Heikki Mäkynen

Exercise enhances vasorelaxation in experimental obesity associated hypertension

OBJECTIVE Regular exercise is recommended for the non-pharmacological treatment of hypertension, but the mechanisms underlying the lowering of blood pressure remain controversial. Therefore, we studied the effects of 22-week-long training on blood pressure, arterial reactivity, and metabolic abnormalities in a model of genetic obesity and moderate hypertension. METHODS Obese and lean Zucker rats were subjected to treadmill exercise from 8 to 30 weeks of age. Blood pressures were measured by the tail-cuff method, and urine was collected in metabolic cages. At the end of the study, the samples for biochemical determinations were taken, and reactivity of isolated mesenteric and carotid arterial rings was examined in standard organ chambers. RESULTS The exercise prevented the elevation of blood pressure which was observed in non-exercised obese Zucker rats, and also reduced blood pressure in the lean rats. The relaxations of norepinephrine-preconstricted mesenteric and carotid arterial rings to acetylcholine and nitroprusside were clearly improved by exercise in the obese rats. In the lean rats exercise enhanced vasorelaxation to nitroprusside in the mesenteric and carotid rings, and to acetylcholine in the carotid preparations. The exercise-induced improvement of endothelium-mediated dilatation to acetylcholine was abolished by nitric oxide synthesis inhibition with NG nitro-L-arginine methyl ester, but not by cyclooxygenase inhibition with diclofenac or functional inhibition of endothelium-dependent hyperpolarization by precontractions with KCl. The urinary excretion of the systemic prostacyclin metabolite (2,3-dinor-6-ketoprostaglandin F1 alpha) was increased two-fold by exercise in the obese and lean rats, whereas that of the thromboxane A2 metabolite (11-dehydrothromboxane B2) remained unaffected. Treadmill training reduced blood glucose, cholesterol, and triglycerides, but did not affect the high levels of insulin in obese Zucker rats. CONCLUSIONS These results suggest that the antihypertensive effect of long-term exercise in experimental obesity related hypertension is associated with improved vasodilatation. This is expressed as enhanced relaxation via endogenous and exogenous nitric oxide, and increased endothelial prostacyclin production. The improved control of arterial tone after training could be attributed to the alleviation of hyperlipidemia and insulin resistance, whereas hyperinsulinaemia per se remained unaffected.

Endothelial function in spontaneously hypertensive rats: influence of quinapril treatment

1 Angiotensin converting enzyme (ACE) inhibition has been shown to restore the impaired endothelial function in hypertension, but the mediators underlying the promoted endothelium‐dependent dilatation have not been fully characterized. Therefore, we investigated the effects of 10‐week‐long quinapril therapy (10 mg kg−1 day−1) on responses of mesenteric arterial rings in vitro from spontaneously hypertensive rats (SHR) and normotensive Wistar‐Kyoto (WKY) rats. 2 Endothelium‐dependent relaxations of noradrenaline (NA)‐precontracted rings to acetylcholine (ACh) and adenosine 5′‐diphosphate (ADP) were similar in WKY rats and quinapril‐treated SHR and more pronounced than in untreated SHR. The nitric oxide (NO) synthase inhibitor NG‐nitro‐L‐arginine methyl ester (L‐NAME) attenuated the relaxations in both WKY groups and quinapril‐treated SHR, and completely inhibited them in untreated SHR. When endothelium‐dependent hyperpolarization was prevented by precontraction of the preparations with potassium chloride (KC1), no differences were found in relaxations to ACh and ADP between the study groups. In addition, in NA‐precontracted rings the L‐NAME‐ and indomethacin‐resistant relaxations to ACh were partially prevented by apamin, an inhibitor of calcium‐activated potassium channels. 3 Interestingly, in quinapril‐treated SHR but not in the other groups, exogenous bradykinin potentiated the relaxations to ACh in both NA‐ and KCl‐precontracted arterial rings. 4 Contractile sensitivity of endothelium‐intact rings to NA was reduced in SHR by quinapril, and was more effectively increased by L‐NAME in quinapril‐treated than untreated SHR. 5 In conclusion, since the relaxations to ACh and ADP in quinapril‐treated SHR were augmented in the absence and presence of NO synthesis inhibition but not under conditions which prevented hyperpolarization, enhanced endothelium‐dependent relaxation after long‐term ACE inhibition can be attributed to increased endothelium‐dependent hyperpolarization. However, the potentiation of the response to ACh by exogenous bradykinin in quinapril‐treated SHR, as well as the increased attenuating effect of the endothelium on NA‐induced contractions in these animals appear to result from enhanced endothelium‐derived NO release.

Endothelial Function in Deoxycorticosterone-NaCl Hypertension Effect of Calcium Supplementation

BACKGROUND Dietary calcium intake has been suggested to correlate inversely with blood pressure in humans and experimental animals. However, the effects of calcium supplementation on hypertensive disturbances of the endothelium have not been well characterized. METHODS AND RESULTS Wistar-Kyoto rats made hypertensive by deoxycorticosterone (DOC)-NaCl treatment, but a concurrent increase in chow calcium content from 1.1% to 2.5% markedly attenuated the rise in blood pressure. The function of isolated mesenteric arterial rings in vitro was investigated at the close of the 10-week study. In norepinephrine-precontracted rings, the relaxations to acetylcholine (ACh) and ADP, as well as to nitroprusside, 3-morpholinosydnonimine, and isoproterenol were attenuated in hypertensive rats on 1.1% calcium supplementation. In the presence of NG-nitro-L-arginine methyl ester (L-NAME), the relaxations to ACh in hypertensive animals on normal calcium were practically absent, whereas in normotensive rats and calcium-supplemented hypertensive rats, distinct relaxations to higher concentrations of ACh were still present. These responses were reduced by 30% to 50% with apamin, a blocker of Ca2+-activated K+ channels, and were further inhibited by blockade of ATP-dependent K+ channels with glyburide. Interestingly, relaxations elicited by ACh and ADP during precontraction with 60 mmol/L KCl (preventing endothelium-dependent hyperpolarization) were not impaired in hypertensive animals. The contractile sensitivity of endothelium-intact arterial rings to 5-hydroxytryptamine and norepinephrine was higher in hypertensive rats on either normal or high-calcium diet, whereas the increase in contractile sensitivity caused by L-NAME corresponded in all groups. CONCLUSION High-calcium diet markedly opposed experimental DOC-NaCl hypertension, an effect associated with improved arterial relaxation, while abnormalities of vascular contractile properties remained unaffected. In particular, the hyperpolarization-related component of endothelium-dependent arterial relaxation, mediated via opening of arterial K+ channels, could be augmented by calcium supplementation in DOC-NaCl hypertension.

Dietary calcium and magnesium supplements in spontaneously hypertensive rats and isolated arterial reactivity

1 High calcium diet attenuates the development of hypertension but an associated undesirable effect is that Mg2+ loss to the urine is enhanced. Therefore, we studied the effects of high calcium diet alone and in combination with increased magnesium intake on blood pressure and arterial function. 2 Forty‐eight young spontaneously hypertensive rats (SHR) were allocated into four groups, the dietary contents of Ca2+ and Mg2+ being: 1.1%, 0.2% (SHR); 2.5%, 0.2% (Ca‐SHR); 2.5%, 0.8% (CaMg‐SHR); and 1.1%, 0.8% (Mg‐SHR), respectively. Development of hypertension was followed for 13 weeks, whereafter electrolyte balance, lymphocyte intracellular free calcium ([Ca2+]i), and mesenteric arterial responses in vitro were examined. Forty normotensive Wistar‐Kyoto (WKY) rats were investigated in a similar manner. 3 Calcium supplementation comparably attenuated the development of hypertension during normal and high magnesium intake in SHR, with an associated reduced lymphocyte [Ca2+]i and increased Mg2+ loss to the urine. 4 Endothelium‐dependent arterial relaxation to acetylcholine was augmented in Ca‐SHR and CaMg‐SHR, while the relaxations to isoprenaline and the nitric oxide donor SIN‐1 were similar in all SHR groups. Relaxation responses induced by the return of K+ to the organ bath upon precontractions in K+‐free solution were used to evaluate the function of arterial Na+, K+‐ATPase. The rate of potassium relaxation was similar in Ca‐SHR and CaMg‐SHR and faster than in untreated SHR. 5 Contractile responses to high concentrations of potassium and noradrenaline, and the ability of vascular smooth muscle to sequester Ca2+, which was evaluated by eliciting responses to caffeine or noradrenaline after loading periods in different Ca2+ concentrations, were comparable in all SHR groups. In SHR with increased magnesium intake, and in WKY rats with calcium or magnesium supplementation, no detectable effects on blood pressure and arterial function were observed. 6 In conclusion, high calcium diet attenuated the development of hypertension in SHR, with an associated augmented endothelium‐dependent relaxation, promoted recovery rate of ionic gradients across the cell membrane via Na+, K+‐ATPase, and reduced basal [Ca2+]i. Dietary magnesium supplementation, whether combined with normal or high calcium intake, had no beneficial effects on blood pressure or arterial function.

Effects of calcium and potassium supplements on arterial tone in vitro in spontaneously hypertensive rats

Calcium and potassium intakes inversely correlate with blood pressure in experimental hypertension. Therefore, we examined the effects of calcium and potassium supplements alone and in combination on arterial tone in spontaneously hypertensive rats (SHR). Wistar‐Kyoto (WKY) rats served as normotensive controls. Calcium and potassium contents in the control diet were both 1%, while those in supplemented chows were 3% and 3.5%, respectively. The sodium content of all diets was moderately elevated to 1.1%. After 12 weeks of the study systolic blood pressures in SHR on high calcium and on high potassium diets were markedly lower (about 53 and 58 mmHg, respectively) than in hypertensive controls, while combined supplementation of these cations reduced blood pressure even further (about 69 mmHg). Responses of mesenteric arterial rings in vitro were examined at the end of the study. Both high calcium and high potassium diets improved the impaired relaxation to acetylcholine (ACh) in SHR, while the combination of these supplements completely normalized this response. Cyclo‐oxygenase inhibition by diclofenac augmented the relaxation to ACh in hypertensive controls but not in the other groups. Nevertheless, enhanced endothelium‐mediated dilatation was still observed in the presence of diclofenac and the nitric oxide synthase inhibitor NG‐nitro‐L‐arginine methyl ester (L‐NAME) in all supplemented groups. Interestingly, additional blockade of Ca2+‐activated K+ channels by tetraethylammonium abolished the improved relaxation to ACh in SHR on high calcium and on high potassium, but distinct responses were still observed in WKY rats and SHR on the combined supplement. When hyperpolarization of smooth muscle was prevented by precontraction of the preparations with 50 mM KCl, only marginal differences were observed in the diclofenac and L‐NAME‐resistant relaxations to ACh between the study groups. Finally, endothelium‐independent vasorelaxations of noradrenaline‐precontracted rings to nitroprusside, isoprenaline and cromakalim were comparably augmented by all supplements. In conclusion, the vascular mechanisms underlying the antihypertensive effect of high calcium and high potassium diets during moderately elevated sodium intake in SHR may involve enhanced arterial hyperpolarization, increased smooth muscle sensitivity to nitric oxide and decreased production of vasoconstrictor prostanoids. The administration of these cations in combination was more effective than either of them alone in reducing blood pressure and restoring arterial tone.

Comparison of the effects of supplementation with whey mineral and potassium on arterial tone in experimental hypertension

OBJECTIVE The aim of this work was to compare the effects of supplementation of rat chow diet with potassium (K+) and whey mineral concentrate (Whey), a diet rich in milk minerals, on blood pressure and arterial responses in vitro in spontaneously hypertensive rats (SHR). METHODS Thirty young SHR and twenty Wistar-Kyoto rats (WKY) were allocated into five groups: SHR, Whey-SHR, K(+)-SHR, WKY and Whey-WKY. Whey-supplementation was performed by adding 25% whey mineral concentrate to the chow, which in particular increased the intake of potassium (from 1.0% to 3.6%) and also that of calcium (from 1.0% to 1.3%) and magnesium (from 0.2% to 0.3%) in the rats. The K(+)-SHR were given extra potassium chloride (KCl) so that the final potassium content in the chow was 3.6%. Blood pressures were measured indirectly by the tail-cuff method. Responses of mesenteric arterial rings were examined in standard organ chambers after 12 study weeks. RESULTS During the 12-week study systolic blood pressures in control SHR increased steadily from 160 to about 230 mmHg, while supplementation with either Whey or potassium had a clear antihypertensive effect of about 50 mmHg in the hypertensive rats. Blood pressures in the WKY and Whey-WKY groups remained comparable during the whole study. In noradrenaline-precontracted arterial rings, endothelium-dependent relaxation to acetylcholine (ACh), as well as endothelium-independent relaxations to nitroprusside and isoprenaline were attenuated in untreated SHR, while all these dilatory responses were similarly improved by Whey and potassium supplementation. The cyclooxygenase inhibitor diclofenac, which reduces the synthesis of dilatory and constricting prostanoids, clearly enhanced the relaxation to ACh in untreated SHR, but was without effect in the other groups. In the presence of the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester the relaxation to ACh was markedly reduced in all SHR groups, whereas in the two WKY groups, distinct relaxations to ACh were still present. The remaining responses were partially prevented by tetraethylammonium, an inhibitor of calcium-activated potassium channels, and the difference between untreated and potassium-supplemented SHR was abolished. When endothelium-mediated hyperpolarization of smooth muscle was prevented by precontracting the preparations with 50 mM KCl, only marginal differences were observed in relaxations to ACh between untreated SHR and the other groups. Interestingly, the impaired endothelium-independent relaxations to cromakalim, a hyperpolarizing vasodilator acting via ATP-sensitive potassium channels, were normalized by Whey mineral and potassium diets. CONCLUSION Supplementation with Whey mineral and a comparable dose of potassium similarly opposed the development of experimental genetic hypertension, an effect which was associated with improved arterial dilatory properties. Both supplements augmented the hyperpolarization-related component of arterial relaxation, increased the sensitivity of smooth muscle to nitric oxide, and decreased the production of vasoconstrictor prostanoids. Therefore, the beneficial effects of the Whey diet could be attributed to increased intake of potassium in SHR.

Losartan and enalapril therapies enhance vasodilatation in the mesenteric artery of spontaneously hypertensive rats.

We studied the effects of 10-week long enalapril and losartan treatments (4 and 15 mg kg(-1) day(-1), respectively) on mesenteric arterial function in vitro in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). The relaxations of noradrenaline-precontracted rings to acetylcholine, nitroprusside and cromakalim were similar in WKY and enalapril- and losartan-treated SHR, and more pronounced than in untreated SHR. The responses to acetylcholine were attenuated by N(G)-nitro-L-arginine methyl ester in WKY and drug-treated SHR, but were completely inhibited in untreated SHR. When hyperpolarization of smooth muscle was prevented by KCl-induced precontractions, no differences were found in the relaxations to acetylcholine and nitroprusside between the groups, and the dilatations to cromakalim were abolished. Moreover, in noradrenaline-precontracted rings of drug-treated SHR, the addition of tetraethylammonium attenuated the nitric oxide synthase and cyclooxygenase-resistant relaxations to acetylcholine and abolished the enhanced dilatations to nitroprusside. In conclusion, since the enhancement of vasorelaxation in enalapril- and losartan-treated SHR was abolished by conditions preventing hyperpolarization, the improved vasodilatation following these therapies could be attributed to enhanced vasodilatation via K+ channels in this model of hypertension.

Three levels of dietary calcium-effects on blood pressure and electrolyte balance in spontaneously hypertensive rats

SummaryThe effects of three levels of calcium intake on blood pressure (BP) and electrolyte balance were studied for 12 weeks in spontaneously hypertensive rats (SHR): the chow of the SHR-1 group contained 1.1% calcium, and that of the supplemented groups 2.1% (SHR-2) and 3.1% (SHR-3) calcium. Wistar-Kyoto rats on a 1.1% calcium diet (WKY-1) served as normotensive controls. After 10 and 12 weeks BP was significantly lower in both calcium-supplemented groups than in the SHR-1 group, the SHR-2 and SHR-3 groups not deviating from each other. Platelets and lymphocytes were used as experimental cell models to study the effects of the calcium diets on intracellular free calcium ([Ca2+]i) level, which was measured by the fluorescent indicator quin-2. At the end of the study [Ca2+]i was lower in both cell types in SHR-2 and SHR-3 than in SHR-1, the supplemented groups being comparable to each other. In platelets [Ca2+]i still remained higher in the calcium-treated than the WKY-1 group, while in lymphocytes the levels were similar between SHR-2, SHR-3 and WKY-1. Plasma sodium, calcium and magnesium levels did not differ in the SHR groups, but plasma potassium was higher in both supplemented groups than in SHR-1. Plasma renin activity was comparable in SHR-1, SHR-2 and WKY-1, but was suppressed in the SHR-3 group. Creatinine clearance in the SHR-3 group was higher than in SHR-1 and SHR-2, but still remained lower than in WKY 1. High calcium intake was associated with a dose-dependent increase in urinary magnesium excretion, while the excretions of sodium and potassium were proportional to the intakes. The tissue Na+ :K+ ratio in abdominal aorta and tail artery was reduced in SHR-2,.but only a nonsignificant tendency was observed in SHR-3 when compared with the SHR-1 group.In summary, high calcium intake reduces [Ca2+]i in both platelets and lymphocytes in SHR, suggesting that alterations in cellular calcium regulation may explain the BP-lowering effect of calcium supplementation. Elevation of dietary calcium level from 1.1% to 2.1% is associated with a lowering of the Na+ :K+ ratio in the arterial wall. A further increase in calcium content from 2.1% to 3.1% appears to have a favourable effect on renal function in SHR, but also aggravates magnesium loss into the urine. During the higher supplemented calcium intake, suppression of the renin-angiotensin system seems to be involved in the lowering of BP.

Effect of celiprolol therapy on arterial dilatation in experimental hypertension

1 It has recently been suggested that therapy with β‐adrenoceptor blockers reduces peripheral arterial resistance via enhanced vascular dilatation. Therefore, we studied the effects of celiprolol, which is a specific β1‐antagonist that has a weak β2‐agonist action, on arterial tone in spontaneously hypertensive rats (SHR) and Wistar‐Kyoto (WKY) rats. 2 Two doses of celiprolol (5 and 50 mg kg−1 day−1) were administered to the SHR, while the WKY rats received only the higher dose of the drug. During the 12‐week treatment period the higher dose attenuated the increase in blood pressure by approximately 20 mmHg in SHR, whereas the lower dose was without significant antihypertensive effect. Celiprolol therapy did not affect blood pressure in the normotensive WKY rats. 3 Responses of mesenteric arterial rings in vitro were examined at the end of the study. Interestingly, endothelium‐mediated relaxations of noradrenaline (NA)‐precontracted rings to acetylcholine (ACh) in the absence and presence of the cyclo‐oxygenase inhibitor, diclofenac, were equally enhanced in both celiprolol‐treated SHR groups. The nitric oxide synthase inhibitor NG‐nitro‐L‐arginine methyl ester (L‐NAME) practically abolished the relaxations to ACh in all SHR irrespective of whether they had received celiprolol, whereas in WKY rats L‐NAME only attenuated the responses to ACh. However, no differences were found between the SHR groups in relaxations to ACh when hyperpolarization of smooth muscle was prevented by precontractions induced by 50 mM KCl. Vasorelaxation of NA‐precontracted rings to the exogenous nitric oxide donor, nitroprusside, was also moderately augmented in both celiprolol‐treated SHR groups, while the relaxation to β‐adrenoceptor agonist, isoprenaline, remained equally impaired in all SHR whether or not they had received celiprolol. No differences were observed between the two WKY groups in the responses to ACh, nitroprusside or isoprenaline. 4 Contractile sensitivity of mesenteric arterial rings to the receptor‐mediated agonists, NA and 5‐hydroxytryptamine, was comparable in all study groups. 5 In conclusion, SHR treatment with either the low or the higher dose of celiprolol was accompanied by enhancement of both endothelium‐dependent and endothelium‐independent nitric oxide‐mediated arterial relaxation, possibly via a hyperpolarization mechanism. Interestingly, this effect appeared to be independent of the reduction in blood pressure.

Enhancement of arterial relaxation by long-term atenolol treatment in spontaneously hypertensive rats.

1 The effects of long‐term atenolol (25 mg kg−1 day−1) therapy on arterial function were studied in spontaneously hypertensive rats (SHR) and Wistar‐Kyoto (WKY) rats. The 14‐week treatment attenuated the increase in blood pressure by approximately 30 mmHg in SHR, but did not affect blood pressure in WKY rats. 2 Responses of mesenteric arterial rings in vitro were examined at the end of the study. The relaxation to acetylcholine was similar in WKY rats and atenolol‐treated SHR and more pronounced than in untreated SHR, whereas the relaxation to the nitric oxide donor 3‐morpholinosydnonimine (SIN‐1) was comparable in all study groups. Moreover, after maximal relaxations to acetylcholine, marked recontractions developed in untreated SHR but not in the other groups. Vasorelaxation to isoprenaline was also attenuated in SHR and was moderately improved by the atenolol therapy. 3 Arterial relaxation induced by return of potassium to the organ bath upon precontractions elicited by potassium‐free solution were used to evaluate vascular smooth muscle Na+,K+‐ATPase. The rate of potassium relaxation was fastest in WKY rats and was also faster in atenolol‐treated than in untreated SHR. 4 The ability of vascular smooth muscle to sequester calcium was evaluated by eliciting responses to caffeine or noradrenaline after loading periods in different organ bath calcium concentrations. The subsequent contractions were lower in untreated SHR than in WKY rats, and augmented in SHR by the atenolol treatment. 5 Smooth muscle contractions to noradrenaline were comparable in SHR and WKY rats, while atenolol treatment slightly increased the maximal response to this agonist in SHR. Responses to potassium chloride were not affected by atenolol and contractions following cumulative re‐addition of calcium to the organ bath after precontraction with potassium chloride and noradrenaline in calcium‐free solution were comparable in all study groups. 6 In conclusion, the moderate antihypertensive effect of atenolol in SHR was accompanied by enhancement of β‐adrenoceptor‐mediated and normalization of endothelium‐dependent arterial relaxation. Furthermore, ability to sequester calcium into cellular stores, and function of Na+,K+‐ATPase were augmented in vascular smooth muscle. Therefore, the present results suggest that the long‐term blood pressure‐lowering action of atenolol in this type of genetic hypertension is accompanied by improved arterial relaxation and normalization of endothelial function.