Pasi Jolma

Randomized Controlled Study of Effects of Sudden Impact Loading on Rat Femur

Physical loading creating high peak strains on the skeleton at high strain rates is suggested to be the most effective type of activity in terms of bone mineral acquisition. This study assessed the effects of sudden impact loading on mineral and mechanical bone properties in 13‐week‐old Sprague‐Dawley rats. The rats were randomly assigned as sedentary controls (SED, n = 10), control animals receiving low‐intensity exercise (EX, n = 15), and experimental animals receiving low‐intensity exercise combined with sudden impact‐loading (EX + IMP, n = 15). In the EX group, the rats walked in a walking mill at a speed of 10 cm/s for 20 minutes/day, 5 days/week for 9 weeks. In the EX + IMP group, the program was identical to the EX group except for the additional sudden impacts administered to their skeleton during the walking exercise. At the start, there were 50 impacts per session, after which their number was gradually increased to 200 impacts per session by week 6 and then kept constant until the end of the experiment, week 9. These horizontally and vertically directed body impacts were produced by a custom‐made walking mill equipped with computer‐controlled high‐pressure air cylinders. After sacrifice, both femora of each rat were removed and their dimensions, bone mineral content (BMC) by dual‐energy X‐ray absorptiometry, and mechanical properties by femoral shaft three‐point bending and femoral neck compression were determined. The cortical wall thickness increased significantly in the EX and EX + IMP groups as compared with SEDs (+7.6%, p = 0.049 and +10%, p = 0.020, respectively). The EX + IMP group showed +9.0% (p = 0.046) higher cross‐sectional moment of inertia values than the EX group. No significant intergroup differences were seen in the BMC values, while the breaking load of the femoral shaft (EX + IMP vs. SED +8.8%, p = 0.047) and femoral neck (EX + IMP vs. SED +14.1%, p = 0.013) was significantly enhanced by the impact loading. In conclusion, this study indicates that mechanical loading can substantially improve the mechanical characteristics of a rat femur without simultaneous gain in its mineral mass. If this is true in humans too, our finding gives an interesting perspective to the numerous longitudinal exercise studies (of women) in which the exercise‐induced gains in bone mass and density have remained mild to moderate only.

Arterial function in nitric oxide-deficient hypertension: influence of long-term angiotensin II receptor antagonism

OBJECTIVE Since the effects of angiotensin II receptor antagonism on arterial function in nitric oxide (NO)-deficient hypertension are unknown, we investigated the influence of losartan therapy (20 mg kg-1 day-1) on the control of arterial tone in NG-nitro-L-arginine methyl ester (L-NAME; 20 mg kg-1 day-1)-induced hypertension. METHODS Forty Wistar rats were divided into four groups: control, losartan, L-NAME, and losartan + L-NAME. The responses of isolated mesenteric arterial rings were examined in standard organ chambers after 8 treatment weeks. RESULTS Losartan therapy prevented the development of L-NAME-induced hypertension and the associated impairments of endothelium-independent relaxations to nitroprusside, isoprenaline, and cromakalim, vasodilators acting via the formation of NO, activation of beta-adrenoceptors and opening of K+ channels, respectively. In addition, endothelium-dependent relaxations of noradrenaline-precontracted rings to acetylcholine during NO synthase inhibition in vitro were decreased in L-NAME rats, and clearly improved by losartan therapy. The inhibition of cyclooxygenase by diclofenac improved the responses to acetylcholine more effectively in L-NAME than losartan + L-NAME rats, but the relaxations remained decreased in L-NAME rats when compared with losartan + L-NAME rats. When hyperpolarization of smooth muscle was prevented by precontractions induced by high concentration of KCl, the responses to acetylcholine during combined NO synthase and cyclooxygenase inhibition were similar and almost abolished in all groups. Furthermore, superoxide dismutase, a scavenger of superoxide anions, enhanced the acetylcholine-induced relaxations more effectively in L-NAME than losartan + L-NAME rats, although plasma antioxidant capacity was similar in all study groups. CONCLUSION Chronic L-NAME-induced hypertension was associated with attenuated arterial relaxation via endothelium-dependent and -independent mechanisms, both of which were improved by the losartan treatment. The mechanisms whereby losartan enhanced arterial relaxation in this model of experimental hypertension may have included enhanced hyperpolarization and increased sensitivity to NO in smooth muscle, and decreased vascular production of superoxide and vasoconstrictor prostanoids.

AT1 Receptor Blockade Improves Vasorelaxation in Experimental Renal Failure

Abstract— It is not known whether angiotensin II type 1 receptor antagonists can influence the function and morphology of small arteries in renal failure. We investigated the effect of 8‐week losartan therapy (20 mg/kg per day) on isolated mesenteric resistance arteries by wire and pressure myographs in 5/6 nephrectomized rats. Plasma urea nitrogen was elevated 1.6‐fold after nephrectomy, and ventricular synthesis of atrial and B‐type natriuretic peptides was increased 2.2‐fold and 1.7‐fold, respectively, whereas blood pressure was not affected. Losartan did not influence these variables. The endothelium‐mediated relaxation to acetylcholine was impaired in nephrectomized rats in the absence and presence of nitric oxide synthase and cyclooxygenase inhibition. Blockade of calcium‐activated potassium channels by charybdotoxin and apamin reduced the remaining acetylcholine response, and this effect was less marked in nephrectomized than in sham‐operated rats. Relaxation to levcromakalim, a vasodilator acting through adenosine triphosphate‐sensitive potassium channels, was also impaired after nephrectomy. The arteries of nephrectomized rats showed eutrophic inward remodeling: Wall‐to‐lumen ratio was increased without change in wall cross‐sectional area. All changes in arterial relaxation and morphology were normalized by losartan therapy. Aortic ACE content, measured by autoradiography, directly correlated to the plasma level of urea nitrogen, suggesting that renal failure has an enhancing influence on the vascular renin‐angiotensin system. Losartan normalized relaxation and morphology of resistance arteries in experimental renal failure, independent of its influence on blood pressure, impaired kidney function, or volume overload. The mechanism of improved vasodilation by losartan may include enhanced relaxation through potassium channels.

Potassium channel-mediated vasorelaxation is impaired in experimental renal failure

Chronic renal failure is associated with increased cardiovascular morbidity and abnormal arterial tone, but the underlying pathophysiological mechanisms are poorly understood. Therefore, we studied the responses of isolated mesenteric arterial rings from Wistar-Kyoto rats in standard organ chambers 6 wk after subtotal (5/6) nephrectomy or sham operation. Subtotal nephrectomy resulted in a 1.7-fold elevation of plasma urea nitrogen, whereas blood pressure was not significantly affected. Endothelium-mediated relaxations of norepinephrine-precontracted rings to ACh were impaired in renal failure rats. The nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester inhibited relaxations to ACh more effectively in the renal failure group, whereas the cyclooxygenase inhibitor diclofenac did not significantly affect the response in either group. Inhibition of Ca(2+)-activated K(+) channels by charybdotoxin and apamin attenuated NO synthase- and cyclooxygenase-resistant relaxations to ACh in control but not renal failure rats and abolished the difference between these groups. Endothelium-independent relaxations to isoproterenol and cromakalim, vasodilators acting via beta-adrenoceptors and ATP-sensitive K(+) channels, respectively, were impaired in the renal failure group, whereas relaxations to the NO donor nitroprusside were similar in both groups. In conclusion, endothelium-mediated relaxation in renal failure rats was impaired in the absence and presence of NO synthase and cyclooxygenase inhibition but not with prevented smooth muscle hyperpolarization. Endothelium-independent relaxations to isoproterenol and cromakalim were also attenuated after 5/6 nephrectomy. These results suggest that impaired vasodilatation in experimental renal failure could be attributed to reduced relaxation via arterial K(+) channels.Chronic renal failure is associated with increased cardiovascular morbidity and abnormal arterial tone, but the underlying pathophysiological mechanisms are poorly understood. Therefore, we studied the responses of isolated mesenteric arterial rings from Wistar-Kyoto rats in standard organ chambers 6 wk after subtotal (5/6) nephrectomy or sham operation. Subtotal nephrectomy resulted in a 1.7-fold elevation of plasma urea nitrogen, whereas blood pressure was not significantly affected. Endothelium-mediated relaxations of norepinephrine-precontracted rings to ACh were impaired in renal failure rats. The nitric oxide (NO) synthase inhibitor N G-nitro-l-arginine methyl ester inhibited relaxations to ACh more effectively in the renal failure group, whereas the cyclooxygenase inhibitor diclofenac did not significantly affect the response in either group. Inhibition of Ca2+-activated K+ channels by charybdotoxin and apamin attenuated NO synthase- and cyclooxygenase-resistant relaxations to ACh in control but not renal failure rats and abolished the difference between these groups. Endothelium-independent relaxations to isoproterenol and cromakalim, vasodilators acting via β-adrenoceptors and ATP-sensitive K+ channels, respectively, were impaired in the renal failure group, whereas relaxations to the NO donor nitroprusside were similar in both groups. In conclusion, endothelium-mediated relaxation in renal failure rats was impaired in the absence and presence of NO synthase and cyclooxygenase inhibition but not with prevented smooth muscle hyperpolarization. Endothelium-independent relaxations to isoproterenol and cromakalim were also attenuated after 5/6 nephrectomy. These results suggest that impaired vasodilatation in experimental renal failure could be attributed to reduced relaxation via arterial K+ channels.

Hyperuricemia, Oxidative Stress, and Carotid Artery Tone in Experimental Renal Insufficiency

BACKGROUND Hyperuricemia may play a role in the pathogenesis of cardiovascular disease, but uric acid is also a significant antioxidant. We investigated the effects of oxonic acid-induced hyperuricemia on carotid artery tone in experimental renal insufficiency. METHODS Three weeks after 5/6 nephrectomy (NX) or Sham operation, male Sprague-Dawley rats were allocated to 2.0% oxonic acid or control diet for 9 weeks. Blood pressure was monitored using tail cuff, isolated arterial rings were examined using myographs, and blood and urine samples were taken, as appropriate. Oxidative stress and antioxidant status were evaluated by measuring urinary 8-isoprostaglandin F(2 alpha) (8-iso-PGF(2 alpha)) excretion and plasma total peroxyl radical-trapping capacity (TRAP), respectively. RESULTS Plasma creatinine was elevated twofold in NX rats, but neither NX nor oxonic acid diet influenced blood pressure. Urinary 8-iso-PGF(2 alpha) excretion was increased over 2.5-fold in NX rats on control diet. Oxonic acid diet increased plasma uric acid 2-3-fold, TRAP 1.5-fold, and reduced urinary 8-iso-PGF(2 alpha) excretion by 60-90%. Carotid vasorelaxation to acetylcholine in vitro, which could be abolished by nitric oxide (NO) synthase inhibition, was reduced following NX, whereas maximal response to acetylcholine was augmented in hyperuricemic NX rats. Vasorelaxation to nitroprusside was impaired in NX rats, whereas oxonic acid diet increased sensitivity also to nitroprusside in NX rats. CONCLUSIONS Oxonic acid-induced hyperuricemia reduced oxidative stress in vivo, as evaluated using urinary 8-iso-PGF(2 alpha) excretion, increased plasma TRAP, and improved NO-mediated vasorelaxation in the carotid artery in experimental renal insufficiency.

Paricalcitol [19-nor-1,25-(OH)2D2] in the treatment of experimental renal bone disease.

Paricalcitol is a less hypercalcemic vitamin D analog that has been shown to suppress secondary hyperparathyroidism and to prevent the associated histomorphometric changes in bone. In this study, we show that paricalcitol also ameliorates the renal insufficiency‐induced loss of bone mineral and the mechanical competence of bone.

Ng-nitro-l-arginine Methyl Ester–induced Hypertension and Natriuretic Peptide Gene Expression: Inhibition by Angiotensin Ii Type 1 Receptor Antagonism

This study examined the role of angiotensin II in the increase of blood pressure, activation of cardiac natriuretic peptide gene expression, left ventricular hypertrophy, and vascular changes in nitric oxide–deficient hypertension. NG-nitro-l-arginine methyl ester (l-NAME, 20 mg/kg/d), angiotensin II type 1 receptor (AT1) antagonist losartan (20 mg/kg/d), or their combination were administered orally for 8 weeks in Wistar rats. l-NAME elevated systolic blood pressure, which reached its maximum within 4 weeks (200 ± 4 mm Hg). Despite hypertension, l-NAME administration for 8 weeks did not induce left ventricular hypertrophy. Losartan treatment significantly decreased the development of hypertension induced by l-NAME and decreased left ventricular hypertrophy in untreated rats. In contrast, losartan did not prevent the hypertrophic remodeling of the mesenteric resistance arteries induced by l-NAME. l-NAME treatment increased ventricular atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) mRNA levels and immunoreactive BNP levels significantly. Losartan therapy decreased the l-NAME-induced ventricular ANP gene expression by 69% (p < 0.05) and also reduced ventricular BNP mRNA levels so that it did not differ from control. Losartan treatment alone decreased ventricular immunoreactive ANP and BNP levels by 30% (p < 0.05). These results show that ventricular ANP and BNP gene expression are dissociated from the increased ventricular mass in nitric oxide deficiency–induced hypertension. Results suggest that l-NAME-induced hypertension and the associated activation of ventricular ANP and BNP gene expression are, at least in part, mediated by angiotensin II, whereas the resistance vessel hypertrophy following nitric oxide synthase inhibition is angiotensin II independent.

Vascular influences of calcium supplementation and vitamin D-induced hypercalcemia in NaCl-hypertensive rats.

&NA; This 8‐week study investigated the effects of increasing dietary Ca2+ content from 1.0% to 3.0% and hypercalcemia induced by oral 1&agr;‐OH vitamin D3 (1OH‐D3, 1.2 &mgr;g/kg), on arterial tone in NaCl‐hypertensive rats. The high‐Ca2+ diet completely prevented the increase in blood pressure induced by the 6.0% NaCl chow, while plasma total Ca2+ and body weight were not different from controls. The 1OH‐D3 treatment moderately elevated plasma total Ca2+ and attenuated the NaCl‐induced rise in blood pressure, but also impaired weight gain. The tone of isolated mesenteric arterial rings was examined at the end of study. The endothelium‐independent relaxations to nitroprusside, isoproterenol, and cromakalim were impaired in NaCl‐hypertension. Experiments with NG‐nitro‐l‐arginine methyl ester and tetraethylammonium in vitro suggested that both the nitric oxide‐ and hyperpolarization‐mediated components of endothelium‐dependent relaxation to acetylcholine were reduced in NaCl‐hypertensive rats. All of the impaired relaxations in NaCl hypertension were normalized by concomitant Ca2+ supplementation. The 1OH‐D3 treatment did not affect vascular relaxation, but it attenuated maximal contractile responses induced by norepinephrine and KCl by more than 50%. The reduced vasoconstrictor responses could not be explained by increased apoptosis in the vessel wall, but calcification may have played a role, since moderate signs of medial or adventitial calcification were observed in the aortic preparations after the 1OH‐D3 treatment. In conclusion, a high‐Ca2+ diet, which did not cause hypercalcemia, normalized blood pressure and endothelium‐dependent and endothelium‐independent vasorelaxation in NaCl‐hypertensive rats. In contrast, chronic hypercalcemia induced by 1OH‐D3 was associated with moderately lowered blood pressure, possibly because of reduced vasoconstrictor responses in arterial smooth muscle.

Preserved Endothelium-Dependent but Impaired β-Adrenergic Relaxation of the Resistance Vessels in Experimental Renal Failure

Chronic renal failure is associated with increased cardiovascular morbidity and reduced arterial elasticity. Only little information is available on the functional effects of uraemia on resistance arteries. Therefore, we studied the influence of renal failure on rat small mesenteric vessels. The responses of arterial rings were investigated in a Mulvany myograph 6 weeks after 5/6 nephrectomy or sham operation. The subtotal nephrectomy resulted in a 1.9-fold elevation of plasma urea nitrogen but was without significant effect on blood pressure. Endothelium-dependent relaxations, largely mediated via arterial K+ channels, were preserved in the resistance vessels of uraemic rats. Endothelium-independent vasorelaxations, mediated via exogenous nitric oxide and the opening of ATP-sensitive K+ channels, were also unchanged. However, the responses induced by isoprenaline were slightly reduced, indicating impaired relaxation via β-adrenoceptors in experimental renal failure.

Treatment of experimental renal osteodystrophy with pamidronate.

We evaluated the effects of the bisphosphonate pamidronate on bone histomorphometry, structure and strength in male rats with uninephrectomy or with chronic renal disease induced by 5/6 nephrectomy. In rats with chronic renal disease the plasma urea, phosphate and parathyroid hormone levels were significantly increased compared to rats with a uninephroctomy and none of these parameters was affected by pamidronate treatment. In the femoral midshaft, chronic renal disease reduced cortical bone mineral density and content. No difference was observed in the breaking load of the femoral midshaft. In the distal femur, a high-turnover renal osteodystrophy was found but pamidronate suppressed this bone turnover and increased bone mineral content. Treatment had no effect on chronic disease-induced augmentation of osteoid volume or fibroblast surface. These studies show that in this model of stage 3 renal disease, pamidronate increased mineral content in the femoral midshaft and distal metaphysis primarily by adding bone to endocortical and trabecular surfaces but did not reduce osteitis fibrosa.