHR. The relaxant effects of Y-27632 were greater in the K+-contracted strips than in the NE-contracted strips. We conclude that Y-27632 shows the greater relaxant effects on the SHR arteries, and the effects are more evident in the large artery and in the K+-contracted strips.

Estriol improves membrane fluidity of erythrocytes by the nitric oxide-dependent mechanism: an electron paramagnetic resonance study

The present in vitro study was performed to investigate the effects of estriol (E3) on membrane fluidity of erythrocytes by means of an electron paramagnetic resonance (EPR) and spin-labeling method. E3 was shown to significantly decrease the order parameter (S) for 5-nitroxide stearate (5-NS) and the peak height ratio (ho/h-1) for 16-NS obtained from EPR spectra of erythrocyte membranes. This finding indicated that E3 might increase the membrane fluidity of erythrocytes. The effect of E3 was significantly potentiated by the nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine (SNAP), and a cyclic guanosine 3',5'-monophosphate (cGMP) analog, 8-bromo-cGMP. In contrast, the change in the membrane fluidity induced by E3 was antagonized by the NO synthase inhibitor, L-NG-nitroarginine-methyl-ester (L-NAME), and asymmetric dimethyl-L-arginine (ADMA). The results of the present study showed that E3 significantly increased the membrane fluidity and improved the microviscosity of erythrocyte membranes, partially mediated by an NO- and cGMP-dependent pathway. Furthermore, the data might be consistent with the hypothesis that E3 could have a beneficial effect on the rheological behavior of erythrocytes and may play a crucial role in the regulation of microcirculation.