Platelet calcium handling is different in rats with salt-dependent and spontaneous forms of genetic hypertension

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Gender-dependent difference in cell calcium handling in VSMC isolated from SHR: the effect of angiotensin II

OBJECTIVE

To investigate gender-dependent difference in the free cytosolic calcium concentration ([Ca2+ ]i ) response to angiotensin II (Ang II) in vascular smooth muscle cells (VSMC) isolated from spontaneously hypertensive rats (SHR). To further evaluate this gender-dependent difference by studying the role of thapsigargin-sensitive intracellular calcium stores and calcium influx in VSMC isolated from male and female SHR.

DESIGN AND METHODS

Confluent primary cultures of VSMC isolated from male (n = 14) and female (n = 14) SHR aged 10 weeks were used in this study. [Ca2+ ]i was measured by image analysis of single myocytes loaded with Fura-2. [Ca2+ ]i response of VSMC to Ang II was measured in the presence and absence of extracellular Ca2+, to evaluate the influence of Ca2+ influx. To characterize inositol triphosphate (IP3 )-sensitive sarcoplasmic reticulum calcium stores, thapsigargin-sensitive calcium stores were measured in VSMC isolated from SHR of both genders.

RESULTS

VSMC isolated from male SHR were characterized by an augmented [Ca2+ ]i response to angiotensin II in comparison with VSMC isolated from female SHR. Surprisingly, the thapsigargin-stimulated [Ca2+ ]i rise was found to be significantly greater in VSMC isolated from female SHR compared with VSMC isolated from male SHR. On the other hand, the gender-dependent difference in [Ca2+ ]i response to angiotensin II was abolished in the absence of extracellular calcium.

CONCLUSIONS

We demonstrated in VSMC isolated from SHR of both genders that a greater [Ca2+ ]i response to angiotensin II in male than female VSMC is dependent on Ca2+ influx.

Abnormal platelet function and calcium handling in Dahl salt-hypertensive rats

The effect of dietary salt on platelet function and Ca(2+) homeostasis was studied in Dahl (DS) rats, a genetic model of salt-sensitive hypertension. DS rats were fed a high-salt (DSHS) or a low-salt diet (DSLS) for up to 4 weeks, and the effects of salt loading on systolic blood pressure, platelet P-selectin expression, and platelet Ca(2+) homeostasis were measured. The high-salt diet increased blood pressure and markedly increased the amount of ionomycin (IM)-releasable Ca(2+) in platelet intracellular stores (Ca(2+)/IM). The alteration in Ca(2+) stores was not prevented when the hypertension was prevented by treatment with hydralazine and reserpine. The Ca(2+) store filling during platelet exposure to 1 mmol/L Ca(2+) for 5 minutes and the rate of sarcoplasmic/endoplasmic Ca(2+) ATPase-dependent Ca(45) uptake were higher in DSHS compared with that in DSLS. There was a decrease in thrombin-induced Ca(2+) influx in platelets from DSHS; consistent with this, agonist-induced P-selectin expression was decreased. In DSLS, nitric oxide accelerated reloading of platelet Ca(2+) stores after their emptying by thrombin but failed to do so in DSHS. These results indicate that in DS rats, a high-salt diet increases sarcoplasmic/endoplasmic Ca(2+) ATPase activity and the Ca(2+)/IM but decreases the reuptake of Ca(2+) caused by nitric oxide. Decreases in Ca(2+) influx and platelet P-selectin expression might be explained by changes in intracellular Ca(2+) stores in DSHS rats, which apparently is a heritable response to a high-salt diet.

Cytosolic pH and calcium in Dahl salt-sensitive and salt-resistant rats: the relationship to plasma lipids

OBJECTIVE

To search for alterations of cytosolic pH and cell calcium handling in platelets and erythrocytes of Dahl rats susceptible and resistant to salt-induced hypertension.

DESIGN AND METHODS

Blood pressure, plasma lipids, platelet cytosolic calcium concentration ([Ca2+]i) and pH (pHi) together with thrombin-induced changes in these parameters as well as erythrocyte [Ca2+]i and 45Ca influx were determined in Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats aged 9, 15 and 24 weeks, which were fed a low-salt diet (0.3% NaCl), and in animals fed high-salt diet (4% NaCl) for 5-10 weeks since weaning.

RESULTS

With a low salt intake platelet pHi was lower in SS/Jr than it was in SR/Jr rats, whereas basal platelet [Ca2+]i was similar in rats of both strains. The difference in basal pHi between SS/Jr and SR/Jr rats increased progressively with age of animals. A high salt intake from youth did not influence platelet [Ca2+]i in rats of either strain but it caused an earlier decrease in pHi in SR/Jr than it did in SS/Jr rats. Thrombin stimulation induced similar elevations of pHi and [Ca2+]i in rats of both strains, irrespective of age, salt intake and response of blood pressure to salt intake. Erythrocyte 45Ca influx and [Ca2+]i were greater for SS/Jr rats but only the latter parameter was correlated positively to blood pressure. Both regulation of platelet pHi and erythrocyte Ca2+ handling were significantly related to plasma lipid levels.

CONCLUSIONS

Platelets of SS/Jr rats fed a low-salt diet were characterized by a lower basal cytosolic pHi but unchanged [Ca2+]i relative to those of SR/Jr rats. Hypertension induced by high salt intake was associated with increased erythrocyte [Ca2+]i but not with elevation of platelet [Ca2+]i or alteration of response to stimulation with thrombin.