Arterial smooth muscle contractions in spontaneously hypertensive rats on a high-calcium diet.

OBJECTIVE: To study the effects of a high-calcium diet upon blood pressure, vascular smooth muscle contractions and intracellular free calcium in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. DESIGN: Eight-week old animals were placed on a normal-calcium diet (1.1% calcium; SHR and WKY rat groups) or a high-calcium diet (2.1% calcium; Ca-SHR and Ca-WKY rat groups) and observed for 12 weeks. METHODS: Blood pressure was measured indirectly by the tail-cuff method and in vitro smooth muscle responses were studied using a standard organ bath chamber. Platelets were used as a cell model for analysis of intracellular free calcium concentration, measured by the fluorescent indicator Quin-2. RESULTS: The blood pressure of Ca-WKY and WKY rats did not differ, but increased systolic blood pressure was attenuated in Ca-SHR compared with SHR. The concentration-response curves of mesenteric arterial rings for potassium chloride and noradrenaline were not affected by the high-calcium diet in either SHR or WKY rats. The time required for total relaxation after washout of contractile agents (washout time) was shortest in WKY and Ca-WKY rats after both agonists, and shorter in Ca-SHR than in SHR after noradrenaline. Smooth muscle responses were also studied by contracting the preparations with noradrenaline and potassium chloride in a calcium-free solution, after which, calcium was added to the organ bath in increasing concentrations. Calcium contraction responses were similar in WKY and Ca-WKY rats; SHR displayed an attenuated response to calcium addition in mesenteric rings stimulated by both agonists. After potassium chloride as agonist, the responses of SHR and Ca-SHR did not deviate but, after noradrenaline, a significant shift in the calcium contraction curve towards the normotensive curve was observed in Ca-SHR. Intracellular free calcium was clearly lower in WKY rats than in SHR, and was significantly reduced by calcium supplementation in the hypertensive but not the normotensive animals. CONCLUSIONS: A reduction in intracellular free calcium concentration and an effect upon receptor-mediated vascular smooth muscle contraction and excitation-contraction coupling may participate in the blood pressure lowering effect of a high-calcium diet.     

Endothelin-1 and vasopressin signalling in blood vessels of young SHR in comparison to adult SHR.

To examine potential intracellular signalling abnormalities of endothelin-1 (ET-1) and vasopressin (AVP) which may contribute to blood pressure elevation, contractility and inositol phosphate levels in intact arteries and calcium transients in vascular smooth muscle cells were investigated after stimulation with these peptides in pre-hypertensive 5 week-old spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) rats. Contractility of aorta in response to ET-1, AVP and norepinephrine (NE) was blunted in SHR relative to WKY. Contraction of mesenteric resistance arteries induced by ET-1 was similar in both groups, whereas sensitivity in response to NE and AVP was greater in SHR. Basal inositol phosphate in aorta and mesenteric arteries was elevated in SHR, but ET-1 and AVP-stimulated inositol phosphate responses were similar in both groups. Calcium transients induced by ET-1 and AVP in vascular smooth muscle cells were similar in young SHR and WKY. In contrast, in adult rats inositol phosphate responses to ET-1 were blunted in aorta of SHR, but were normal in mesenteric arteries. Inositol phosphate responses to AVP were similar in both rat strains of rats both in aorta and mesenteric arteries except for accumulation of inositol trisphosphate, which was enhanced in mesenteric arteries of SHR. Calcium mobilization in vascular smooth muscle cells from adult SHR also exhibited enhanced responses to AVP. In conclusion, in young SHR, blunted ET-1 and AVP-induced contraction in aorta and enhanced AVP-induced mesenteric artery contraction are associated with normal inositol phosphate production and calcium mobilization. Signal transduction in response to ET-1 and AVP is depressed in aorta of pre-hypertensive SHR after the step of inositol phosphate generation and calcium mobilization. Resistance vessel reactivity to AVP is enhanced in young SHR at steps following inositol phosphate generation and calcium mobilization. These results argue against a role of ET-1, but suggest the possible involvement of AVP in the development of this model of genetic hypertension.     

Structural and functional alterations of mesenteric vascular beds in spontaneously hypertensive rats

The morphology and reactivity of mesenteric arteries from spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar Kyoto rats (WKY) were investigated. Isolated, perfused mesenteric vascular beds were prepared from 6-, 11- and 18-week-old SHR and WKY. At these ages, the walls and media of large mesenteric arteries were significantly thicker in SHR than in WKY. The number of smooth muscle cell layers in the media was significantly larger in SHR than in WKY. This difference between SHR and WKY increased as rats grew older, in parallel with differences in the blood pressure. Flow rate-perfusion pressure curves indicated that the vascular basal resistance to flow increased more profoundly in SHR preparations than in WKY preparations as rats grew older. This may be related to the structural alterations of the resistance vessel wall in SHR. The pressor responses to KCl were greater in SHR preparations than in WKY preparations as rats grew older. This may be caused partly by the increase of the number of smooth muscle cell layers in the media of SHR resistance vessels. The pressor response to norepinephrine (NE) was significantly higher in SHR preparations than in WKY preparations at all ages investigated. In marked contrast to the vascular basal resistance and the pressor response to KCl, the pressor response to NE was extremely exaggerated in SHR at the age of 6 weeks. This extremely high NE response in younger SHR may not be caused by the structural alteration in resistance vessels. It may be caused by a functional change, which is regulated by the signal transduction process in smooth muscle cells of resistance vessels. These results suggest that the development of hypertension in SHR may be caused by genetic structural and functional abnormalities of resistance vessels. Both abnormalities may be caused by the hyperreactivity to NE through an altered signal transduction process in smooth muscle cells of resistance vessels in SHR.     

Enhanced neuropeptide Y immunoreactivity and vasoconstriction in mesenteric small arteries from spontaneously hypertensive rats

Enhanced sympathetic nerve activity is thought to play a role in the pathogenesis of hypertension. The purpose of the present study was to investigate the mechanisms underlying the enhanced vasocontractile response to perivascular stimulation of mesenteric arteries isolated from female spontaneously hypertensive rats (SHR). Innervation of mesenteric small arteries was evaluated by immunohistochemistry and confocal microscopy while functional studies were conducted in a microvascular myograph. The distribution of nerve terminals immunoreactive for tyrosine hydroxylase (TH) and neuropeptide Y (NPY) was similar in mesenteric small arteries from Wistar-Kyoto (WKY) and SHR rats. However, immunointensity of TH or NPY immunoreactivities were much higher in small arteries from SHR compared to WKY. Expressed as percentage of contractions elicited by 124 mM K(+), concentration-response curves for noradrenaline (NA) and NPY were shifted leftward in SHR compared with WKY rats. The combination of noradrenaline (1 microM) and NPY (10 nM) contracted mesenteric arteries from WKY and SHR to higher levels than compared to either contractile agent added alone. The NPY Y(1) receptor antagonist, BIBP 3226, inhibited these contractions with 87 +/- 0.7 and 80 +/- 1.3% (p < 0.05, n = 6) in arteries from WKY and SHR rats, respectively. In arteries incubated with the alpha(1)-adrenoceptor antagonist, prazosin, and preactivated with vasopressin, electrical field stimulation evoked contractions which were more pronounced in mesenteric arteries from SHR compared to WKY rats. BIBP 3226 partially inhibited these contractions. In vasopressin-activated arteries BIBP 3226 caused rightward shifts of the concentration-response curves for NPY in mesenteric arteries from SHR rats, but in addition it also abolished the maximal NPY contraction in arteries from WKY rats. In the presence of BIBP 3226, low concentrations (1 pM to 10 nM) of NPY caused relaxations in arteries from WKY, but not in segments from SHR rats. Mechanical removal of the endothelium abolished NPY relaxation in arteries from WKY. In arteries activated with vasopressin and exposed to either forskolin or sodium nitroprusside, the addition of NPY evoked contractions which were more pronounced in arteries from SHR compared to WKY arteries. The present study suggests that enhanced NPY content and vasoconstriction to NPY in arteries from hypertensive rats can contribute to the enhanced sympathetic nerve activity and vascular resistance in female hypertensive rats. Endothelial cell dysfunction as well as alterations in smooth muscle response to NPY seem to contribute to the enhanced vasoconstriction in arteries from hypertensive animals.     

Growth factors and extracellular signal-regulated kinase in vascular smooth muscle cells of normotensive and spontaneously hypertensive rats

OBJECTIVE: Transforming growth factor-beta1 (TGF-beta1) stimulates vascular smooth muscle cell growth in spontaneously hypertensive rats (SHR), but inhibits cell growth in normotensive Wistar- Kyoto (WKY) rats. The present study was undertaken to test the hypothesis that TGF-beta1 might differentially modulate the activities of mitogen-activated protein (MAP) kinase family members (ERK, JNK and p38) in vascular smooth muscle cells of SHR and WKY rats. METHODS: MAP kinase activity was measured from cultured vascular smooth muscle cells in response to TGF-1 by specific substrate phosphorylation of myelin basic protein, GST-c-Jun and GST-ATF2. RESULTS: Exposure of cultured vascular smooth muscle cells from SHR or WKY rats to TGF-beta1 resulted in a marked increase in the activity of ERK, but not of JNK or p38. The increase of ERK activity stimulated by TGF-beta1 appeared similar in time course and extent in both WKY and SHR cells, with increased activity peaking at 15 min of incubation. Epidermal growth factor (EGF) also stimulated the activity of ERK, in both WKY and SHR cells, but nor of JNK or p38, with stimulation of ERK activity by EGF occurring more rapidly in SHR cells than in those from WKY rats. Co-incubation of SHR cells with TGF-beta1 and EGF showed additive effect on ERK activity. CONCLUSIONS: The results provide the first evidence that TGF-beta1 activates ERK in vascular smooth muscle cells of both normotensive and hypertensive rats. The matching response of ERK activation to TGF-1 in SHR cells suggests that the MAP kinase-signaling pathway remains largely unchanged in the regulation of vascular smooth muscle growth by TGF-1 in spontaneously hypertensive rats.     

Comparison of Endothelin-1 and Noradrenaline Stimulated Inositol Phosphate Formation in Cultured Aortic Smooth Muscle Cells from Spontaneously Hypertensive and Wistar Kyoto Rats

Total [³H]-inositol phosphate formation was measured in cultured aortic smooth muscle cells from 6 and 14 week spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) normotensive rats. Basal inositol phosphate formation was significantly increased in cells cultured from SHR compared to WKY at both 6 and 14 weeks as was basal phosphatidylinositol formation. This difference in basal values was apparent after 9 h or more incubation with [³H]-myoinositol. Both endothelin-1 and noradrenaline stimulated inositol phosphate formation was unchanged in cultured smooth muscle cells from 6-week SHR compared to WKY. In cultured smooth muscle cells from 14-week SHR a decrease was observed in endothelin-1 stimulated inositol phosphate formation compared to controls. Noradrenaline stimulated inositol phosphate formation was increased in cultured cells from 14 week SHR. Endothelin-1 and noradrenaline stimulated inositol phosphate formation does not appear to be involved in the development (at 6 weeks) of hypertension in this model. However, in established hypertension (14 weeks) cells from SHR have altered total [³H] inositol phosphate formation in response to stimulation with noradrenaline and endothelin-1 although these changes are in opposite directions. Therefore, in cultured smooth muscle cells from 14-week rats noradrenaline and endothelin-1 appear to be regulated independently with regard to their effects on the phosphatidylinositol cycle.     

Studies of hypertension-induced vascular hypertrophy in cultured smooth muscle cells from spontaneously hypertensive rats

Mechanisms of vascular hypertrophy induced by hypertension were studied in cultured aortic smooth muscle cells from spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP) and compared with those from normotensive Wistar-Kyoto (WKY) rats. Fetal calf serum-stimulated ornithine decarboxylase (ODC) activity of cultured smooth muscle cells was greater in SHR and SHRSP than in WKY. Beta- but not alpha-adrenergic agonist stimulated ODC activity acutely in cultured smooth muscle cells from WKY, and isoprenaline-induced activation was blocked by the beta-blocker, propranolol, and enhanced by the phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine. These results indicate that cultured vascular smooth muscle cells from SHR and SHRSP are more prone to increase the protein synthesis than those from WKY through the trophic induction of ODC activity and that the regulation of ODC activity by catecholamines is mediated through beta-agonistic effect in cultured smooth muscle cells.     

Epidermal growth factor responsiveness in smooth muscle cells from hypertensive and normotensive rats

Aortic smooth muscle cells from spontaneously hypertensive rats (SHR) exhibit inappropriate proliferation characteristics in culture that suggest a modified response to serum mitogens or growth factors. The present study compares vascular smooth muscle cells from SHR and normotensive Wistar-Kyoto (WKY) rats with respect to their proliferative and functional response to growth factors. Specific attention was focused on the interaction of these vascular smooth muscle cells with epidermal growth factor. An increased growth rate of vascular smooth muscle cells from SHR (vs. WKY rats) was observed when cells were cultured in the presence of serum (10% and 0.5%), but not under serum-free conditions. The additional presence of low serum concentrations (0.5%) was required for epidermal growth factor to elicit a proliferative response, whereupon smooth muscle cells from SHR displayed an increased (vs. WKY rats) growth rate. Saturation binding of [125I]epidermal growth factor to intact smooth muscle cells indicated a twofold increase in receptor density in SHR-derived cells (p less than 0.001 vs. WKY rats) without an alteration in affinity for the growth factor. Cells derived from SHR also exhibited greater functional responsiveness to epidermal growth factor when compared with smooth muscle cells from WKY rats as evidenced by amplifications of both S6 kinase activation, phosphoinositide catabolism, elevation of intracellular pH, and DNA synthesis (nuclear labeling). We conclude that increased responsiveness of SHR-derived smooth muscle cells to epidermal growth factor could contribute to alterations in vascular smooth muscle growth and tone that may be fundamental to the pathogenesis of hypertension and atherosclerosis.     

Angiotensin II induced phosphorylation of myosin light chain in vascular smooth muscle cells from spontaneously hypertensive and normotensive rats

STUDY OBJECTIVE--The aim was to determine the changes in the phosphorylation of myosin light chain induced by angiotensin II in cultured vascular smooth muscle cells derived from normotensive (WKY) and spontaneously hypertensive rats (SHR). DESIGN--Extracts of vascular smooth muscle cells incubated with [32P]orthophosphoric acid were subjected to 4M urea-SDS electrophoresis, followed by autoradiography and laser densitometry. EXPERIMENTAL MATERIAL--Confluent primary cultures of vascular smooth muscle cells from aorta, superior mesenteric arteries and cerebral arteries were used. MEASUREMENTS AND MAIN RESULTS--The basal myosin light chain phosphorylation of SHR did not differ significantly from that of WKY. Stimulation with 1 nM angiotensin II increased incorporation of 32P into the myosin light chain, which peaked at 4 min and then slowly declined until 15 min. Exposure to angiotensin II (0.001-10 nM) for 4 min evoked a dose dependent increase in the phosphorylation of myosin light chain with a maximal response 40-45% above basal values. No significant differences in the response to angiotensin II were detected between cells derived from the two strains. Saralasin, a specific angiotensin II antagonist, did not affect the basal phosphorylation of myosin light chain but completely abolished the effect of angiotensin II. CONCLUSIONS--Angiotensin II enhances the phosphorylation of the myosin light chain from vascular smooth muscle cells in aorta, mesenteric arteries, and cerebral arteries, but there are no differences in response between SHR and WKY.     

Pertussis Toxin Sensitive Endothelin-1 Coupling to Inositol Phosphate Formation Via a Gtp-Binding Protein: Comparison in Shr and Wky Cultured Aortic Smooth Muscle Cells

The effects of pertussis toxin on endothelin-1 and noradrenaline coupling to inositol phosphate (IP) formation was investigated in cultured aortic smooth muscle cells from 14 week SHR and WKY rats. Endothelin-1 (10^-6M) stimulated IP formation was decreased in cells from SHR compared to WKY (WKY 1117±157, SHR 668±85% of basal). Pre-incubation with pertussis toxin produced a significant and similar reduction in endothelin stimulated IP production in both SHR (54% reduction) and WKY (55%). However, the observed reduction in endothelin-1 stimulated IP accumulation was still apparent in SHR when compared to WKY. Pertussis toxin preincubation followed by removal of extracellular calcium reduced further the endothelin responses by similar amounts in SHR and WKY cells, but SHR stimulated IP formation remained significantly decreased compared to WKY. The extent of pertussis toxin ADP-ribosylation of Gia was similar in both SHR and WKY cells. Endothelin-1 produced a reduction in the extent of ADP-ribosylation of Gia and this was of similar magnitude in both SHR and WKY cell membranes. In contrast, noradrenaline stimulated IP formation was unaffected by pertussis toxin pre-incubation. It was concluded that SHR cells do not appear to have an alteration in endothelin-1 activated, pertussis toxin sensitive G-protein coupling to IP formation or in the dependence of inositol phosphate formation on extracellular calcium.     

Effect of calcium antagonists on aortae isolated from normotensive and hypertensive rats: relaxation and calcium influx blockade

Verapamil and diltiazem were equally potent (ie, similar EC50s) in relaxing potassium-contracted aortas of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. The mechanical EC50s produced approximately 50% calcium influx blockade, suggesting a causal link between relaxation and calcium influx blockade. Nitrendipine was about 250 times more potent in relaxing aortic smooth muscle in SHR than in WKY rats (EC50s in -log [M] were 14.10 +/- 0.30 and 11.70 +/- 0.54, respectively). This difference was not affected by endothelial denudation, and was present when nitrendipine was used by preincubation rather than during established potassium chloride contractions. In spite of the different relaxant potency of nitrendipine in SHR and WKY rats, both strains showed similar EC50s for calcium influx blockade for this compound (9.21 +/- 0.36 in SHR and 8.75 +/- 0.26 in WKY). The dissociation between aortic smooth muscle relaxation and calcium influx blockade after nitrendipine was more pronounced in the SHR strain. This suggests that mechanisms other than or in addition to calcium influx blockade play a role in the relaxation of potassium-contracted vascular smooth muscle with dihydropyridine compounds, but not with other calcium antagonists.     

Effect of centrifugal force and catecholamines on glycosaminoglycans synthesis of vascular smooth muscle cells in culture

To evaluate the effect of hypertension on glycosaminoglycan (GAG) synthesis, cultured vascular smooth muscle cells (CVSMCs) from the aorta of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were exposed to centrifugal forces and catecholamines. GAG synthesis of CVSMCs was measured by the incorporation of [3H]glucosamine into GAGs which were secreted into the culture medium for 24 h. Basal level of GAG synthesis was much higher in SHR than in WKY, when expressed in terms of DNA contents. When exposed to centrifugal force, CVSMCs from rats of both strains synthesized more GAGs. GAG synthesis was enhanced by both noradrenaline (NA) and adrenaline (Ad) in WKY. The enhanced GAG synthesis in WKY by NA or Ad was prevented by pretreatment with propranolol, but not prazosin. In SHR, NA and Ad did not enhance GAG synthesis at this concentration of catecholamines. However, the effects of propranolol or prazosin on GAG synthesis in SHR, when incubated with either NA or Ad, were compatible with the phenomena observed in WKY. Adding dibutyryl cyclic AMP to the culture medium enhanced GAG synthesis in rats of both strains. These data suggest that not only the mechanical stress of high intra-arterial pressure but also beta receptor stimulation, via increasing cyclic AMP, enhance GAG synthesis of vascular smooth muscle cells in hypertension.     

Calcium currents are altered in the vascular muscle cell membrane of spontaneously hypertensive rats

Calcium currents were recorded during whole-cell voltage clamp in cultured azygos venous muscle cells from 1-3-day-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Different holding potentials were used to separate total cell current into its transient (T) and sustained or long-lasting (L) components. In recordings from 30 WKY and 30 SHR vascular cells, total cell calcium current was the same between cells from normotensive (167 +/- 20 pA) and hypertensive (139 +/- 15 pA) rats. However, the relative proportion of T and L calcium currents was different between WKY and SHR cells. In WKY cells, the peak amplitude of the L current was less than that of the T current (42 +/- 30% of total current), whereas in SHR cells, the L current was greater (62 +/- 3% of total current). Calcium currents in vascular muscle cells from SHR were activated and inactivated at more positive potentials than in cells from WKY. This study directly compares transmembrane calcium current in isolated cells from WKY and SHR blood vessels and shows that the proportions of T and L calcium channels activated by depolarization are altered in this genetic model of hypertension.     

Atrial natriuretic peptide: binding and cyclic GMP response in cultured vascular smooth muscle cells from spontaneously hypertensive rats

Atrial natriuretic peptide (ANP) is vasodilatory and natriuretic, but whereas increased plasma ANP levels occur in spontaneously hypertensive rats, their elevated vascular resistance suggests inappropriate target tissue responsiveness to ANP. This study examines ANP-receptor binding properties (at 25 degrees C and 4 degrees C) in cultured vascular aortic smooth muscle cells from spontaneously hypertensive (SHR) and control Wistar-Kyoto (WKY) rats. [I125]-human ANP saturation (0.0625-12.0 nmol) profiles were analyzed using nonlinear regression (LIGAND). Vascular smooth muscle cells from WKY possessed both high affinity (KD1 0.3 nmol; R1 33 fmol/10(5) cells) and low affinity (KD2 15 nmol; R2 400 fmol/10(5) cells) binding sites for ANP. In contrast, for smooth muscle cells from SHR, two receptor forms could not be resolved using identical analytical protocols. Parameter estimates at 25 degrees C and 4 degrees C were not different for either SHR or WKY. The number of receptors for SHR (Bmax approximately 100 fmol/10(5) cells) was lower than the total number of receptors for WKY (high plus low affinity approximately 430 fmol/10(5) cells). The intermediary KD value (approximately 1.0 nmol) for ANP binding in SHR suggests an ANP-receptor interconversion from high affinity to low affinity in smooth muscle cells from SHR. Competition-binding experiments also revealed a decreased affinity for ANP in SHR-derived smooth muscle cells. The cyclic GMP response (intracellular accumulation and extracellular levels) was decreased in SHR smooth muscle cells compared to WKY, although this difference was evident only after prolonged (one hour) stimulation with ANP. Our data indicate a reduced sustained vascular responsiveness to ANP in hypertension.     

In vivo DNA synthesis is not uniformly increased in arterial smooth muscle of young spontaneously hypertensive rats.

We compared the distribution of DNA synthesis over the arterial tree of young normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) with marginally elevated blood pressure. Six-week-old male SHR and WKY rats were therefore infused with 5-bromo-2'-deoxyuridine (BrdUrd) for 2 days and the nuclear incorporation of the thymidine analogue in the media of various arteries was determined by immunohistochemistry. In WKY rats, 2.5% of the arterial smooth muscle nuclei in elastic, muscular and resistance arteries incorporated BrdUrd. In SHR, DNA synthesis was more marked in large arteries than in resistance arteries. It was in addition significantly larger in the aorta, superior mesenteric, renal and femoral arteries of the SHR than in those of the WKY rats. However, nuclear incorporation of BrdUrd in vivo did not differ between SHR and WKY rats in aortic endothelium, carotid arterial smooth muscle, nor in mesenteric or renal resistance arteries. Between 6 and 20 weeks of age, the number of nuclear profiles per media cross-section did not increase in large arteries of WKY rats and SHR. During this period of time, however, carotid artery and thoracic aorta weight and DNA content increased. SHR large arteries gained more DNA than those of WKY rats. These data indicate that DNA synthesis is uniformly distributed over the arterial system in young WKY rats and that DNA synthesis is elevated in the smooth muscle of large arteries of 6-week-old SHR but not in their resistance arteries.     

Malfunction of arterial sarcoplasmic reticulum leading to faster and greater contraction induced by high-potassium depolarization in young spontaneously hypertensive rats.

The contribution of sarcoplasmic reticulum was studied with regard to the increase in arterial contraction induced by a high-potassium depolarization in spontaneously hypertensive rats (SHR). The 20 mmol/l potassium-induced contraction of femoral arteries was faster and greater in 6-week-old SHR than in age-matched normotensive Wistar-Kyoto (WKY) rats. Relaxation after washing the arteries with a Krebs solution was slower in SHR than in WKY rats. When the sarcoplasmic reticulum of SHR arteries had been depleted of calcium by caffeine in a calcium-free solution, the rate of high-potassium-induced contraction of the calcium-depleted SHR arteries was slowed, the same result as that with non-calcium-depleted WKY arteries. In ryanodine-treated arteries, the rate and magnitude of high-potassium-induced contraction were enhanced slightly in SHR and greatly in WKY rats, resulting in no final difference between SHR and WKY rats. Ryanodine slowed the relaxation rate in WKY rats but not in SHR. These results suggest that the diminution in ability of sarcoplasmic reticulum to sequester calcium may be responsible for the faster rate and greater magnitude of high-potassium-induced contraction with the slower relaxation in SHR arteries. We postulated that genetic malfunction of sarcoplasmic reticulum causes the increased contraction of arterial smooth muscle leading to the enhanced vasoconstriction and elevated blood pressure in SHR.     

Calcium channel alterations in genetic hypertension

We proposed earlier that voltage-dependent calcium (Ca2+) current is altered in single azygos venous cells from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). In this study, the effects of different intracellular concentrations of ethylene glycol-bis-N,N,N',N',-tetraacetic acid (EGTA) on Ca2+ currents were investigated. Vascular muscle cells from SHR and WKY rats were equilibrated with pipette solution containing 0.1 mM or 10 mM EGTA. Increasing the EGTA concentration from 0.1 to 10 mM in SHR vascular cells significantly enhanced the peak amplitude of the longer lasting (L) current from 87 +/- 12 pA to 152 +/- 8 pA, while the transient (T) current amplitude was not significantly different (52 +/- 7 pA and 36 +/- 7 pA, respectively). In WKY rat vascular muscle cells, the amplitudes of the T and L currents were not significantly different with the same comparison of intracellular EGTA concentrations. These observations suggest that relatively low intracellular Ca2+ concentrations can more strongly modulate Ca2+ current through the L channel in SHR than WKY rat vascular muscle cells.     

Vascular reactivity in the spontaneously hypertensive rat. Effect of high pressure stress and extracellular calcium

The role of extracellular calcium and high blood pressure stress in altered vascular adrenergic responsiveness in rings of femoral artery from spontaneously hypertensive rats (SHR) was investigated. A model in which partial ligation of the external iliac artery prevents the increase in blood pressure to the ipsilateral femoral artery was used to assess the effect of the increase in pressure stress on these alterations. Age-matched (5-week-old) male Wistar-Kyoto rats (WKY) and SHR were used in the study. Partial ligation was performed before a substantial increase in blood pressure occurred (6 weeks of age), and studies on vascular reactivity were carried out at 10 to 12 weeks of age when the SHR were considered hypertensive (indirect systolic blood pressure greater than 150 mm Hg). Maximal contractility of rings of unprotected femoral artery from the SHR in response to KCl in either a normal (2.5 mM) or low (0.25 mM) calcium Krebs solution was significantly greater (p less than 0.05) than was that of protected vessels from the SHR or protected and unprotected vessels from the WKY; however, no difference in the sensitivity to KCl was observed. Isoproterenol-induced relaxation was significantly attenuated in rings of vascular smooth muscle from unprotected femoral arteries of the SHR (p less than 0.05), while the responses of protected vessels from the SHR were similar to controls. Equilibration of vascular smooth muscle in a low calcium Krebs solution resulted in an increase in beta-adrenergic mediated relaxation in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)