Electric muscle stimulation in the hind leg of the spontaneously hypertensive rat induces a long-lasting fall in blood pressure

The influence of prolonged low-frequency, low-intensity electric stimulation of the gastrocnemius muscle or of the biceps femoris muscle on blood pressure and heart rate was investigated in unanaesthetized, spontaneously hypertensive rats (SHR). In both groups, elevations of blood pressure and heart rate were elicited during the 60 min of muscle stimulation. After cessation of the stimulation, a depressor response developed within 60 min. Thirty to sixty minutes post-stimulation the fall in blood pressure was 19 ± 3 and 17 ± 4 mmHg, respectively (mean ± SE) compared with controls. In both groups, the depressor response lasted for over 5 h. In addition, the gastrocnemius-stimulated animals also developed a post-stimulatory bradycardia. In one group of SHR the sciatic nerve was anaesthetized with bupivacaine. The arousal response during stimulation was similar to that in the other groups, but after termination of stimulation blood pressure returned to the control level without any further drop. To investigate further the neurotransmitters involved, one group of gastrocnemius-stimulated SHR was given naloxone by infusion during the stimulation. A modest post-stimulatory blood pressure fall also occurred in this group, but it lasted only 90 min. Another SHR group was pre-treated with parachlorophenylalanine, a serotonin synthesis blocker, which completely abolished the post-stimulatory depressor response. These results indicate that prolonged muscle stimulation gives rise to a post-stimulatory long-lasting drop in blood pressure and that this response is mediated by somatic nerve afferents. Involvement of the endorphin and serotonin systems is also suggested.     

Electrical stimulation of the gastrocnemius muscle in the spontaneously hypertensive rat increases the pain threshold: role of different serotonergic receptors

In a previous study, prolonged low-frequency muscle stimulation in the hind leg of the fully conscious spontaneously hypertensive rat (SHR) was shown to induce a long-lasting reduction of blood pressure. It was also shown that opioid and serotonergic (5-HT) systems were involved. More recently, we have shown that the 5-HT₁ receptors are involved in the post-stimulatory decrease in blood pressure. In the present study, the influence of this type of muscle stimulation on the pain threshold was investigated. Pain perception was measured as the squeak threshold to noxious electric pulses. After cessation of the stimulation, an analgesic response was elicited within 60 min and peak analgesia developed after 120 min, being 139 ±10% (P < 0.01) of the prestimulatory control value. The increased pain threshold lasted for another 2 h. One group of SHR was pretreated with PCPA, a serotonin synthesis blocker, which completely abolished the post-stimulatory analgesia. To analyse further the involvement of different serotonin systems, drugs with selective affinity for 5-HT receptors were used. In one group a prestimulatory dose of metitepine maleate (a 5-HT_1&2 receptor antagonist) abolished the post-stimulatory elevation of the pain threshold. The prolonged analgesic response was still present after prestimulatory treatment with ritanserin or ICS 205–930 (5-HT₂ and 5-HT₃ blocking agents respectively). In another group of experiments, the serotonin receptor antagonists were administered post-stimulation to animals with fully elicited analgesia. None of the antagonists used could reverse the elevation of pain threshold towards prestimulatory levels. Thus, intact 5-HT systems were necessary to elicit the analgesia to muscle stimulation and the response was mediated by the 5-HT₁ receptor. However, the results indicate that serotonin is not required to maintain the analgesia once it has been elicited.     

Role of different serotonergic receptors in the long-lasting blood pressure depression following muscle stimulation in the spontaneously hypertensive rat

In a previous study prolonged low-frequency muscle stimulation in the hind leg of the spontaneously hypertensive rat (SHR) was shown to induce a reduction in blood pressure (about 15 mmHg) that lasted for many hours. We showed in that study that endorphin and serotonin systems were involved. In the present study drugs with selective affinity for the serotonin (5-HT) receptors were used to analyse further the involvement of different serotonin systems. In one group of SHR, a prestimulatory dose of metitepine maleate (a 5-HT1 and 5-HT2 receptor antagonist) completely abolished the post-stimulatory depressor response. The long-lasting depressor response was still present, although less pronounced, after a bolus dose of the 5-HT2 blocking agent ritanserin (R 55667) at the start of stimulation. The 5-HT3 receptor antagonist ICS 205-930 did not influence the response at all, nor did the selective 5-HT1a receptor agonist 8-OH-DPAT enhance the depressor response. These results indicate that the reduction in blood pressure after muscle stimulation is mainly mediated by the 5-HT1 receptor.     

Na+, K+-pump activity and skeletal muscle contractile deficits in the spontaneously hypertensive rat

Skeletal muscles in an animal model of genetic hypertension (the spontaneously hypertensive rat, SHR) exhibit significant deficits in contractile performance. These deficits appear to be unrelated to the rise in blood pressure. Slow-twitch soleus muscles show a decrease in specific muscle tension and a reduced resistance to muscle fatigue during prolonged contractile activity. We tested the hypothesis that the reduced fatigue resistance occurs as a consequence of an impaired ability to maintain or restore Na⁺ and K⁺ balance across the sarcolemma during repeated contractions. This may result from a genetically based increase in the Na⁺ permeability of SHR muscles, coupled with a reduction Na⁺, K⁺ pump capacity as the animals mature. Soleus muscles in adult SHR exhibit a significant increase in intracellular Na⁺ content and a significant decrease in intracellular K⁺ content at rest. ⁸⁶Rb⁺ uptake in Na⁺-loaded hypertensive muscles is 45% less than predicted from the number of ouabain-binding sites available. Activation of Na⁺, K⁺ pumps using adrenaline or insulin produces a significantly smaller hyperpolarization in hypertensive soleus than in control muscles. Control soleus muscles are hyperpolarized for at least 10 min after a 4 min period of high-frequency activity, but hypertensive soleus muscles remain at resting polarity. Nonetheless, the number of ouabain-binding sites in hypertensive muscles is significantly greater than in control soleus, and binding affinities are similar. This apparent deficit in pump capacity might lead to a greater and more prolonged increase in extracellular K⁺ during repetitive contractions, and an associated decline in tension. Recently, we have been able to prevent the abnormal decrease in hypertensive soleus fatigue resistance by long-term treatment (8 weeks) with the Ca²⁺ blocker amlodipine. The therapy prevented or reversed the contractile deficits, but did not restore the responsiveness of the Na⁺, K⁺ pump to hormonal stimulation. The current data suggest that both a reduction in Na⁺, K⁺-pump capacity and changes in Ca²⁺ distribution play a role in the development of contractile deficits in hypertensive muscles.     

Receptor cleavage and P-selectin-dependent reduction of leukocyte adhesion in the spontaneously hypertensive rat

The SHR, a genetic model for hypertension and the metabolic syndrome, has attenuated leukocyte adhesion to the postcapillary endothelium by an unknown mechanism. Based on recent evidence of elevated levels of MMPs in plasma and on microvascular endothelium of the SHR with cleavage of several receptor types, we hypothesize that the reduced leukocyte-endothelial interaction is a result of enhanced proteolytic cleavage of P-selectin on the postcapillary endothelium and PSGL-1 on leukocytes. The attenuated rolling interactions of SHR leukocytes with the endothelium were restored by chronic treatment with a broad-spectrum MMP inhibitor (CGS) for 24 weeks. The SHR MMP levels, in plasma and mesentery, as well as the systolic blood pressure, decreased significantly with treatment. In the SHR mesentery, labeling of P-selectin in the postcapillary venules by immunohistochemistry demonstrated, on average, a 31% lower extracellular P-selectin density compared with the normotensive WKY. A significantly lower extracellular PSGL-1 density on the membranes of SHR neutrophils compared with the WKY also supported our hypothesis. In vivo stimulation of the mesenteric postcapillary venules with histamine demonstrated that the SHR had an attenuated response, as measured by leukocyte rolling velocity on the endothelium. The reduced P-selectin and PSGL-1 density, on SHR postcapillary endothelium and on SHR leukocytes, respectively, was restored significantly by chronic MMP inhibition. The impaired ability of SHR leukocytes to reduce rolling velocity upon inflammatory stimulation led to fewer firmly adhered leukocytes to the endothelium as a contributor to immune suppression.     

硝苯地平导致高血压大鼠血管平滑肌细胞的凋亡

血管平滑肌细胞的凋亡 (程序性死亡 )在血管管壁结构的调节中是一个基本的必经的过程。目前一些抗高血压药物的药理学效应就是促进血管平滑肌细胞的凋亡。血管平滑肌细胞凋亡的生物化学的规律是复杂的。我们选择一些抗高血压的药物作用于培养的正常血压大鼠 (ＷＫＹ)和高血压大鼠(ＳＨＲ)的血管平滑肌细胞。观察这些药物能否导致这两种大鼠平滑肌细胞的凋亡。我们选择的药物是 :Ｌ型Ｃａ通道拮抗剂—硝苯地平 ;一氧化氮的供体—硝普钠 ;毛喉素 (一种腺苷酰环化酶的活化剂 )。或一种肌浆网选择性抑制Ｃａ2 +—三磷酸腺苷酶。用这些药物对比它…     

The first suckling episode in the rat: the role of endogenous activity at mu and kappa opioid receptors

The present study examined the role of endogenous activity at mu and kappa opioid receptors in attachment to and ingestion of milk from a surrogate nipple in cesarean-delivered newborn rats prior to regular suckling experience. Selective opioid antagonist drugs were injected into the cisterna magna (IC administration) or lateral ventricles (ICV administration). Blockade of endogenous activity at mu opioid receptors by IC administration of the selective antagonist CTOP reduced attachment time and markedly increased disengagements from the nipple. CTOP also increased the intensity of suckling measured as milk intake per min attached to the nipple, when milk was available from the nipple in a free-access regime, and enhanced intake when milk was infused through an intraoral cannula aside from the suckling context. The ICV administration of the selective kappa antagonist nor-BNI considerably increased latency to grasp the surrogate nipple, while time on the nipple and milk intake were decreased. The presented data suggest that populations of mu and kappa receptor-containing neurons, differentiable by the route of antagonist administration, play an important role in initiation and maintenance of suckling behavior in the newborn rat during its first encounter with the nipple and milk. The kappa opioid system is predominantly involved in the initiation of the newborn's behavior directed toward the nipple providing milk. The role of the mu opioid system seems more complicated: it transforms initial oral grasp responses into sustained attachment to the nipple and maintains the intake of milk at a certain physiological level.     

Increased oxytocinergic system activity in the cardiac muscle in spontaneously hypertensive SHR rats

IntroductionThe present study aimed to determine whether the presence of cardiac hypertrophy due to arterial hypertension is associated with a change in the activity of the oxytocinergic system in cardiomyocytes.Material and methodsThe experiments were performed on male, spontaneously hypertensive rats (SHR, n = 10) and normotensive Wistar-Kyoto rats (WKY, n = 12). Blood samples were collected from both SHR and WKY animals to asses plasma oxytocin (OT) concentration; the rats were sacrificed by decapitation. Samples of the left and right ventricles were harvested for the analysis of the OT and oxytocin receptor (OTR) protein by ELISA, and OT and OTR mRNA expression by RT-PCR. Immunohistopathological studies were performed to confirm the presence of OTR receptors in the cardiac muscle of the ventricles.ResultsPlasma OT concentration did not differ between SHR and WKY rats. In the SHR rats, the expression of OT mRNA and the OT protein level was higher in the left and the right ventricle, while OTR mRNA expression was significantly lower in both the left and the right ventricle. However, the level of OTR protein was higher only in the left ventricle of the SHR rats. The presence of OTR receptors was confirmed by immunohistochemical analysis in the muscle of the right and left ventricle.ConclusionsThe presence of arterial hypertension is associated with increased activity of the oxytocinergic system in the heart, especially in the area of the left ventricle. These findings support the important role of this system in the maintenance of cardiovascular homeostasis.     

Role of opioid receptors in the long-lasting blood pressure depression after electric muscle stimulation in the hind leg of the rat

In a previous study, electrically induced contractions of the gastrocnemius muscle in conscious spontaneously hypertensive rats were shown to induce a blood pressure reduction of 15-20 mmHg lasting several hours. We showed in that study that endogenous opioid systems were involved. In this study, drugs with selective affinity for different opioid receptors were used to analyse further the involvement of endogenous opioid systems in the post-stimulatory drop in blood pressure in spontaneously hypertensive rats. Prestimulatory intracerebroventricular administration of beta-FNA (a mu-receptor antagonist) did not significantly influence the response at all, nor did a lower intravenous dose of naloxone reverse the post-stimulatory drop in blood pressure. High-dose naloxone (15 mg kg-1) increased post-stimulatory blood pressure by around 10 mmHg. About 50% of the drop thus remained after this treatment. A similar, partial reversal of the decreased blood pressure was seen after intravenous administration of a delta-receptor antagonist, ICI 154,129. However, the depressor response was completely reversed by a low intravenous dose of MR 2266 BS (a kappa-receptor antagonist). These results suggest that the reduction in blood pressure after muscle stimulation is mainly mediated by the opioid kappa-receptor. A certain involvement of the delta-receptor is also indicated.     

Changes of imidazoline receptors in spontaneously hypertensive rats

The role of imidazoline receptors in the regulation of vascular function remains unclear. In this study, we evaluated the effect of agmatine, an imidazoline receptor agonist, on systolic blood pressure (SBP) in spontaneously hypertensive rats (SHRs) and investigated the expressions of imidazoline receptors by Western blot. The isometric tension of aortic rings isolated from male SHRs was also estimated. Agmatine decreased SBP in a dose‐dependent manner in SHRs but not in the normal group [Wistar–Kyoto (WKY) rats]. This reduction in SBP in SHRs was abolished by BU224, a selective antagonist of imidazoline I₂‐receptors. Higher expression of imidazoline receptors in SHR was observed. Moreover, agmatine‐induced relaxation in isolated aortic rings precontracted with phenylephrine or KCl. This relaxation was also abolished by BU224 but was not modified by efaroxan, an imidazoline I₁‐receptor antagonist. Agmatine‐induced relaxation was also attenuated by PNU 37883, a selective blocker of vascular ATP‐sensitive potassium (K_ATP) channels. Additionally, vasodilatation by agmatine was reduced by an inhibitor of protein kinase A (PKA). We suggest that agmatine can lower blood pressure in SHRs through activation of the peripheral imidazoline I₂‐receptor, which is expressed more highly in SHRs.     

Immune response modulation in the spontaneously hypertensive rat

Spontaneously hypertensive male and female rats (SHR) were compared with Wistar/Kyoto (W/K) controls at 15 wk and 80 wk of age. Treatment of the young and old hypertensives with thymosin, fraction 5, lowered the blood pressure within 4 wk of the start of treatment. Following 10 wk of injections, the blood pressures of the hypertensive rats remained at a depressed level for about 6 wk. The thymic hormone raised the depressed spontaneous T-cell rosette formation of the aged hypertensive rat and increased the lymph node T-cell response to the mitogens, Con A and PHA. Thymosin administration over a period of 7 wk increased the size of the aged hypertensive thymus. No similar effect was observed in the W/K. Spleen cell production of prostaglandin E (PgE) was markedly higher in the young hypertensive and immune complex deposition was found in the glomeruli and tubules of the aged SHR kidneys. Thymosin lowered the high level of PgE to normal and decreased the immune complex deposition in the kidney. IgG1 levels were considerably depressed in the SHR as compared to the W/K. Following thymosin administration levels of IgG1 increased 2-fold in both rat strains. Plaque-forming cells from the spleens of the untreated SHR were about 3-fold less than those of the age-matched W/K. Following treatment with thymosin the number of plaque-forming cells of both groups demonstrated a substantial further decrease. Spontaneous hypertension in rats is similar, in certain respects to autoimmune-like diseases in humans with a depression in T-cell activity as well as immune complex deposition; both conditions being altered by exposure to a thymic extract.     

苯丙氨酸对自发性高血压大鼠血管平滑肌细胞内钙的影响

目的 探索苯丙氨酸对自发性高血压大鼠血管平滑肌细胞内钙的影响，方法 将ＳＨＲ和ＷＫＹ的血管平滑肌细胞在不同浓度苯丙酸培养液中培养一定时间后，加入（＾４５ＣａＣｌ２）温育一定时间后，测定进入细胞内（＾４５Ｃａ＾２＋）的量；而后将在不同浓度苯丙氨酸培养液中培养一定时间后的ＳＨＲ和ＷＫＹ的血管平滑肌细胞同ＰＳＳ液温育，采用Ｆｕｒａ－２／ＡＭ荧光指示剂测定不同浓度苯丙氨酸对细胞内游离Ｃａ＾２＋的影响。结果     

Immobilization stress induces vasodepressor and altered neuroendocrine responses in the adult stroke-prone spontaneously hypertensive male rat

The effects of acute (1 h) and daily repeated immobilization stress (14 days, twice-daily, 1 h) were studied on arterial blood pressure and heart rate and on the blood levels of several hormones in the adult (5 months old) stroke-prone spontaneously hypertensive rat (SHRSP) and in the age-matched normotensive Wistar-Kyoto (WKY) rat. The major result was the development of a long-lasting vasodepressor response in the SHRSP, while the same acute or repeated immobilization stress in the WKY rat led to the development of a prolonged vasopressor response. Differential changes to stress were also observed in practically all neuroendocrine axes with the exception of the pituitary-adrenal axis. The vasodepressor response to immobilization stress in SHRSP may be related to an exaggerated defence-like reaction causing an enhanced vasodilation in the skeletal muscle beds associated with a tachycardia similar to that in the normotensive control rats.     

Patterns of maternal behavior in the spontaneously hypertensive rat

Maternal behavior of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats was assessed over the preweanling period (days 1-21). Ten litters of each strain were observed during the dark phase of the light:dark cycle using a scan sampling technique. Cages were observed periodically throughout the day and the momentary behavior of the dam was recorded on a checklist of 9 different behaviors. These behaviors included: nursing entire litter, nursing part of litter, contact with entire litter, contact with part of litter, pup carrying/retrieval, licking pups, sniffing pups, nest building, and away from litter. A second set of ten litters per strain was observed in the same manner during the light phase of the light:dark cycle thus providing around the clock data throughout the entire preweanling period. For purposes of data analysis, the 21-day preweanling period was divided into 7 3-day blocks. The mean relative frequency with which each behavior was observed from SHR and WKY mothers was determined for each block and for each phase of the light:dark cycle separately. SHR mothers were with their pups and nursing them more frequently than WKY mothers during the light phase of the circadian cycle. Complimentary to this finding, WKY mothers were observed away from their pups more frequently than SHR mothers during both light and dark phases. Finally, SHR mothers were observed to lick their pups more often than WKY mothers during both phases of the light:dark cycle. These findings were consistent across the entire preweanling period.(ABSTRACT TRUNCATED AT 250 WORDS)     

The juxtaglomerular apparatus of the spontaneously hypertensive rat

Spontaneously hypertensive rats (SHR) are used to study the pathogenesis of essential hypertension. This study investigates the role of the renin-angiotensin system (RAS) in the pathogenesis of hypertension in SHRs and the morphometry of the JGA by a three-dimensional computer reconstruction program "GLOM" and electron microscopy. Systolic blood pressure (SBP) (tail cuff method) was higher in SHRs compared to controls (P less than 0.001). Plasma renin concentration (PRC) was lower in SHRs than in controls (P less than 0.001). Reconstruction of the JGA revealed granulated JG cells in the afferent and efferent arterioles and in the vascular tree away from the JGA area. Electron microscopy showed granulated JG cells in the afferent and efferent arterioles. The percentage volume of the granulated JG cells in SHR was significantly higher than in controls (P less than 0.01). A relationship was found between the percentage volume of granulated JG cells and the SBP in SHRs (r = 0.933, P less than 0.05). The wall/lumen perimeter ratio was also significantly higher in the SHRs compared to the controls (P less than 0.05). Low PRC in SHRs has been reported by several workers. The apparent hyperactivity of the JGA may indicate failure of renin release or an abnormal synthesis/secretion rate.     

Arterial myogenic properties of the spontaneously hypertensive rat

When subject to a transmural pressure gradient resistance arteries develop a spontaneous, intrinsically initiated contraction which varies according to the pressure stimulus and occurs in the absence of vasoconstrictor agonists. Such pressure-dependent active changes in vascular tone are indicative of the vascular myogenic response and contribute to autoregulation and the setting of total peripheral resistance and hence blood pressure regulation. The myogenic behaviour of blood vessels provides the background tone upon which other vasomotor influences act. Hypertension is associated with a raised vascular resistance and in this article the evidence for increased myogenic activity contributing to the raised vascular resistance is reviewed. Although there are some cases that provide evidence for exaggerated myogenic responsiveness in resistance arteries taken from hypertensive animals it is not possible to conclude that enhanced myogenic contractile responses within normal pressure ranges contribute to the raised total peripheral resistance. However, the myogenic tone of the resistance arteries of the various vascular beds is subject to differing modulatory influences in hypertensive animals and their normotensive controls which may contribute to the aetiology of hypertension.     

Morphological alterations in skeletal muscle of spontaneously hypertensive rats

The Extensor digitorum longus (EDL) and the soleus muscles of spontaneously hypertensive rats (SHR) were studied in comparison with those of their normal counterparts, the Wistar Kyoto (WKY) rats. Quantitative assessment of capillaries and muscle fibre typing was done with optical microscopy, while the study of capillary abnormalities was performed by ultrastructural observation. There were no differences in fibre type proportion or in capillarity indexes between the SHR and the control rats. A reduction in the area of IIB fibres was found in the EDL muscle of the hypertensive animals. The ultrastructural study showed abnormalities in the capillaries of both muscles in SHR, the cross section of the endothelial cells was enlarged; there was irregular distribution of caveolae and pinocytic vesicles, the capillary basement membrane showed irregular width, with parts engrossed and reduplicated. Some pericytes were prominent. There were macrophages present in the interstitial space. In some muscle fibres there was disorganization of the sarcomere structure, swelling of the sarcotubular system, abundant autophagic vacuoles, and proliferative satellite cells. There were abundant collagen fibrils. The presence of cellular rests, autophagic vacuoles and loss of sarcolemma indicated necrosis. It can be concluded, that in SHR, muscle capillaries showed alterations that may be the substrate of functional rarefaction, although anatomical rarefaction (number reduction) could not be demonstrated. In EDL and soleus muscles of SHR, signs of a mild myopathy with focal fibrosis were present.     

Renomedullary interstitial cells in the spontaneously hypertensive rat.

Renomedullary interstitial cells (RIC) are known to synthesize and release prostaglandins which may play a significant role in the development or severity of hypertension. The medulla of the spontaneously hypertensive rat contains RIC which are morphologically very similar to those previously described in the normotensive rat. The granularity of the RIC, however, was increased in the spontaneously hypertensive rat compared to normotensive Wistars (9.6 +/- 2.34 versus 5.3 +/- 2.05 granules per cell, respectively, p less than 0.001) or treated spontaneously hypertensive rats (7.2 +/- 1.65 granules per cell, p less than 0.001). Granule counts also increased in the presence of mild and moderate degrees of renal arteriolar sclerosis, but decreased in long standing hypertension with more severe and extensive lesions involving both arteries and arterioles. These results are consistent with the hypothesis that the RIC respond to an elevation of blood pressure in the spontaneously hypertensive rats by increased release of antihypertensive substances. In addition, the decrease in granularity of the RIC in the presence of extensive renal arteriolar and arterial damage suggests reduced ability to compensate for the elevated blood pressure and thus may contribute to the acceleration of hypertension.