Release of serotonin in the locus coeruleus of normotensive and spontaneously hypertensive rats (SHR)

The aim of the present work was to clarify whether differences exist between the release of endogenous serotonin in the locus coeruleus of normotensive and hypertensive rats. The locus coeruleus was superfused with artificial cerebrospinal fluid (aCSF) through a push-pull cannula and serotonin and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were determined in the superfusate by HPLC combined with electrochemical detection. Compared with normotensive Wistar-Kyoto (WKY) rats, the basal release rate of serotonin in the locus coeruleus of spontaneously hypertensive rats (SHR) was increased more than twofold. Intravenous infusion of noradrenaline (4 μg/kg min) increased mean arterial blood pressure to the same extent in hypertensive and normotensive rats. The pressor response was associated with an increased serotonin release. In WKY rats, the release of serotonin in the locus coeruleus evoked by noradrenaline infusion was more pronounced than in SHR. In WKY rats, intravenous infusion of sodium nitroprusside (150 μg/kg min) led to a fall in blood pressure which was less pronounced and lasted shorter than in SHR. The depressor response was associated with decreased serotonin release. In WKY rats, the decrease in serotonin release evoked by sodium nitroprusside was more pronounced and lasted longer than in SHR. Neither noradrenaline nor sodium nitroprusside influenced the outflow of 5-HIAA. The sensory stimuli noise and tail pinch led to a slight rise in arterial blood pressure which was similar in WKY rats and SHR. These stimuli enhanced the release rate of serotonin and the outflow of 5-HIAA to the same extent in the locus coeruleus of normotensive and hypertensive rats. The findings suggest that the enhanced release of serotonin in the locus coeruleus of genetically hypertensive rats reflects a mechanism counteracting the disturbed blood pressure homeostasis. Stressors influence blood pressure and release of serotonin in the locus coeruleus of SHR and WKY rats to the same extent. Received: 16 November 1998 / Accepted: 22 February 1999     

Possible mechanisms underlying the hypertensive response to clonidine in freely moving, normotensive rats

Possible mechanisms underlying the hypertensive response to intracerebroventricular (i.c.v.) or intravenous (i.v.) injection of clonidine were investigated in freely moving, normotensive rats. In conscious rats, clonidine (2-20 micrograms) injected i.c.v. caused a dose-dependent and long-lasting pressor response associated with bradycardia. A similarly long-lasting pressor response was induced following an initial rapid rise in mean blood pressure after i.v. bolus injections of clonidine (5-50 micrograms/kg). In pentobarbital-anesthetized rats, the prolonged pressor responses to i.v. and i.c.v. injected clonidine at high doses were significantly smaller than those in conscious rats. Low doses of clonidine caused only depressor responses which developed gradually. No significant changes in concentrations of plasma norepinephrine and epinephrine were found during the pressor period after i.c.v. injection of clonidine (20 micrograms). Systemic (2 mg/kg, i.v.) or central (100 micrograms, i.c.v.) pretreatment with phentolamine abolished only the prolonged pressor response to both i.c.v. (20 micrograms) and i.v. (50 micrograms/kg) injected clonidine. The prolonged pressor response to clonidine (20 micrograms, i.c.v.) was enhanced by pretreatment with hexamethonium (25 mg/kg, i.v.), methylatropine (1 mg/kg, i.v.) or atropine (1 mg/kg, i.v.) and it was not affected by pretreatment with saralasin (300 micrograms/kg and 25 micrograms/kg/min, i.v.), d(CH2)5Tyr(Me)-arginine-vasopressin, a vasopressin antagonist (50 micrograms/kg, i.v.) or naloxone (1 mg/kg, i.v.). Neither adrenalectomy nor adrenal demedullation had an effect on the pressor response to clonidine (20 micrograms, i.c.v.). In adrenalectomized rats, systemic pretreatment with hexamethonium (25 mg/kg, i.v.) caused a potentiation of the pressor response to clonidine (20 micrograms, i.c.v.). These results suggest that clonidine induces the pressor response through activation of central alpha-adrenoceptors, probably the alpha 2 subtype, without an increase in sympatho-adrenomedullary activity. It is speculated that the response may be mediated by vasoactive humoral substance(s).     

Differential amino acid transmission in the locus coeruleus of Wistar Kyoto and spontaneously hypertensive rats

In addition to differences in their blood pressure, Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) are known to differ in their emotional behaviour. The neurochemistry underlying these differences is not well understood. In the present study the release rates of the two main regulatory amino acids in the locus coeruleus, glutamate and gamma-aminobutyric acid (GABA), were monitored in WKY rats and SHR to investigate whether basal and/or challenged neurotransmission differs between these strains. The strains differed in their basal blood pressure (WKY 102±2 mmHg, SHR 140±4 mmHg), as well as in their emotional behaviour, since WKY rats displayed enhanced anxiety-related behaviour in the open field test (time in centre: WKY 197±40 s/30 min, SHR 741±93 s/30 min). Basal glutamate and GABA release rates did not differ between WKY rats and SHR. A rise in blood pressure induced by intravenous infusion of noradrenaline for 10 min enhanced GABA release in WKY rats by 60%, while no effect was observed in SHR. Glutamate release did not respond to experimental hypertension in both strains. Intravenous infusion of sodium nitroprusside led to a fall in blood pressure, which was less pronounced and was of shorter duration in WKY rats than in SHR. The depressor response had no effect on amino acid release in the locus coeruleus of both strains. Mild stress induced by noise or tail pinch led to slight rises in arterial blood pressure (10 mmHg and 20 mmHg respectively), which were similar in WKY rats and SHR. Tail pinch enhanced the release rates of glutamate and GABA in the locus coeruleus of WKY rats and SHR; however, no strain differences were noted. Noise stress did not significantly influence amino acid release. These findings demonstrate that SHR and WKY rats differ in GABAergic neurotransmission, which is revealed in response to specific cardiovascular challenges, but not to mild stressors. The observed lack of GABA response to blood pressure elevation in SHR may reflect a disturbed mechanism counteracting high blood pressure, possibly contributing to hypertension in this strain.     

Central alpha 1-adrenoceptors and cardiovascular control in normotensive and spontaneously hypertensive rats

We investigated the antagonistic properties of saralasin in acute and chronic angiotension II (ANG II)-dependent hypertension. Two models of experimental hypertension were studied: (a) Rats acutely infused i.v. with ANG II to raise the blood pressure (BP) by about 35 mmHg. (b) One-clip, two-kidney renal hypertensive rats. In both experimental models increasing doses of saralasin were infused i.v., and three parameters were evaluated at each dose level: (1) fall of BP, (2) plasma concentration of saralasin, and (3) plasma concentration of ANG II. It was found that saralasin led to a more pronounced fall of BP in malignant than in benign renal hypertension. To reduce BP by about 20 mmHg, saralasin plasma concentrations had to exceed those of ANG II about 2000-fold in renal hypertension and about 7-fold in rats infused with ANG II. It is concluded that saralasin antagonises ANG II more effectively in acute than in chronic hypertension.     

Characterization of noradrenaline release in the locus coeruleus of freely moving awake rats by in vivo microdialysis

Rationale The origin and regulation of noradrenaline (NA) in the locus coeruleus (LC) is unknown.Objectives The neurochemical features of NA overflow (nerve impulse dependence, neurotransmitter synthesis, vesicle storage, reuptake, ₂-adrenoceptor-mediated regulation) were characterized in the LC.Methods Brain microdialysis was performed in awake rats. Dialysates were analyzed for NA.Results NA in the LC decreased via local infusion of Ca²⁺-free medium (–42±5%) or the sodium channel blocker tetrodotoxine (TTX) (–47±8%) but increased (333±40%) via KCl-induced depolarization. The tyrosine hydroxylase (TH) inhibitor -methyl-p-tyrosine (250 mg kg^–1, i.p.) and the vesicle depletory drug reserpine (5 mg kg^–1, i.p.) decreased NA. Therefore, extracellular NA in the LC satisfies the criteria for an impulse flow-dependent vesicular exocytosis of neuronal origin. Local perfusion of the ₂-adrenoceptor agonist clonidine (0.1–100 M) decreased NA (E_max=–79±5%) in the LC, whereas the opposite effect (E_max=268±53%) was observed with the _2A-adrenoceptor antagonist BRL44408 (0.1–100 M). This suggests a tonic modulation of NA release through local _2A-adrenoceptors. The selective NA reuptake inhibitor desipramine (DMI) (0.1–100 M) administered into the LC increased NA in the LC (E_max=223±40%) and simultaneously decreased NA in the cingulate cortex, confirming the modulation exerted by NA in the LC on firing activity of noradrenergic cells and on the subsequent NA release in noradrenergic terminals.Conclusion Synaptic processes underlying NA release in the LC are similar to those in noradrenergic terminal areas. NA in the LC could represent local somatodendritic release, but also the presence of neurotransmitter release from collateral axon terminals.     

Nitric oxide-releasing aspirin inhibits vasoconstriction in perfused tail artery of normotensive and spontaneously hypertensive rats

The aim of this study was to investigate the capacity of the 2-(acetyloxy)benzoic acid 3-(nitrooxymethyl)phenyl ester (NCX 4016), a nitric oxide (NO)-releaser derivative of aspirin, to decrease blood pressure in spontaneously hypertensive rats (SHR) and to counteract the adrenergic vasoconstriction in perfused tail artery of these animals. Oral treatment for 10 consecutive days with NCX 4016 (100 micromol/kg) in SHR and their genetic controls Wistar Kyoto (WKY) rats resulted in a reduction of blood pressure in SHR but not in WKY rats. In SHR, the NCX 4016 treatment increased the serum nitrite/nitrate and diminished the serum thromboxane B2, whereas aspirin did not change blood pressure but abolished the serum thromboxane B2. Perfused tail arteries excised from vehicle-treated SHR exhibited a significant impairment of endothelium-dependent vasorelaxant function. These vessels, prepared from SHR or WKY rats treated orally with NCX 4016 (10, 30 and 100 micromol/kg for 7 consecutive days), revealed a dose-dependent decrease in vasoconstriction in response to transmural nerve stimulation and norepinephrine, whereas aspirin was ineffective. Furthermore, in tail arteries of both SHR and WKY rats treated orally with NCX 4016 (100 micromol/kg for 7 consecutive days), the cGMP increased significantly. In conclusion, NCX 4016, by releasing NO and increasing cGMP in vascular tissue, reduces sympathetic-mediated vasoconstriction in resistance vessels and lowers blood pressure in SHR.     

Age-related changes of in vivo beta-adrenergic responsiveness in normotensive and spontaneously hypertensive rats

The variations of plasma cyclic AMP concentration caused by propranolol and isoproterenol were studied in order to investigate endogenous stimulation and responsiveness of the beta-adrenoceptor-cyclic AMP system in vivo, in 7 and 18 week old unanaesthetized male SHR and WKY. In both age groups, the basal cyclic AMP level was higher in SHR than in controls but was significantly reduced to a comparable value in the two strains after intraperitoneal injection of 2.5 mg X kg-1 propranolol, a dose which markedly depressed or even abolished the effect of isoproterenol. Cumulative dose-response curves of plasma cyclic AMP concentration obtained in another group of rats after successive subcutaneous injections of isoproterenol showed that the ED50 value of this drug was higher in SHR than in WKY and increased in a parallel manner in the two strains between 7 and 18 weeks of age (respectively from 0.21 +/- 0.01 to 0.34 +/- 0.02 mumol X kg-1 in WKY and from 0.32 +/- 0.02 to 0.52 +/- 0.03 mumol X kg-1 in SHR). At the same time blood pressure increased much less markedly in WKY (from 115 +/- 4 to 119 +/- 2 mm Hg) than in SHR (from 134 +/- 3 to 179 +/- 3 mm Hg). Altogether these results show that endogenous beta-adrenergic stimulation of the cyclic AMP system was higher in SHR of both ages in spite of a diminished responsiveness of this system to exogenous isoproterenol.     

Exaggerated pressor response to centrally administered renin in freely moving, spontaneously hypertensive rats

The cardiovascular responses to intracerebroventricular (i.c.v.) injection of renin were compared between freely moving normotensive Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The i.c.v. injection of renin (0.05-1.0 mU) produced a dose-dependent and a long-lasting rise in mean blood pressure associated with variable changes in heart rate (HR) in both WKY and SHR. However, the blood pressure and HR were not affected by intravenously injected renin (0.1 mU). The pressor response to i.c.v. injected renin was greater in SHR than in WKY, the dose-response curve for renin in SHR being to the left of that in WKY. Central (i.c.v.) pretreatment with [Sar1, Ile8]angiotensin II (50 micrograms) largely abolished the pressor response to i.c.v. injected renin in both WKY and SHR. The i.c.v. injection of angiotensin II (ANG II) (10-100 ng) induced a dose-dependent pressor response which was antagonized by central pretreatment with [Sar1, Ile8]ANG II (50 micrograms). The pressor response to ANG II was also greater in SHR than in WKY. These results suggest that the pressor response to centrally administered renin as well as to ANG II, which is mediated via ANG II receptors located in the brain, is enhanced in SHR.     

The effect of ketanserin on cardiovascular reflexes in conscious normotensive and spontaneously hypertensive rats

The effect of ketanserin (3 mg/kg i.v.) on the baroreceptor heart rate reflex and the Bezold-Jarisch reflex was examined in conscious Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). In the control situation (before ketanserin treatment), reflex bradycardia in response to phenylephrine (baroreflex) and phenyldiguanide (Bezold-Jarisch reflex) were impaired in SHR as compared with WKY, while reflex tachycardia in response to nitroprusside was similar in the two groups. However, after ketanserin administration in SHR, there was a reversal of the baroreflex-mediated tachycardia in response to nitroprusside into a bradycardic response. The nitroprusside-induced bradycardia was not caused by the release of 5-HT stimulating chemosensitive vagal afferents since the 5-HT3 receptor antagonist MDL 72222 did not block this response. In the same SHR, the Bezold-Jarisch reflex evoked by phenyldiguanide and the phenylephrine-induced bradycardia were potentiated by ketanserin. All the above effects of ketanserin were less evident in the WKY. Ketanserin did not alter vagal efferent function in anaesthetized SHR since it did not affect bradycardia induced by electrical stimulation of the vagus nerve. Therefore, it is suggested that ketanserin has sensitised cardiac vagal afferent mechanisms in SHR, which led to a normalization of reflex bradycardic function to a level normally observed in conscious normotensive WKY (i.e. prior to ketanserin treatment).     

Angiotensin II monoclonal antibody: blood pressure effects in normotensive and spontaneously hypertensive rats

The angiotensin II (AII) specific monoclonal antibody KAA8--a selective functional AII antagonist and a hypotensive agent in high renin hypertensive rats--did not lower blood pressure in conscious normotensive rats (NRs) and spontaneously hypertensive rats (SHRs). The non-peptide AII receptor antagonist DuP 753 decreased blood pressure significantly in SHRs but not in NRs. These results suggest the involvement of locally released AII in the control of blood pressure in SHRs but not in NRs.     

Plasma catecholamines and neuropeptide-Y as indices of sympathetic nerve activity in normotensive and stroke-prone spontaneously hypertensive rats

The suitability of plasma catecholamines (CAs) and neuropeptide-Y (NPY) as biochemical indices of sympathetic nerve activity (SNA) has been investigated, and these parameters have been compared between adult normotensive (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). Plasma norepinephrine (NE), epinephrine (E) and NPY were measured in venous and arterial blood samples taken from conscious, unrestrained rats. Under resting conditions, both CAs were significantly higher in SHRSP than in WKY; plasma E in particular was raised threefold. SHRSP had higher plasma levels of NPY in arterial blood but not in venous blood. Acute hydralazine-induced hypotension caused a slight rise in NPY and striking increases of CAs, which were accentuated in SHRSP. Ganglion blockade with pentolinium reversed these increases but the differences in basal plasma CA levels between strains still persisted. Barbiturate anaesthesia had little effect on plasma levels of NPY or NE, but plasma E levels were depressed, particularly in SHRSP, so that the strain difference in plasma E taken from venous blood was no longer apparent. The results indicate that plasma levels of CAs but not NPY are useful indices of SNA in conscious rats. Comparisons between WKY and SHRSP after drug treatment demonstrate a major contribution by the adrenal medulla to plasma CA levels in SHRSP which, under resting conditions, may not be sympathetically evoked.     

Comparative effects of tramadol on vascular reactivity in normotensive and spontaneously hypertensive rats

The aim of the present study was to determine the effects of tramadol on vascular reactivity in aortic rings from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Aortic rings, with or without endothelium, were obtained from male WKY rats and SHR (15-20 weeks old) and prepared for isometric tension recording. Aortic rings were precontracted with phenylephrine (10 micromol/L) or 40 mmol/L KCl and then exposed to cumulative concentrations of tramadol (0.1-1 mmol/L). Tramadol produced a concentration-dependent relaxation of precontracted aortic rings from WKY rats and SHR, which was not dependent on functional endothelium. Vascular relaxation was significantly greater in rings from SHR than WKY rats. The concentration of tramadol necessary to produce a 50% reduction of the maximal contraction to phenylephrine (IC(50)) in rings with and without endothelium from SHR was 0.47 +/- 0.08 and 0.44 +/- 0.03 mmol/L, respectively (P = 0.76). Tramadol attenuated the contracture elicited by Ca2+ in depolarized tissue, suggesting that it may inhibit L-type Ca2+ channels. However, pretreatment with nicardipine (1 micromol/L) prevented the relaxation induced by tramadol in aortic rings from WKY rats and partially reduced its inhibitory effect in aortic rings from SHR. 6. Pretreatment of endothelium-denuded aorta with glybenclamide (3 micromol/L), 4-aminopyridine (3 mmol/L), tetraethylammonium (3 mmol/L) and naloxone (100 micromol/L) did not affect tramadol-induced vasodilation of aortic rings from either WKY rats or SHR. Intravenous administration of tramadol (10 mg/kg) to conscious SHR significantly reduced both systolic and diastolic blood pressure from 171.4 +/- 5.3 to 129.3 +/- 5.3 (P = 0.002) and from 125.0 +/- 6.5 to 57.8 +/- 8.9 mmHg (P = 0.003), respectively.     

Isoproterenol and glucagon effects in perfused hearts from spontaneously hypertensive and normotensive rats

The metabolic effects of isoproterenol and glucagon were compared in perfused hearts from spontaneously hypertensive (SH) Wistar rats and normotensive (NT) Wistar controls. Animals were used when either 13–15 weeks of age or when 40–44 weeks of age. Isoproterenol (3 × 10^?8 M) or glucagon (10^?7 M) was infused over a 3-min period of time. The hearts were subjected to freeze-clamping and emersion in liquid nitrogen and the powdered tissue was assayed for cyclic AMP, phosphorylase activity and glycogen. Glycerol release into the perfusate was also measured. The alterations in these parameters produced by isoproterenol were the same in both the SH and NT hearts. However, the hearts from the SH rats were less responsive to glucagon than those from the NT animals. Infusion of glucagon in isolated hearts of SH rats resulted in increases in cyclic AMP levels and phosphorylase activation which were only 25–50 per cent of those observed with the hearts from the NT rats. Glycerol release from the hearts of the SH rats was delayed after glucagon infusion and reached its maximum response 1 min after the maximum response was achieved in the hearts from the NT rats (45 sec). Infusion of glucagon resulted in a 30 per cent decrease in glycogen content of the hearts from NT rats but produced no significant change in the glycogen content of the hearts from SH rats. Cyclic AMP phosphodiesterase activity was found to be the same in the hearts of the two types of animals. These data demonstrate a diminished response to glucagon in hearts from SH rats possibly due to an alteration in the glucagon receptors.     

Vascular reactivity in spontaneously hypertensive normotensive and hypotensive rats

1 Three strains of rats spontaneously hypertensive (HRS), normotensive (NS) and hypotensive (HOS), were selected by repeated inbreeding.2 In order to explain the different blood pressure levels observed, we have studied their vascular reactivity to noradrenaline and angiotensin II. Experiments were performed in anaesthetized ganglion-blocked, vagotomized rats.3 The reactivity to noradrenaline was significantly higher in HRS than in NS and HOS rats.4 The reactivity to angiotensin II appeared identical in the three strains used.     

Haemodynamic and humoral effects of omega-conotoxin GVIA in normotensive and spontaneously hypertensive rats.

Omega-conotoxin GVIA, a 27-amino acid peptide, has been shown to be a potent and selective inhibitor of N-type voltage-operated calcium channels (VOCCs). A single intravenous dose of 10 micrograms/kg conotoxin slowly lowered blood pressure by 41.3 +/- 4.4 and 73.3 +/- 4.6 mm Hg in conscious Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) respectively without changing the heart rate. Plasma renin activity was significantly increased after conotoxin. In anaesthetized rats, conotoxin (3 and 10 micrograms/kg) lowered blood pressure and heart rate and produced a marked increase in renal vascular conductances. Baroreceptor heart rate reflex experiments using methoxamine and sodium nitroprusside before and after treatment with conotoxin showed that conotoxin almost totally abolished the sympathetic component of the reflex without affecting the vagal tone to the heart in both rat strains. Because conotoxin does not affect directly the vasculature and heart contractile properties, we suggest that the control of presynaptic calcium influx and of neurotransmitter release mostly depends on conotoxin-sensitive N-type VOCCs in the peripheral sympathetic system of the rat.     

Histamine- and acetylcholine-induced changes in the permeability of the blood-brain barrier of normotensive and spontaneously hypertensive rats

Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats were given saline, histamine (1.25, 2.5 or 5.0 micrograms/kg) or acetylcholine (1.0 or 2.0 micrograms/kg). Both agents caused a dose-dependent decrease in blood pressure which was greater in the spontaneously hypertensive animals. The permeability of the blood brain-barrier was measured with 131I-labelled serum albumin (RISA) or with 99mTc-sodium pertechnetate (TcO4-). The lowest dose of histamine caused a decrease in permeability of the blood-brain barrier to serum albumin in normotensive rats and acetylcholine caused an increase. Only the largest dose of histamine increased the permeability of the blood-brain barrier to serum albumin in spontaneously hypertensive rats. All doses of histamine and acetylcholine increased the permeability of the blood-brain barrier to sodium per technetate in nomotensive rats and the two lower doses of histamine increased the permeability in spontaneously hypertensive animals. This provides another example of the dissociation of change in the systemic blood pressure and change in the permeability of the blood-brain barrier.     

Cardiovascular responses to morphiceptin in spontaneously hypertensive and normotensive rats

In the urethane anesthetized spontaneously hypertensive rats (SHR) intraventricular injections of morphiceptin produced dose-related increase in heart rate and blood pressure. In contrary, intraventricular administration of morphiceptin in Wistar rats induced a fall in blood pressure and heart rate. Yohimbine antagonized pressor responses to morphiceptin in SHR. Naloxone counteracted both the stimulatory effects of morphiceptin in SHR as well as hypotensive responses in Wistar rats. Bilateral vagotomy blocked depressant action of morphiceptin in normotensive but failed to alter the pressor effects in SHR rats. After systemic injections of morphiceptin a fall in heart rate and blood pressure was obtained in both strains.     

Release of endogenous histamine in the hypothalamus of anaesthetized cats and conscious,freely moving rabbits

Summary The hypothalamus of anaesthetized cats and conscious, freely moving rabbits was superfused with CSF through double-walled, push-pull cannulae and the release of endogenous histamine was determined in the superfusates by a radioenzymatic assay.In the posterior hypothalamic area of the anaesthetized cat, the rate of release of endogenous histamine varied rhythmically; phases of high rate of release appeared at 60 min cycles. The release of histamine was increased by electrical stimulation of the superfused area, as well as by hypothalamic superfusion with potassium-rich CSF.In the conscious rabbit, the anterior hypothalamic area and the posterior hypothalamic nucleus were superfused simultaneously. In both regions, the resting release of histamine varied rhythmically at approximately 70 min cycles. Phases of high or low-rate of release in the anterior hypothalamic area coincided with the corresponding phases in the posterior hypothalamic nucleus.The rhythmic release of endogenous histamine in the hypothalamus, as well as the ability of depolarizing stimuli to enhance the release of the amine support the idea that histamine acts as a neurotransmitter in the central nervous system.This work was supported by the Deutsche ForschungsgemeinschaftPart of the results has been presented at the International Histamine Symposium, Okayama, July 1981     

Ca2+ buffering function of sarcoplasmic reticulum in rat tail arteries: comparison in normotensive and spontaneously hypertensive rats

The superficial buffer barrier function of the sarcoplasmic reticulum (SR) during rest and that during stimulation with Bay k 8644, an agonist of L-type Ca2+ channels, were compared in endothelium-denuded strips of tail arteries from 13-week-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR), by measuring the effects of cyclopiazonic acid (CPA) and thapsigargin that inhibit SR Ca2+-ATPase and the effect of ryanodine that depletes SR Ca2+. The addition of 10 microM CPA induced a transient contraction that was not significantly different between WKY and SHR. The CPA-induced contraction was strongly inhibited by 100 nM nifedipine and was abolished by Ca2+-free solution in both strains. Thapsigargin (100 nM) or ryanodine (10 microM) induced similar, small transient contractions in the two strains. The addition of Bay k 8644 (1-100 nM) almost failed to induce a contraction in both WKY and SHR. When the strips were preincubated with 10 microM CPA, 100 nM thapsigargin or 10 microM ryanodine, Bay k 8644 induced similar concentration-dependent contractions in the two strains. The amount of Ca2+ stored in the SR, as estimated from the 20 mM caffeine-induced contraction, was not significantly different between WKY and SHR. Our results suggest that the SR of rat tail arteries can buffer a large amount of Ca2+ that enters the cell during the rest and the Bay k 8644 stimulation, and these functions are not altered in SHR.