The elevation of the cytoplasmic calcium ions in vascular smooth muscle cells in SHR--measurement of the free calcium ions in single living cells by lasermicrofluorospectrometry

Differential growth of craniofacial and tibial bones to sympathetic hyperactivity-related hypertension in rats

OBJECTIVE

To evaluate the effect of sympathetic nervous system hyperactivity on craniofacial skeletal growth in growing spontaneously hypertensive rats (SHRs).

DESIGN

Craniofacial skeletal growth was compared between male SHR and Wistar-Kyoto rats (WKR) using linear measurements on lateral and transverse cephalometric radiographs at the age of 12 weeks. Tibia length was measured as an index of whole body growth. Body weight　and blood pressure were measured from 3 to 12 weeks of age. Bone microstructure in the mandibular condyle and tibia between the two groups was compared at the age of 12 weeks using microcomputed tomography.

RESULTS

The SHRs had a significantly lower body weight than WKRs from 7 weeks of age, and tibial length was significantly smaller in the SHRs than in the WKR at 12 weeks of age. In all SHRs, blood pressure was significantly higher than in WKRs from 3 to 12 weeks of age. Cephalometric analyses revealed decreased measurements of the neurocranium, viscerocranium, and mandible in SHRs, and mandibular growth was most negatively affected in this group. Lastly, in SHRs, microcomputed tomography analyses revealed decreased bone mineral density and bone volume/tissue volume in the mandibular condyle but not in the tibia.

CONCLUSION

In growing SHRs, hypertension related to the hyperactivity of the sympathetic nervous system reduced craniofacial skeletal growth more than the growth of the tibia.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Altered mitochondrial function, capacitative calcium entry and contractions in the aorta of hypertensive rats

OBJECTIVE

It has been suggested that Ca entry through store-operated Ca channels (SOCs) is regulated by a dynamic interplay between the endoplasmic reticulum Ca stores and the mitochondria. These relationships drive the activation and inactivation of SOCs, yet it remains unclear whether this regulation of SOCs by mitochondria is altered in the aorta of spontaneously hypertensive rats (SHRs).

METHODS

We performed a thorough study of the mitochondrial membrane potential, the ability of mitochondria to deal with cytosolic Ca, capacitative Ca entry (CCE), and stromal interaction molecule 1 (STIM1) and calcium release-activated calcium modulator 1 (orai1) protein expression, as well as the contractile capacity of aortic rings, in normotensive Wistar Kyoto rats (WKYs) and SHRs.

RESULTS

Changes were observed in aortic tissue and cultured vascular smooth muscle cells isolated from SHRs relative to WKYs, including more depolarized mitochondria, stronger CCE upon the addition of Ca, larger cytosolic Ca transients (cytosolic Ca concentration) or aortic ring contraction elicited by endoplasmic reticulum depletion and a significant increase in STIM1 protein expression but not of orai1.

CONCLUSION

These results suggest that the impaired Ca buffering capacity of partially depolarized mitochondria dysregulates CCE, leading to overfilling of the endoplasmic reticulum Ca store through enhanced STIM1/orai1 interactions and an increase in aorta contractions in SHRs. Thus, understanding the implications of the alterations to STIM1/orai1, and their relationship to mitochondria, may aid drug development and therapeutic strategies to treat hypertension, as well as its long-term sequelae in poorly controlled patients.

Alveolar bone healing process in spontaneously hypertensive rats (SHR). A radiographic densitometry study

Hypertension is one of the most important public health problems worldwide. If undiagnosed or untreated, this pathology represents a systemic risk factor and offers unfavorable conditions for dental treatments, especially those requiring bone healing.

OBJECTIVES

The purpose of this study was to demonstrate, by analysis of bone mineral density (BMD), that the alveolar bone healing process is altered in spontaneously hypertensive rats (SHRs).

MATERIAL AND METHODS

Wistar rats and SHRs were submitted to extraction of the upper right incisor and were euthanized 7, 14, 21, 28 and 42 days after surgery. Right maxillae were collected, radiographed and analyzed using Digora software. BMD was expressed as minimum (min), middle (med) and maximum (max) in the medium (MT) and apical (AT) thirds of the dental alveolus.

RESULTS

The results were compared across days and groups. Wistar showed difference in med and max BMD in the MT between 7 and 28 and also between 14 and 28 days. The AT exhibited significant difference in med and min BMD between 7 and 28 days, as well as difference in min BMD between 28 and 42 days. SHRs showed lower med BMD in the MT at 28 days when compared to 21 and 42 days. Differences were observed across groups in med and min BMD at day 28 in the MT and AT; and in max BMD at 14, 21 and 42 days in the MT.

CONCLUSIONS

These results suggest that the alveolar bone healing process is delayed in SHRs comparing with Wistar rats.

Gene expression and functional activity of sodium/calcium exchanger enhanced in vascular smooth muscle cells of spontaneously hypertensive rats

Effects of hypertension on the function of the Na+/Ca2+ exchanger (NCX) were investigated by analyzing vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats. Angiotensin II-induced 45Ca2+ efflux from VSMCs mediated by NCX was enhanced by up to 3-fold in SHR compared with WKY, whereas ionomycin-induced Ca efflux mediated by NCX was not different between SHR and WKY. The decline rate from the peak value of intracellular 45Ca2+ concentration ([Ca2+]i) mobilized by angiotensin II was decelerated by removal of extracellular sodium (Na+o) in SHR but not in WKY. Gene expressions of NCX subtype 1 and angiotensin II receptor type1A assessed by quantitative RT-PCR were increased by 1.3- and 1.5-fold, respectively in SHR compared with WKY. NCX protein was also increased 1.6-fold in SHR compared with WKY. MEK inhibitor, PD98059, partly blocked the Nao-dependent acceleration of the [Ca2+]i recovery rate and tyrosine kinase inhibitor, genistein, diminished it in SHR. Genistein decreased angiotensin II-induced Nao- dependent 45Ca2+ efflux. However, angiotensin II did not enhance the tyrosine phosphorylation of NCX. These results suggest that acceleration of Ca2+ efflux from VSMCs of SHR was at least partly due to the enhancement of functional activity of NCX via increased gene expression and tyrosine phosphorylation in connection with hypertension.

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.

Coexisting independent sodium-sensitive and sodium-insensitive mechanisms of genetic hypertension in spontaneously hypertensive rats (SHR)

Some essential hypertensive patients and genetic hypertensive rat strains have less than the normal levels of Mg2+ tightly bound to the plasma membranes of their erythrocytes and other cells, i.e., the magnesium binding defect (MgBD). This binding defect appears to cause increased passive permeability of the membrane to Na+ and thereby its increased intracellular concentration, particularly if the Na+-extrusion enzyme systems of the cell are also defective. The Na+-Ca2+ exchange system in the cell membrane exports Na+ and imports Ca2+, increasing the tone of the smooth muscle cell and thus producing hypertension (HTn). This HTn is Na+-sensitive. Evidence supporting this postulate was obtained by determining the intraerythrocyte total concentrations of Na+, Ca2+, K+, and Mg2+ in two strains of spontaneously hypertensive rats (SHR and SS/Jr rats, having the MgBD together with the other requisites of the Na+-sensitive pathway) and their respective controls (WKY and SR/Jr rats, in which this complete pathway is absent). The Na+ and Ca2+ concentrations in the hypertensive rats were increased, and that of K+ was decreased. The concentrations of these cations were very similar in the two hypertensive strains. The level of membrane tightly bound Ca2+ in SHR erythrocyte membranes was significantly higher than those in the other three rat strains, which were not statistically different from each other. These results support previously reported evidence of the existence of a novel HTn-generating mechanism in the SHR rat, in which the intracellular Ca2+ concentration is increased as the result of the enhanced diffusion of this ion into the cell and the accompanying deficiency of the Ca2+ extrusion enzyme systems. This pathway is therefore Na+-insensitive, i.e., Ca2+-sensitive.Key words: essential hypertension, Na+-sensitive hypertension, Na+-insensitive hypertension, Ca2+-sensitive hypertension.

Potassium channel function in vascular disease

Potassium ion (K(+)) channel activity is a major regulator of vascular muscle cell membrane potential (E(m)) and is therefore an important determinant of vascular tone. There is growing evidence that the function of several types of vascular K(+) channels is altered during major cardiovascular diseases, such as chronic hypertension, diabetes, and atherosclerosis. Vasoconstriction and the compromised ability of an artery to dilate are likely consequences of defective K(+) channel function in blood vessels during these disease states. In some instances, increased K(+) channel function may help to compensate for increased vascular tone. Endothelial cell dysfunction is commonly associated with cardiovascular disease, and altered activity of nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor could also contribute to changes in resting K(+) channel activity, E(m), and K(+) channel-mediated vasodilatation. Our current knowledge of the effects of disease on vascular K(+) channel function almost exclusively relies on interpretation of data obtained by using pharmacological modulators of K(+) channels. As further progress is made in the development of more selective drugs and through molecular approaches such as gene targeting technology in mice, specific K(+) channel abnormalities and their causes in particular diseases should be more readily identified, providing novel directions for vascular therapy.

Contribution of sarcoplasmic reticulum calcium uptake and L-type calcium channels to altered vascular responsiveness in the aorta of renal hypertensive rats

This study examined whether alterations in intracellular or extracellular Ca2+ mobilization were related to differences in caffeine and phenylephrine (PHE)-induced contractions between two-kidney. one-clip hypertensive (2K-1C) and normotensive (2K) rat aortas. After depletion and reloading of intracellular Ca2+ stores, caffeine and PHE-induced contractions in Ca2+-free solution were increased in 2K-1C. Thapsigargin reduced the contraction to caffeine in 2K-1C and 2K with similar sensitivity. PHE-induced contraction in 1.6-mM Ca2+ solution was decreased in 2K-1C, and nifedipine was less effective in lowering this response. The responsiveness to extracellular Ca2+ was decreased in 2K-1C hypertensive rat aortas. Our results indicate an increased intracellular Ca2+ stores that are not related to alteration in Ca2+-ATPase function and a lower contribution of L-type channels to the contraction of 2K-1C aortas.

Combined SSCP and heteroduplex analysis of the human plasma membrane Ca(2+)-ATPase isoform 1 in patients with essential hypertension

In recent theories concerning the pathogenesis of essential hypertension, altered calcium homeostasis plays an important role. Increased intracellular Ca(2+) levels have repeatedly been reported in different cell types of hypertensive subjects. In vascular smooth muscle cells the plasma membrane Ca(2+)-ATPase (PMCA) represents the most important Ca(2+)-ejection system. Modifications of this pump therefore have been assumed to increase contractile tone of small vessels. For this reason, the purpose of this study was to determine if genetic alterations in the hPMCA1 gene might be associated with arterial hypertension. For detection of polymorphisms all 22 PMCA1 exons from 44 patients with essential hypertension (based on rigorous clinical data in addition to a positive family history) and from 40 normotensives without a family history of hypertension were PCR amplified and subsequently subjected to combined single-strand conformation polymorphism (SSCP) and heteroduplex (HTX) analysis. Despite the high sensitivity of almost 100%, differences could not be identified between hypertensives and normotensives within the two groups. These data indicate that at least in this population PMCA1 polymorphisms are presumably not related to common forms of essential hypertension. Furthermore, the high degree of evolutionary conservation of the PMCA1 gene underlines the pivotal role of this ATPase for cell physiology.

Sarcoplasmic reticulum in vascular cells in hypertension and during proliferation

1. Multiple sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and two types of sarcoplasmic reticulum Ca2+ channels, the ryanodine receptor and the inositol 1,4,5 triphosphate (IP3) receptor are expressed. The heterogeneity of the Ca2+ pumps and Ca2+ channels in vascular cells will be discussed. 2. An age-related change in expression of the SERCA isoforms is observed in smooth muscle cells. 3. The sarcoplasmic reticulum Ca(2+)-uptake rate and the level of SERCA 2 mRNA are different in thoracic than in abdominal aortas and in aortas from spontaneously hypertensive rats than from normotensive rats. 4. Proliferation of vascular smooth muscle cells is associated with major changes in intracellular Ca(2+)-handling mechanisms.

Strain-associated differences in hypoxic chemosensitivity of the carotid body in rats

Studies in humans indicate genetic effects on the ventilatory response to hypoxia, but the site of these effects is unknown. The present study explores the question of whether there are genetically directed effects on the intrinsic hypoxic chemosensitivity of the carotid body. The approach was to study these responses in two inbred rat strains [spontaneously hypertensive rats (SHR) and Fischer 344 (F-344)] and to measure in vivo carotid chemosensitivity as the change in carotid sinus nerve (CSN) activity during progressive, isocapnic hypoxia and the isolated, in vitro responses of excised superfused carotid bodies, loaded with the fluorimetric indicator fura 2, measured as the cytosolic calcium response to moderate hypoxia (PO2 = 55 mmHg). CSN responses in F-344 rats (n = 12) were uniformly low, with a shape parameter A of 13.8 +/- 6.59 (SE), whereas responses in SHR (n = 15) were sevenfold higher (108 +/- 24.1; P < 0.002) and showed greater variation. In vitro, intracellular calcium responses of superfused carotid bodies estimated from the fluorimetric ratio (340/380 nm) showed a greater peak increase during hypoxia in carotid bodies from SHR (140 +/- 4.7%) than from F-344 rats (114 6.0%; P < 0.01). Our results indicate strain-related differences in hypoxic chemosensitivity that are intrinsic to the carotid body and that could mediate genetic effects on ventilatory responsiveness to hypoxia.

Erythrocytic protein kinase C activity in primary hypertension

OBJECTIVES

Increased protein kinase C activity has been reported in erythrocytes from patients with primary hypertension and also from hypertensive rats. In this phenomenological study, we investigated whether a possible increased activity was the result of an augmented amount of enzyme molecules or a more active enzyme.

DESIGN

Collect blood samples, separate erythrocytes from other blood cells. After partial purification of protein kinase C in the erythrocyte lysate, assay the enzyme activity under optimal conditions using a specific peptide substrate.

SETTING

Central Hospital in Eskilstuna and University Hospital in Uppsala, Sweden.

SUBJECTS

Healthy individuals: 47 persons (20 women and 27 men). Ten patients with untreated primary hypertension.

MAIN OUTCOME MEASURES

Erythrocytes were separated from leucocytes and platelets by passing through a cellulose column followed by repeated washings. Some proteins in the erythrocyte lysate interfering with protein kinase C assay were removed by chromatography on DEAE-cellulose.

RESULTS

The mean protein kinase C activity in erythrocytes from healthy individuals was 0.18 +/- 0.02 pmol [32P]phosphate min(-1) x 10(6) cells, regardless of sex and age. The corresponding value for patients with primary hypertension was 0.16 +/- 0.04 pmol [32P]phosphate min(-1) x 10(6) cells.

CONCLUSIONS

The amount of protein kinase C, measured as the activity at optimal assay conditions, in erythrocytes from patients with primary hypertension is not critical for the development of moderate hypertension.

Intracellular free calcium abnormalities in fibroblasts from non-insulin-dependent diabetic patients with and without arterial hypertension

As arterial hypertension is frequently associated with diabetes, it is possible that altered intracellular free calcium ([Ca2+]i) handling, as reported in non-insulin-dependent diabetic patients, is accounted for by abnormalities caused by hypertension rather than diabetes. Our aim was to investigate [Ca2+]i transients triggered by two extracellular agonists, bradykinin and angiotensin II, with or without chronic insulin exposure, in cultured skin fibroblasts from 10 normotensive and 10 hypertensive non-insulin-dependent patients, matched for age, body mass index, and metabolic control, with fibroblasts from 10 healthy control subjects. Long-term cultured fibroblasts were loaded with fura 2-AM for measurement of [Ca2+]i. Resting [Ca2+]i levels were similar in the three groups of subjects. [Ca2+]i spikes stimulated by angiotensin II (0.1 mumol/L) and bradykinin (1 mumol/L) were significantly greater in hypertensive non-insulin-dependent diabetic patients (216 +/- 43 and 374 +/- 39 nmol/L, respectively) than in normotensive patients (174 +/- 16 and 267 +/- 55 nmol/L) and control subjects (188 +/- 29 and 320 +/- 78 nmol/L). Also, ionomycin evoked a greater [Ca2+]i response in hypertensive than normotensive non-insulin-dependent diabetic patients and in control subjects. Chronic insulin exposure increased by 70% to 90% the [Ca2+]i response to both angiotensin II and bradykinin in control subjects and normotensive non-insulin-dependent diabetic patients but not in hypertensive patients. The presence of abnormalities in [Ca2+]i transients in fibroblasts from only hypertensive non-insulin-dependent diabetic patients supports the possibility that these defects are a feature of concomitant arterial hypertension rather than of diabetes or its disturbed metabolic milieu.

Altered sodium-calcium exchange in afferent arterioles of the spontaneously hypertensive rat

Studies were performed to determine if there is a derangement in Na-Ca exchange activity in afferent (AA) and efferent (EA) arterioles from 3- and 9-week-old spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Cytosolic calcium concentration ([Ca2+]i) was assessed using microscope-based photometry in fura-2 loaded arterioles bathed in a Ringer's solution. Baseline [Ca2+]i was similar in the AA of 3- and 9-week-old WKY and SHR. In AA from 3-week-old rats, [Ca2+]i increased by 89 +/- 15 nM in WKY and by 73 +/- 13 nM in SHR during decreases in bath sodium concentration ([Na+]e) from 150 to 2 mM (Na+ replaced with n-methyl-D-glucamine). In 9-week-old hypertensive SHR (SBP = 150 mm Hg), increases in [Ca2+]i were attenuated (24 +/- 3 nM) relative to 3-week-old WKY and SHR, and 9-week-old WKY (90 +/- 9 nM; P < 0.05). Likewise, the rate of removal of Ca2+ in the continued presence of 2 mM Nae (Ca2+ sequestration and/or extrusion) was markedly reduced in AA of 9-week-old SHR (-0.15 +/- 0.03 nM/second) versus 3-week-old SHR (-0.72 +/- 0.12 nM/second) and 3- and 9-week-old WKY (-0.49 +/- 0.10 and -0.67 +/- 0.14 nM/second). In other experiments, AAs were preincubated in 1 mM ouabain to increase intracellular [Na+]. This maneuver augmented the increase in [Ca2+]i obtained with removal of Na+e; however, the responses obtained in 9-week-old SHR arterioles were still attenuated compared to those obtained in arterioles for 3- and 9-week-old WKY and 3-week-old SHR. These results suggest that exchanger number and/or sensitivity to the transmembrane Na gradient was reduced in the SHR AA. In EA, baseline [Ca2+]i was similar in 3- and 9-week-old WKY and SHR. In contrast to AA, the magnitude of Na-dependent and Na-independent changes in [Ca2+]i was not different in the EA of 3- and 9-week-old WKY and SHR. These results indicate that regulation of Na-Ca exchange activity may differ between AA and EA segments. Furthermore, diminished Na-Ca exchange and Na-independent Ca2+ sequestering/extrusion mechanisms could contribute to altered AA [Ca2+]i in the SHR.

Angiotensin II-stimulated phospholipase C responses of two vascular smooth muscle-derived cell lines. Role of cyclic GMP

Vascular smooth muscle cells of the spontaneously hypertensive rat (SHR) are known to show increased responsiveness to angiotensin II (Ang II) compared with cells of normotensive control Wistar-Kyoto rats (WKY). We investigated the hypothesis that differential levels of cGMP lead to the different responsiveness of the cells, using vascular smooth muscle cells in culture. cGMP levels in extracts of SHR-derived cells were lower than those of WKY-derived cells. This was true for both unstimulated cells and cells treated with equal concentrations of either sodium nitroprusside or S-nitroso-N-acetylpenicillamine. Stimulation of cells with Ang II did not affect levels of cGMP but increased levels of inositol 1, 4, 5-trisphosphate (IP3) and Ca2+, which were greater in SHR- than in WKY-derived cells. When SHR and WKY cells were preincubated with different concentrations of S-nitroso-N-acetylpenicillamine to generate similar cGMP levels in each cell type, the subsequent IP3 response to Ang II was the same in the two cell types. To reduce any influence of cGMP on responses, we permeabilized the cells with alpha-toxin. Stimulation of alpha-toxin-permeabilized the cells with high Ca2+ revealed an IP3 response in SHR- but not WKY-derived cells. Similarly, permeabilized SHR cells responded to Ang II but WKY cells did not. However, GTP and GTP gamma S elevated IP3 in both cell types. Taken together, these results indicate that the low response of WKY cells can be accounted for by the inhibitory influence of cGMP. However, when this inhibition is removed by permeabilization, further differences between the cells are revealed that will contribute to the elevated SHR response.

Alterations in rat interlobar artery membrane potential and K+ channels in genetic and nongenetic hypertension

The renal vasculature plays an important role in the control of blood pressure. K+ channels have been demonstrated to regulate smooth muscle membrane potential and thereby control smooth muscle tone. However, few data are available on K+ channel function in the renal vasculature of hypertensive animals. This study details changes in K+ currents and membrane potential in genetic and nongenetic models of hypertension. The patch-clamp technique and Ca(2+)-imaging fluorescence were used to examine the differences in Wistar-Kyoto (WKY), Sprague-Dawley (SD), spontaneously hypertensive (SHR), and deoxycorticosterone acetate (DOCA) hypertensive single cells of rat kidney interlobar arteries. In current-clamp experiments, SHR and DOCA hypertensive cells were approximately 20 mV more depolarized than the control cells. In voltage-clamp experiments with 4-amino-pyridine and niflumic acid present to inhibit voltage-dependent K+ (K(v)) and Ca(2+)-activated CI- (CI(Ca)) currents, SHR and DOCA hypertensive Ca(2+)-activated K+ (K(Ca)) currents were significantly larger and activated at more negative potentials than the control. Conversely, with charybdotoxin and niflumic acid present to inhibit K(Ca) and CI(Ca) currents, SHR and DOCA hypertensive K(v) current was significantly smaller than the control. Finally, basal and angiotensin II-stimulated peak intracellular free [Ca2+] was greater in the SHR and DOCA hypertensive cells compared with control cells. These results suggest that membrane potential and the activity of K(Ca) and K(v) channels are altered in hypertensive rat renal interlobar arteries and may play a role in the regulation of renal blood flow under physiological and patho-physiological conditions.

Spontaneous and agonist-induced contractions and endothelium-dependent relaxation in aortae from SHRSP and WKY rats under various levels of passive force

1. The influence of the passive force on the contraction and endothelium-dependent relaxation in aortae of normotensive Wistar Kyoto (WKY) rats and stroke-prone spontaneously hypertensive rats (SHRSP) were compared. 2. Force changes of endothelium-intact and -removed preparations were measured isometrically by a force-displacement transducer. Endothelium-dependent relaxation was observed by applying acetylcholine to the preparation precontracted in the presence of 5 x 10(-7) mol/L noradrenaline. 3. The preparations showed spontaneously developed tension (tone) that increased with the increase in the passive force. The effect of passive force was greater in preparations from SHRSP. Contraction initiated by noradrenaline was also increased by passive force up to 30 mN, then showed a tendency to decrease. 4. Endothelium-dependent relaxation was depressed as the passive force was increased. Preparations from SHRSP showed impaired endothelium-dependent relaxation and were influenced by passive force to a lesser degree when compared with preparations from WKY rats. 5. Relaxation by sodium nitroprusside was influenced by passive force to a much lesser extent than that by acetylcholine. 6. Indomethacin potentiated endothelium-dependent relaxation and blocked the effect of passive force in both preparations. 7. The difference in relaxation and the effect of passive force is primarily caused by the difference in the release of endothelium-derived contracting factor, which is thought to be a product of the cyclo-oxygenase pathway of the arachidonic acid cascade.

Effects of chronic oral administration of levcromakalim on in vitro contractile responses of arterial smooth muscle

It has been shown that oral administration of 0.038-0.15 mg/kg levcromakalim elicits a dose-related antihypertensive response in spontaneously hypertensive rats (Clapham et al., Arzneim. Forsch. 41 (1991) 385). In the present study, we examined the effects of long term administration of a high dose of levcromakalim on in vitro vascular contractility. Levcromakalim (2.25 mg/kg/day) was administered to the rats for 2 weeks and the thoracic aorta was then isolated. The levcromakalim treatment markedly reduced the relaxant effect of levcromakalim itself on norepinephrine-induced contraction. Relaxant effects of sodium nitroprusside and 8-bromo-cGMP were also attenuated by the levcromakalim treatment, although the relaxant effects of verapamil and forskolin were unchanged. The levcromakalim treatment decreased the threshold concentration for KCl and norepinephrine to induce contraction. The chronic levcromakalim treatment did not affect the cGMP production due to 3-isobutyl-1-methylxanthine and/or sodium nitroprusside. The aorta isolated from spontaneous hypertensive rats did not exhibit spontaneous activity in normal solution. After treatment with levcromakalim, however, the aorta showed spontaneous rhythmic contractions. Verapamil (10 microM) completely suppressed the spontaneous activity and decreased the basal tension below the original level. Similar to the effects of chronic treatment with levcromakalim, high-K+ solution (15.4 mM) augmented the contractile response to norepinephrine in the aorta of normotensive rats and induced rhythmic contractions in the aorta of spontaneously hypertensive rats. These results suggest that chronic treatment with a high dose of levcromakalim attenuates not only the effects of levcromakalim itself but also the cGMP-mediated relaxation, possibly by desensitizing the K+ channel.

Stimulation of two vascular smooth muscle-derived cell lines by angiotensin II: differential second messenger responses leading to mitogenesis

1. We show here that angiotensin II (AII) and endothelin-1 (ET-1) stimulate [3H]-thymidine incorporation in a smooth muscle cell line derived from aortae of spontaneously hypertensive rats (SHR), but not in cells derived from normotensive controls (WKY). We have used the differential response of the two cell lines to investigate the relationship between second messenger systems and the mitogenic response. 2. AII produced an increase in accumulation of inositol 1,4,5-triphosphate which was greater in the SHR-derived cell line than in the WKY cells. 3. AII gave an increase in cytosolic Ca2+ in each of the cell lines, with both a larger peak (15-30 s) and plateau response (2 min) in the SHR cells. ET-1 gave an enhanced response in the SHR-derived cells with respect to the peak but not the plateau of cytosolic Ca2+. 4. Phospholipase D activity was studied by monitoring the formation of [3P]-phosphatidylbutanol in 32Pi prelabelled cells. AII stimulation gave a larger phospholipase D response in the SHR-derived cells, while ET-1 gave a larger response in WKY-derived cells. 5. Stimulation of SHR-derived cells with 100 nM AII for 1 h, followed by 19 h in the absence of agonist, stimulated [3H]-thymidine incorporation over the next 4 h. When the 1 h stimulation with AII was in the presence of increasing concentrations of butanol, which diverts the product of the phospholipase D pathway, there was a loss of stimulated [3H]-thymidine incorporation which was significant at 10 mM butanol and at 30-50 mM reached a maximum loss of 40%. 6. Contrasting with this there was no apparent loss of ET-l-stimulated thymidine incorporation when butanol was present at concentrations up to 40 mM.7. These results suggest that phospholipase D is one of several pathways in the mitogenic response of SHR-derived vascular smooth muscle cells to All.

Glucose-induced alterations of cytosolic free calcium in cultured rat tail artery vascular smooth muscle cells

We have previously suggested that hyperglycemia per se may contribute to diabetic hypertensive and vascular disease by altering cellular ion content. To more directly investigate the potential role of glucose in this process, we measured cytosolic free calcium in primary cultures of vascular smooth muscle cells isolated from Sprague-Dawley rat tail artery before and after incubation with 5 (basal), 10, 15, and 20 mM glucose. Glucose significantly elevated cytosolic free calcium in a dose- and time-dependent manner, from 110.0 +/- 5.4 to 124.5 +/- 9.0, 192.7 +/- 20.4, and 228.4 +/- 21.9 nM at 5, 10, 15, and 20 mM glucose concentrations, respectively. This glucose-induced cytosolic free calcium elevation was also specific, no change being observed after incubation with equivalent concentrations of L-glucose or mannitol. This glucose effect was also dependent on extracellular calcium and pH, since these calcium changes were inhibited in an acidotic or a calcium-free medium, or by the competitive calcium antagonist lanthanum. We conclude that ambient glucose concentrations within clinically observed limits may alter cellular calcium ion homeostasis in vascular smooth muscle cells. We suggest that these cellular ionic effects of hyperglycemia may underlie the predisposition to hypertension and vascular diseases among diabetic subjects and/or those with impaired glucose tolerance.

Myofilament calcium sensitivity of normotensive and hypertensive resistance arteries

We measured intracellular Ca2+ and isometric force simultaneously in endothelium-denuded mesenteric resistance arteries of 12- to 15-week-old male spontaneously hypertensive rats (SHR). Wistar-Kyoto (WKY) rats, and Wistar rats. Basal Ca2+ did not differ among vessels of these strains (SHR, 86.6 +/- 4.5 nmol/L; WKY, 78.5 +/- 4.7 nmol/L; Wistar, 83.1 +/- 3.9 nmol/L). Myofilament Ca2+ sensitivity was determined by measuring the intracellular Ca2+ and force responses to cumulative addition of extracellular Ca2+ (0.025 to 2.5 mmol/L) in the presence of 100 mmol/L K+ or 10 mumol/L norepinephrine after depletion of releasable intracellular Ca2+ stores. With 100 mmol/L K+, no between-strain differences in active stress, intracellular Ca2+, or myofilament Ca2+ sensitivity were observed. With 10 mumol/L norepinephrine, the active stress response of SHR vessels to 0.025 and 0.05 mmol/L Ca2+ was increased compared with both normotensive strains. The intracellular Ca2+ response was not different in vessels of SHR and WKY rats but was depressed in Wistar vessels. Myofilament Ca2+ sensitivity of SHR was elevated compared with both WKY and Wistar rats (P < .05) (ED25 for SHR, 74.4 +/- 5.1 nmol/L; WKY, 89.8 +/- 5.5 nmol/L; Wistar, 86.9 +/- 3.4 nmol/L). No strain differences in intracellular Ca2+ or active stress responses of SHR and WKY vessels were detected during cumulative addition of norepinephrine with constant extracellular Ca2+ (1.5 mmol/L). These results indicate that no hypertension-associated defect in vascular Ca2+ handling exists in mesenteric arteries of the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction mechanisms of the vasoconstriction in hypertension

We tested the hypothesis that vascular smooth muscle in genetic hypertension is characterised by hypereactivity to vasoactive agonists, by abnormalities in Ca2+ handling and the phosphoinositide signalling system. Activation of these signal transduction mechanisms by noradrenaline and endothelin-1 was compared in isolated perfused tail arteries from adult hypertensive and normotensive Wistar Kyoto rats. Basal cytosolic Ca2+ was greater in arteries from hypertensive rats, but basal perfusion pressure and basal inositol phosphate accumulation were unchanged. Contractile responses and Ca2+ mobilisation after noradrenaline, but not endothelin-1, were enhanced in arteries from hypertensive rats. Total inositol phosphates accumulation was similar in hypertensive and normotensive rats after either noradrenaline or endothelin-1 stimulation. In both hypertensive and normotensive rats, for a given Ca2+ mobilisation, higher contractile responses and higher levels of inositol phosphates were observed after endothelin-1 than noradrenaline stimulation. In conclusion, changes in contractility associated with modifications in the Ca2+ handling between hypertensive and normotensive rats suggested that alterations in the signal-transduction system occur with hypertension. The different effects of endothelin-1 and noradrenaline could be related to interactions with other signalling pathways.

Pressure promotes DNA synthesis in rat cultured vascular smooth muscle cells

High blood pressure is one of the major risk factors for atherosclerosis. In this study, we examined the effects of pressure on cell proliferation and DNA synthesis in cultured rat vascular smooth muscle cells. Pressure without shear stress and stretch promotes cell proliferation and DNA synthesis in a pressure-dependent manner. Pressure-induced DNA synthesis was inhibited significantly by the phospholipase C (PLC) inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate, the protein kinase C inhibitor H-7, 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, staurosporine, and the tyrosine kinase inhibitor ([3,4,5-trihydroxyphenyl]methylene)propanedinitrile. To clarify whether activation of PLC and calcium mobilization are involved in pressure-induced DNA synthesis, production of 1,4,5-inositol trisphosphate (IP3) and intracellular Ca2+ was measured. Pure pressure increased IP3 and intracellular Ca2+ in a pressure-dependent manner. The increases in both IP3 and intracellular Ca2+ were inhibited significantly by 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate. This study demonstrates a novel cellular mechanism whereby pressure regulates DNA synthesis in vascular smooth muscle cells, possibly via activation of PLC and protein kinase C.

Intracellular calcium concentration and activation of the Na+/H+ exchanger in essential hypertension

To investigate the relationship between changes in intracellular calcium concentration ([Ca2+]i) and agonist-induced activation of the Na+/H+ exchanger in essential hypertension (EH), platelet [Ca2+]i and pHi were measured in 24 patients with EH (14 males) aged 48 +/- 2 years and 23 matched normotensive controls (NT) (12 males) aged 45 +/- 3 years. Measurements were done with spectrofluorimetry using the dyes Fura-2 for [Ca2+]i and BCECF for pHi. [Ca2+]i and pHi were measured in the resting condition and after stimulation in vitro with 0.1 U/ml human thrombin. The thrombin-induced rise in pHi was Na+ dependent and amiloride sensitive, indicating that it was mediated by the Na+/H+ exchanger. Unstimulated [Ca2+]i was higher in patients with EH than in NT (60 +/- 3 vs. 48 +/- 1 nmol/liter, P < 0.005), but there were no differences in resting pHi between both groups (7.16 +/- 0.01 vs. 7.16 +/- 0.008). In the presence of 1 mmol/liter external calcium (Ca2+o), thrombin-induced increment in [Ca2+]i was significantly greater in patients with EH than in NT (281 +/- 21 vs. 206 +/- 19; P < 0.05) as was the pHi increment (EH: 0.137 +/- 0.01; NT: 0.095 +/- 0.01; P < 0.05). Both agonist-induced increments in [Ca2+]i and in pHi were correlated with mean arterial pressure (MAP) only in the EH group (r = 0.58, P < 0.005 and r = 0.59, P < 0.005, respectively). The agonist-induced rise in pHi was positively correlated with the rise in [Ca2+]i both in the EH group (r = 0.65, P < 0.001) and in the NT (r = 0.55, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Arterial smooth muscle responses in adult and moderately aged spontaneously hypertensive rats

In order to further clarify differences between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats as well as the effects of ageing, vascular smooth muscle responses of mesenteric arterial rings and intracellular free calcium concentration ([Ca2+]i) in platelets and lymphocytes were studied in 20-week-old and 32-week-old animals. Arterial contractile responses induced by noradrenaline and potassium chloride were comparable in 20-week-old SHR and WKY rats, whereas at 32 weeks of age maximal contractile force generation to both of these agents was clearly lower in SHR. In both age groups the calcium entry blocker nifedipine was more effective in inhibiting potassium chloride-evoked responses in SHR than in WKY rats, and arterial relaxation responses by endothelium-dependent (acetylcholine) and endothelium-independent (nitroprusside, isoprenaline) mechanisms were more pronounced in WKY rats when compared with SHR. The ability of vascular smooth muscle cells to sequester calcium was evaluated by first depleting cellular calcium stores with maximal contractions to noradrenaline in calcium-free buffer, whereafter calcium was returned to the organ bath. After a 10 min. calcium loading period the arterial rings were rechallenged with noradrenaline. Both in 20-week-old and 32-week-old rats these responses were less marked in SHR than in WKY rats, suggesting reduced ability of smooth muscle cells to sequester calcium. In addition, platelets and lymphocytes were used as cell models to examine [Ca2+]i in the experimental groups by the fluorescent indicator quin-2. In these two cell types [Ca2+]i was higher in SHR than in WKY rats in both of the age groups studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in Ca2+ mobilization in platelets from stroke-prone spontaneously hypertensive rats

Intracellular free Ca2+, [Ca2+]i, levels were measured in platelets from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY) using fura-2AM. In the presence of extracellular Ca2+ (1 mM), [Ca2+]i levels in unstimulated platelets of 2- and 9-month-old SHRSP were both significantly higher than those of the age matched WKY. In the absence of extracellular Ca2+, the levels in platelets from 9-month-old SHRSP were also higher than any other groups examined. Receptor-linked Ca2+ influxes of old SHRSP were smaller when thrombin or collagen was given to the platelets. Phorbol 12-myristate 13-acetate (TPA) enhanced more prominently the Ca2+ influx into old SHRSP platelets than into old WKY platelets. These results strongly suggest that the Ca2+ permeability across plasma membrane is increased in young as well as old SHRSP platelets, where the resting [Ca2+]i level is highly sustained because of an impaired Ca2+ uptake mechanism and possible enhancement of protein kinase C activity.

Enhanced phospholipase D activity in vascular smooth muscle cells derived from spontaneously hypertensive rats

When cultured in the presence of fetal calf serum, aortic vascular smooth muscle cells (VSMC) derived from spontaneously hypertensive rats (SHR) grow faster than those from normotensive control Wistar-Kyoto (WKY) rats. In order to investigate the mechanism underlying this growth abnormality, we measured phospholipase D (PLD) activity in VSMC taken from both SHR and WKY rats. Upon stimulation with serum, platelet-derived growth factor (PDGF) and porbol 12-myristate 13-acetate (TPA), phosphatidylethanol (PEt) was produced in the presence of ethanol. The responses of the VSMC from SHR (SHR-cells) to all stimuli were significantly greater than those of the VSMC from WKY rats (WKY-cells), which suggests an enhanced PLD activity in the SHR-cells. Since PLD is regarded as an enzyme involved in signal transduction leading to cell proliferation, this PLD hyper-reactivity in the SHR-cells may account at least partially for the growth abnormality in the SHR-cells.

Modulation of rabbit aortic Ca(2+)-activated K+ channels by pinacidil, cromakalim, and glibenclamide

Rabbit aortic smooth muscle microsomes were isolated and large-conductance Ca(2+)-activated K+ (BK) channels incorporated into planar lipid bilayers. The selectivity sequence and relative permeability ratios for monovalent cations was K+ (1.0) > Rb+ (0.68) > NH4+ (0.14) >> Na+, Cs+ (< 0.05). Application of pinacidil or cromakalim (0.05-10 microM) shifted the probability of opening (Po)-voltage relationship in the hyperpolarizing direction. The concentrations of pinacidil and cromakalim required to increase Po 50% of the maximum value at -40 mV were 0.96 +/- 0.04 and 0.52 +/- 0.03 microM, respectively. Neither pinacidil nor cromakalim altered the voltage sensitivity of the channel (11-13 mV/e-fold change in Po). Kinetic analysis of data at -40 mV demonstrated that pinacidil (1 microM) decreased the length of time the channel dwelled in its long-closed state by 50% from 173 +/- 50 to 86 +/- 19 ms. No significant change was observed for the open time constant (20 ms). Glibenclamide (10 microM) had no effect on Po of BK channels. However, glibenclamide reversed the pinacidil- or cromakalim-stimulated increase in Po of BK channels. These data suggest that both cromakalim and pinacidil increased the probability of opening of single rabbit aortic large-conductance Ca(2+)-activated K+ channels and that this channel modulation may contribute to the vasorelaxant properties of these drugs.

Regulation of Ca2+ transport by platelet-derived growth factor-BB in rat vascular smooth muscle cells

The present study investigates the effects of platelet-derived growth factor (PDGF) isoform BB (PDGF-BB) on cytosolic Ca2+ concentration ([Ca2+]i), Ca2+ transport, and Ca2+ pools in rat vascular smooth muscle (VSM) cells. VSM cells from thoracic aorta of Milan normotensive rats were enzymatically dispersed, cultured in 10% serum medium, and made quiescent by 72 hours in 0.3% serum medium. [Ca2+]i, Ca2+ influx, Ca2+ efflux, and exchangeable cell Ca2+ pool were evaluated by ratiometric fluorescent and radioisotope techniques. Ca2+ transport showed time-dependent changes during stimulation with PDGF-BB. The initial early responses to this peptide were transient rise in [Ca2+]i, a 30% decrease in Ca2+ influx, and a 3.6-fold increase in the rate constant for active Ca2+ efflux. Stimulation of Ca2+ efflux and inhibition of Ca2+ influx were associated with a substantial 30% reduction in the cell Ca2+ pool. This initial stimulation of Ca2+ efflux is concomitant with Ca2+ mobilization into the cytosol and is due to activation of Na(+)-independent Ca2+ efflux via the Ca2+ pump. After a 10-minute stimulation, Ca2+ influx returned to the basal value, whereas Ca2+ efflux remained 2.2-fold above control values, leading to a decline in [Ca2+]i below basal levels and a further decrease in the cell Ca2+ pool. Nearly half of this late Ca2+ efflux appears to be driven by Na(+)-Ca2+ exchange, as evidenced by its external Na+ dependence. After a 120-minute stimulation with PDGF-BB, nifedipine-sensitive Ca2+ influx is increased 37% above basal levels, and Ca2+ efflux remains elevated. During prolonged stimulation by PDGF-BB, both Ca2+ influx and efflux are stimulated, resulting in a new intracellular Ca2+ homeostasis marked by the recovery of the cell Ca2+ pool but a lowered [Ca2+]i. These final events coincide with the initiation of cell proliferation in VSM cells by PDGF-BB.

Vascular smooth muscle cell calcium fluxes. Regulation by angiotensin II and lipoproteins

This study examined 45Ca uptake, 45Ca efflux, and the distribution of exchangeable 45Ca in confluent, quiescent cultures of aortic smooth muscle cells (VSMCs) from normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). These parameters were investigated under basal conditions and after addition of angiotensin II (Ang II) and low (LDL) and high (HDL) density lipoproteins. Basal 45Ca uptake was approximately 50% greater in VSMCs from SHRs (p < 0.005 versus WKY). Calcium antagonists (diltiazem or nifedipine) abolished this difference. The 45Ca uptake response to Ang II was approximately twofold greater in SHR than in WKY VSMCs (p < 0.05), and Ang II-induced increments of 45Ca uptake were weakly inhibited (by approximately 15-25%) by calcium antagonists. Lipoproteins also stimulated 45Ca uptake in VSMCs, and the apparent affinity of this process was approximately fivefold greater for LDL than for HDL. Calcium antagonists did not inhibit either LDL- or HDL-induced 45Ca uptake. SHR and WKY VSMCs did not differ with respect to 45Ca uptake induced by either LDL or HDL. The initial size of the slowly exchangeable pool of intracellular Ca2+ was approximately 35% greater in SHR VSMCs (p < 0.05 versus WKY). Ang II-induced mobilization of intracellular calcium (measured as the decrease in 45Ca content of the slowly exchangeable pool) was threefold greater in SHR VSMCs (p < 0.005 versus WKY). LDL and HDL marginally stimulated 45Ca efflux from this pool (< or = 20% above control) and to comparable extents in both SHR and WKY VSMCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in sarcoplasmic reticulum Ca(2+)-ATPase and alpha-smooth muscle actin gene expression in aortas of normotensive and spontaneously hypertensive rats

The expression of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase gene and the SR Ca2+ pump function were investigated in thoracic aortas of 5- and 17-week-old normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). The relative level of the two isoforms of SR Ca(2+)-ATPase mRNA expressed in the aorta (i.e., SERCA 2a and SERCA 2b) was determined by quantitative S1 nuclease protection analysis and normalized to the level of alpha-smooth muscle (alpha-Sm) actin mRNA. The level of alpha-Sm actin mRNA itself was normalized to the level of 18S ribosomal RNA using slot-blot hybridization assays. Total SR Ca2+ pump activity was estimated by measuring the rate of oxalate-supported Ca2+ uptake in homogenates. At 5 weeks, the amount of SERCA 2a and SERCA 2b mRNA, normalized to 18S ribosomal RNA, and the ratio of alpha-Sm actin mRNA to 18S RNA were identical in SHR and WKY rats. The Ca2+ pump activity was similar in the two strains of rats at 5 weeks. From 5 to 17 weeks, the amount of SERCA 2a mRNA increased in both strains while the level of SERCA 2b mRNA remained constant. The Ca2+ pump activity was unchanged in SHRs and tended to decrease in WKY rats. Accordingly, the change in the ratio of the SR Ca(2+)-ATPase mRNA isoforms does not appear to influence SR function. The level of alpha-Sm actin mRNA and SERCA 2a mRNA increased in parallel from 5 to 17 weeks in both strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in activities of thromboxane A2 receptor antagonists in smooth muscle cells

Thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors were characterized in rat vascular smooth muscle cells (VSMC). The specific binding of [3H]SQ 29,548 was inhibited by KW-3635, a novel non-prostanoic TXA2 antagonist, SQ 29,548 and BM-13505 (daltroban). SQ 29,548 showed a single class of binding sites with a Ki value of 1.6 nM. The inhibition patterns were better fit to two-component curves for KW-3635 (Ki values of 0.45 nM and 42 nM) and BM-13505 (2.3 nM and 20 nM). U46619, a TXA2 agonist, induced an increase in intracellular calcium concentration ([Ca2+]i), which was inhibited by these antagonists. KW-3635 and SQ 29,548 did not induce any increase in [Ca2+]i, whereas BM-13505 was found to induce a smaller increase in [Ca2+]i. The BM-13505-induced increase in [Ca2+]i was also inhibited by pretreatment with KW-3635, SQ 29,548 and BM-13505. The results demonstrate that BM-13505 has partial agonistic activity on TXA2/PGH2 receptors, and KW-3635 and SQ 29,548 do not. SQ 29,548 and BM-13505 inhibited both U-46619- and BM-13505-induced increases in [Ca2+]i to a similar degree. Alternatively, KW-3635 inhibited a U46619-induced increase in [Ca2+]i more effectively than a BM-13505-induced increase. These results suggest the heterogeneity of functional binding sites or subtypes of TXA2/PGH2 receptors present in VSMC.

Hypotensive drugs and the cellular energetic potential

Clinical and experimental investigations have revealed changes of the Na, K, Ca and Mg concentration in serum and cells in patients with essential arterial hypertension (EAH). Some hypotensive drugs correct, more or less, the electrolytic abnormalities at the same time as the decrease of arterial pressure. Epidemiological studies have shown a lower arterial blood pressure in populations with decreased intake of Na and an increased intake of K, Ca and Mg. However the most frequently encountered electrolyte anomaly in patients with EAH is hypophosphatemia, mainly induced by a shift of inorganic phosphate from serum into the cells. Probably the increased influx of inorganic phosphate from the serum, increases the arterial cellular energetic potential. As hypotensive drugs lead to a reduction of the inorganic phosphate shift from serum into the cells one may suggest that the obtained decrease of arterial blood pressure may be at least partially due to a diminished energetic potential of certain cells.

Transcriptional regulation of calcium-activated neutral protease in cardiomyocytes of hypertensive rats

Increased enzymatic activity of calcium-activated neutral protease II (CANP II) has been reported previously in cardiac tissues of rats with 2 kidney, 1 clip Goldblatt hypertension (2K, 1C-HT); this was associated with elevated intracellular free Ca(++). Because it was suggested that increased levels of the enzyme were responsible for the enhanced activity, CANP II mRNA expression was assessed in cardiomyocytes isolated from 2K, 1C-HT rats and from a genetic model of hypertension, the spontaneously hypertensive rat (SHR). Utilizing a rabbit probe for the large subunit of CANP II (pLM 1006 cDNA), a 3.7 kilobase mRNA band was visualized in Northern blots of poly A+ RNA. Densitometric analysis of the blots revealed that there was a significant (p < 0.005) increase in the levels of CANP II large subunit mRNA in cardiomyocytes of 2K, 1C-HT rats when compared with controls. Interestingly, CANP II mRNA levels were comparable in cardiomyocytes of SHR and Wistar Kyoto (WKY) rats. Results of nuclear runoff transcription assays indicated that enhanced expression of CANP II mRNA in 2K, 1C-HT rat hearts was regulated at the transcriptional level. The data support specific CANP II gene activation in the hearts of renal hypertensive rats.

Phospholipase C responses in cells from spontaneously hypertensive rats

We tested the hypothesis that increased systemic vascular resistance in spontaneously hypertensive rats may be secondary to enhanced phospholipase C activity in response to vasoconstrictor stimuli. Activation of phospholipase C by angiotensin II (Ang II), thromboxane A2, arginine vasopressin, and endothelin-1 was compared in cultured glomerular mesangial cells and mesenteric vascular smooth muscle cells taken from 13- to 14-week-old hypertensive and normotensive Wistar-Kyoto rats (blood pressure, 185 +/- 1 versus 135 +/- 2 mm Hg). Phospholipase C was assessed by measuring cytosolic free calcium and by the accumulation of radiolabeled inositol phosphates. Basal cytosolic calcium did not differ between mesangial cells taken from both strains but was greater in smooth muscle cells from hypertensive rats (210.1 +/- 8.2 versus 149.2 +/- 4.7 nM). The responsiveness of cytosolic calcium and inositol phosphate accumulation to Ang II was significantly enhanced in mesangial cells from hypertensive rats (10(-7) M Ang II: peak increase of calcium, 1,266 +/- 181 versus 603 +/- 93 nM; percent increment of inositol phosphates at 1 minute, 266 +/- 26 versus 98 +/- 10%). Vascular smooth muscle cells from hypertensive rats, when compared with normotensive rats, showed a similar augmentation of Ang II-stimulated intracellular calcium and inositol phosphates. Thromboxane A2-induced enhancement of intracellular calcium and inositol phosphate accumulation in vascular smooth muscle cells was also greater in hypertensive animals. However, the responses to vasopressin and endothelin in mesangial or vascular smooth muscle cells did not differ between the normotensive and hypertensive animals. There was no significant difference in Ang II receptor number and affinity between hypertensive- and normotensive-derived mesangial cells. We conclude that genetically increased blood pressure in rats may be secondary to enhanced post-receptor signaling in glomerular mesangial cells activated by Ang II and to enhanced signaling in vascular smooth muscle cells stimulated by either Ang II or thromboxane A2.

Increased Na+/H+ exchange activity in vascular smooth muscle cells of spontaneously hypertensive rats and possible involvement of protein kinase C

1. Na+ influx into cultured vascular smooth muscle cells (VSMC) obtained from spontaneously hypertensive rats (SHR) and from Wistar-Kyoto rats (WKY) was measured. Na+ influx via the Na+/H+ exchange system was measured as the rate of 22Na+ influx into cultured VSMC sensitive to ethylisopropylamiloride (EIPA), a specific inhibitor of the exchange system. 2. The total 22Na+ influx rate in SHR was significantly higher than in WKY (6.08 +/- 0.16 vs 4.13 +/- 0.09 nmol/min per mg protein; P less than 0.001; n = 14). The EIPA (1 X 10(-4) mol/L)-sensitive 22Na+ influx rate in SHR was significantly higher than that in WKY (4.32 +/- 0.27 vs 2.17 +/- 0.14 nmol/min per mg protein; P less than 0.001; n = 14). There was no difference in EIPA-insensitive 22Na+ influx between SHR and WKY. The EIPA-sensitive 22Na+ influx rate into VSMC was significantly decreased in SHR but not in WKY by the addition of 1 X 10(-4) mol/L 1-(5-isoquinoline-sulfonyl)-methylpiperazine (H-7), an inhibitor of protein kinase C (PK-C). 3. These results suggest that the increase in Na+ influx in SHR may be due to elevation of the Na+/H+ exchange activity, and possible involvement of PK-C in the increased Na+/H+ exchange activity in VSMC from SHR.

Rise and fall of agonist-evoked platelet Ca2+ in hypertensive rats

We previously reported an enhanced peak response of intracellular free Ca2+ to thrombin in platelets of spontaneously hypertensive rats in comparison with normotensive Wistar-Kyoto rats. In the present study, we compared the platelet intracellular Ca2+ response to the receptor-linked agonist thrombin with the response to the Ca2+ ionophore ionomycin. Basal intracellular Ca2+ was higher in hypertensive platelets as was leakage of fura-2. We confirmed the previous finding that the thrombin-induced intracellular Ca2+ peak is greater in platelets of hypertensive rats and noted that the rate of recovery from peak intracellular Ca2+ is significantly greater in this model. In contrast, the peak platelet intracellular Ca2+ response to ionomycin (50 nM and 5 microM) was not different between the two strains, and the rate of recovery from the peak response was only slightly depressed in hypertensive rats after the low dose of ionomycin. Internal Ca2+ discharge capacity, assessed by the intracellular Ca2+ response to a maximal dose of ionomycin in Ca(2+)-free medium, was not different between platelets of the two strains. Thus, activated platelet intracellular Ca2+ is not altered in the hypertensive rat when the nonphysiological ionophore ionomycin is used as agonist. However, a heightened intracellular Ca2+ response is observed when the receptor-mediated agonist thrombin is used. These results are consistent with the hypothesis that differences in receptor-linked second messenger pathways underlie the altered intracellular Ca2+ response in platelets of genetically hypertensive rats and may contribute to differences both in the mobilization of Ca2+ and in its fall.

Role of sarcolemmal membrane sodium-calcium exchange in vascular smooth muscle tension

A body of information obtained by experiments with intact tissues, isolated cells, and sarcolemmal vesicles indicates, beyond a reasonable doubt, that a specific Na(+)-Ca2+ exchange system exists in vascular smooth muscle. However, its role in the regulation of cytosolic free-Ca2+ concentration and cell tension under physiological conditions remains unclear. Under pharmacological conditions in which the Na(+)-K+ pump is inhibited either by digitalis glycosides or K(+)-free medium, Na(+)-Ca2+ exchange may be modulated by increases in cytosolic free Na+ to increase the cytosolic free-Ca2+ concentration and cell tension. Under pathological conditions in which the cytosolic Na+ concentration is increased as a result of inhibition of the Na(+)-K+ pump by endogenous ouabain or a digitalis-like factor, or activation of the Na(+)-H+ exchange or passive permeability of Na+, the Na(+)-Ca2+ exchange activity of vascular smooth muscle and the nerve terminal may play an important role in the development and/or maintenance of hypertension. These and other premises remain to be confirmed or discounted.

Erythropoietin increases cytosolic free calcium concentration and thrombin induced changes in cytosolic free calcium in platelets from spontaneously hypertensive rats

Using fura-2 cytosolic free calcium concentrations were measured in intact washed platelets from 9 spontaneously hypertensive rats (SHR) and from 9 age-matched normotensive Wistar-Kyoto rats (WKY). In resting platelets cytosolic free calcium concentration was significantly higher in SHR than in WKY (171.8 +/- 64.4 nM vs 93.1 +/- 59.0 nM, p less than 0.05). After preincubation with erythropoietin cytosolic free calcium concentration was significantly higher in SHR than in WKY (197.5 +/- 83.2 vs 93.0 +/- 60.1, p less than 0.01). Using platelets from SHR erythropoietin increased mean resting cytosolic free calcium concentration by 14.9% (p less than 0.05) and mean thrombin induced changes of cytosolic free calcium by 58.3% (p less than 0.01). In contrast, erythropoietin caused no significant increase in the resting calcium concentration or in thrombin induced changes of cytosolic free calcium in platelets from WKY. It is concluded that erythropoietin is involved in the pathogenesis of hypertension by elevating cytosolic free calcium concentration.

A new method of determining intracellular free Ca2+ concentration using Quin2-fluorescence

We determined intracellular free Ca2+ concentration by fluorescence spectroscopy and the time-resolved measurements of 2-[(2-amino-5-methylphenoxy) methyl]-6-methoxy-8-aminoquinoline-N,N,N',N'-tetraacetic acid, tetrapotassium salt (Quin2) incorporated in suspended mouse leukemia L1210 cells. The paper reports the following two points. (1) Various fluorescence spectrum patterns in cell suspensions dissolved with Quin2 acetoxy methylester were compared with those of the complex in buffer solution containing esterase. (2) The fluorescence lifetime of Quin2 bound to Ca2+ was approx. 4.5-11 times longer (10 +/- 1 ns) than that (1.5 +/- 0.5 ns) of Quin2. The fraction of the long lifetime component was plotted against the concentration of CaCl2 in buffer solution. From the results obtained, it was found that approx. 35 nM Ca2+ was contained in each L1210 cell.

Reduced calcium sensitivity of dihydropyridine binding to calcium channels in spontaneously hypertensive rats

To explore the role of calcium channels in hypertension, dihydropyridine ([3H]PN200-110) binding to heart, brain, and skeletal muscle microsomes of 4-, 8- and 15-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats was measured. At a constant Ca2+ ion concentration (pCa 3.0), maximal binding (Bmax) of dihydropyridine binding to heart and brain microsomes was significantly enhanced in 8- and 15-week-old SHR compared with WKY rats (p less than 0.01), whereas this phenomenon was not observed in 4-week-old SHR and WKY rats. Bmax and dissociation constant (Kd) values for skeletal muscle microsomes from SHR showed no difference compared with WKY rats irrespective of age. Dihydropyridine binding to heart microsomes, brain microsomes, and solubilized skeletal muscle microsomes exhibited strong calcium dependence. The Ca2(+)-dependent dihydropyridine binding curves for heart showed a Hill slope, and pK 0.5 values for 15-week-old SHR and WKY rats were 0.70 +/- 0.12 and 4.66 +/- 0.12 versus 0.72 +/- 0.12 and 5.66 +/- 0.08 (n = 4, mean +/- SD), respectively, indicating that 15-week-old SHR require 10-fold higher calcium concentration than WKY rats to promote dihydropyridine binding. The pK 0.5 values of calcium for brain and solubilized skeletal muscle calcium channels in 15-week-old SHR were also significantly lower than in WKY rats. This difference first became apparent in SHR and WKY rats as early as 4 and 8 weeks after birth. These results suggest that enhancement of calcium channel density might occur in the heart and brain of SHR in response to elevated blood pressure and that reduced calcium sensitivity of dihydropyridine binding to calcium channels might be a primary characteristic of this rat strain.

Cytosolic free calcium of aorta in hypertensive rats. Chronic inhibition of angiotensin converting enzyme

Cytosolic free calcium concentration ([Ca2+]i) and muscle tension were simultaneously measured in aortic tissue isolated from spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto (WKY) rats, and SHR chronically treated with a novel angiotensin converting enzyme inhibitor, CS-622. In the presence of 2.5 mM Ca2+ in the bathing solution, aortic [Ca2+]i measured with fura-2 was higher in SHR than in WKY rats, and it was almost the same in CS-622-treated SHR and untreated WKY rats. Increase of external Ca2+ concentration from zero to 2.5 mM elicited a contraction in SHR aortas but not in aortas from both CS-622-treated SHR and untreated WKY rats. When the aortas were contracted by 60 mM K+, however, [Ca2+]i as well as developed tension was similar in the three groups. CGP-28392 (10(-6) M), a Ca2+ channel activator, induced a rhythmic activity superimposed on a gradual increase of [Ca2+]i and tension in SHR aortas but not in the aortas of CS-622-treated SHR or untreated WKY rats. Nicardipine (10(-7) M) decreased the resting [Ca2+]i and the resting tone in SHR aortas, but not in WKY rat aortas. These results suggest that SHR aortas have a higher myogenic tone due to increased [Ca2+]i than WKY rat aortas and that the increased [Ca2+]i is attributed to alterations of dihydropyridine-sensitive Ca2+ channels in SHR aortas. Further, the decrease of the vascular tone induced by long-term administration of the angiotensin converting enzyme inhibitor may be due to a reduction of increased [Ca2+]i in SHR.

1,2-diacylglycerol content in thoracic aorta of spontaneously hypertensive rats

Phosphoinositide metabolism participates in the control of cell calcium homeostasis. Because a notable neutral lipid (1,2-diacylglycerol) is generated from phosphoinositide hydrolysis and is assumed to be a secondary messenger, we determined 1,2-diacylglycerol content and its fatty acid profiles in the thoracic aorta of spontaneously hypertensive rats (SHR) and compared it with those of normotensive Wistar-Kyoto (WKY) rats. After the aorta was exposed to 10(-5) M norepinephrine as a stimulant, 1,2-diacylglycerol content in SHR was significantly higher by 33% than in WKY rats at 4 weeks of age, whereas there was no difference in 1,2-diacylglycerol content between the two strains at 20 weeks of age. Before norepinephrine stimulation, there was no significant difference in 1,2-diacylglycerol level between the two strains at 4 weeks of age. Analysis on a gas chromatograph showed that 1,2-diacylglycerol was composed of similar molecular species of fatty acids in aortas obtained from SHR and WKY rats. On the other hand, the cholesterol content of aortas was higher in SHR than in WKY rats at 20 weeks of age, whereas the difference at 4 weeks was not significant. Phosphatidylcholine, phosphatidylethanolamine, and triglyceride showed no significant difference between the two strains. It is concluded that norepinephrine-induced 1,2-diacylglycerol production increases in the thoracic aorta of SHR before the development of hypertension.