High-calcium diet enhances vasorelaxation in nitric oxide-deficient hypertension

Because the effects of calcium supplementation on arterial tone in nitric oxide-deficient hypertension are unknown, we investigated the influence of elevating dietary calcium from 1.1 to 3.0% in Wistar rats treated with N(G)-nitro-L-arginine methyl ester (L-NAME; 20 mg. kg(-1). day(-1)) for 8 wk. A high-calcium diet attenuated the development of hypertension induced by L-NAME and abrogated the associated impairments of endothelium-independent mesenteric arterial relaxations to nitroprusside, isoproterenol, and cromakalim. Endothelium-dependent relaxations to acetylcholine during nitric oxide synthase inhibition in vitro were decreased in L-NAME rats and improved by calcium supplementation. The inhibition of cyclooxygenase by diclofenac augmented the responses to acetylcholine in L-NAME rats but not in calcium + L-NAME rats. When hyperpolarization of smooth muscle was prevented by KCl precontraction, the responses to acetylcholine during combined nitric oxide synthase and cyclooxygenase inhibition were similar in all groups. Furthermore, superoxide dismutase enhanced the acetylcholine-induced relaxations in L-NAME rats but not in calcium + L-NAME rats. In conclusion, calcium supplementation reduced blood pressure during chronic nitric oxide synthase inhibition and abrogated the associated impairments in endothelium-dependent and -independent arterial relaxation. The augmented vasorelaxation after increased calcium intake in L-NAME hypertension may be explained by enhanced hyperpolarization and increased sensitivity to nitric oxide in arterial smooth muscle and decreased vascular production of superoxide and vasoconstrictor prostanoids.

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.

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.

Control of mesenteric arterial tone in vitro in humans and rats

The majority of the findings concerning arterial physiology and pathophysiology originate from studies with experimental animals, while only limited information exists about the functional characteristics of human arteries. Therefore, the aim of the present work was to compare the control of vascular tone in vitro in mesenteric arterial rings of corresponding size (outer diameter 0.75-1 mm) from humans and Wistar-Kyoto rats. The relaxations to acetylcholine (ACh) were clearly less marked in the mesenteric arteries of humans when compared with rats. However, when calcium ionophore A23187 was used as the vasodilator, the endothelium-mediated relaxations did not significantly differ between these species. The NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) attenuated the relaxations to ACh and A23187 in both groups. The endothelium-independent relaxations to the beta-adrenoceptor agonist isoprenaline and the nitric oxide (NO)-donor nitroprusside were somewhat lower in human arteries, while vasodilation induced by the K+ channel opener cromakalim was similar between humans and rats. Arterial contractile sensitivity to noradrenaline and serotonin was slightly lower in human vessels, whereas contractile sensitivity to KCl was similar between these species. The contractions induced by cumulative addition of Ca2+ with noradrenaline as the agonist were effectively inhibited in both groups by the calcium channel blocker nifedipine, the effect of which was clearly more pronounced in human arteries. In conclusion, the control of vascular tone of isolated arteries of corresponding size from humans and rats appeared to be rather similar. The most marked differences between these species were the impaired endothelium-mediated dilation to ACh and the more pronounced effect of nifedipine on the Ca2(+)-induced contractions in human arteries.

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.