Ca2+ influx mediates enhanced alpha2-adrenergic contraction in aortas from rats treated with NOS inhibitor

Impact of moderate- and high-intensity exercise on the endothelial ultrastructure and function in mesenteric arteries from hypertensive rats

Oxidative stress (OS) influences vascular function and structure in spontaneously hypertensive rats (SHRs). It is also responsible for the decreased nitric oxide (NO) bioavailability that influences endothelial vasodilation. The effects of high-intensity exercise on endothelial function and ultrastructure in hypertension remain unknown. Thus, this study investigated the effects of moderate- and high-intensity exercise on hypertension-associated endothelial dysfunction and ultrastructural remodeling. Moderate-intensity (SHR-M) and high-intensity (SHRH) aerobic exercise training groups were compared in age-matched sedentary SHRs (SHRC) and normotensive Wistar-Kyoto rats (WKY-C). The results showed that the endothelial ultrastructure was impaired in the SHR-H and SHR-C groups. Glutathione peroxidase levels were significantly increased in the SHR-M group compared to the SHR-C group. MDA content was higher in the SHR-H group than in the SHR-C group, but the levels of antioxidant enzymes did not increase accordingly. Apocynin scavenging reactive oxygen species (ROS) ameliorated endothelium-dependent vasodilator function in the SHR-H group. However, the SHR-M and WKY-C groups abolished the increased vasodilation induced by apocynin. L-NAME, a NO synthase inhibitor, was applied to isolated mesenteric arteries (MAs) to evaluate NO contribution. Moderate-intensity exercise reversed the decreased NO contribution to MAs in hypertension, and high-intensity exercise aggravated this change. These data suggest that moderate-intensity exercise ameliorated adverse remodeling of the endothelial ultrastructure and function in hypertension by decreasing oxidative stress and increasing NO contribution. However, high-intensity exercise exacerbated all of these changes by increasing OS and ROS contribution, and decreasing NO contribution.

Alpha(2)-adrenergic receptor signalling in hypertension

Cardiovascular regulation by the sympathetic nervous system is mediated by activation of one or more of the nine known subtypes of the adrenergic receptor family; alpha(1A)-, alpha(1B)-, alpha(1D)-, alpha(2A)-, alpha(2B)-, alpha(2C)-, beta(1)-, beta(2)- and beta(3)-ARs (adrenoceptors). The role of the alpha(2)-AR family has long been known to include presynaptic inhibition of neurotransmitter release, diminished sympathetic efferent traffic, vasodilation and vasoconstriction. This complex response is mediated by one of three subtypes which all uniquely affect blood pressure and blood flow. All three subtypes are present in the brain, kidney, heart and vasculature. However, each differentially influences blood pressure and sympathetic transmission. Activation of alpha(2A)-ARs in cardiovascular control centres of the brain lowers blood pressure and decreases plasma noradrenaline (norepinephrine), activation of peripheral alpha(2B)-ARs causes sodium retention and vasoconstriction, whereas activation of peripheral alpha(2C)-ARs causes cold-induced vasoconstriction. In addition, non-selective agonists elicit endothelium-dependent vasodilation and presynaptic inhibition of noradrenaline release. The evidence that each of these receptor subtypes uniquely participates in cardiovascular control is discussed in this review.

Reduced functional expression of K+channels in vascular smooth muscle cells from rats made hypertensive withNω-nitro-l-arginine

Smooth muscle membrane potential is determined, in part, by K+channels. In the companion paper to this article (Bratz IN, Dick GM, Partridge LD, and Kanagy NL. Am J Physiol Heart Circ Physiol 289: H1277–H1283, 2005), we demonstrated that superior mesenteric arteries from rats made hypertensive with Nω-nitro-l-arginine (l-NNA) are depolarized and express less K+channel protein compared with those from normotensive rats. In the present study, we used patch-clamp techniques to test the hypothesis that l-NNA-induced hypertension reduces the functional expression of K+channels in smooth muscle. In whole cell experiments using a Ca2+-free pipette solution, current at 0 mV, largely due to voltage-dependent K+(KV) channels, was reduced ∼60% by hypertension (2.7 ± 0.4 vs. 1.1 ± 0.2 pA/pF). Current at +100 mV with 300 nM free Ca2+, largely due to large-conductance Ca2+-activated K+(BKCa) channels, was reduced ∼40% by hypertension (181 ± 24 vs. 101 ± 28 pA/pF). Current blocked by 3 mM 4-aminopyridine, an inhibitor of many KVchannel types, was reduced ∼50% by hypertension (1.0 ± 0.4 vs. 0.5 ± 0.2 pA/pF). Current blocked by 1 mM tetraethylammonium, an inhibitor of BKCachannels, was reduced ∼40% by hypertension (86 ± 14 vs. 53 ± 19 pA/pF). Differences in BKCacurrent magnitude are not attributable to changes in single-channel conductance or Ca2+/voltage sensitivity. The data support the hypothesis that l-NNA-induced hypertension reduces K+current in vascular smooth muscle. Reduced molecular and functional expression of K+channels may partly explain the depolarization and augmented contractile sensitivity of smooth muscle from l-NNA-treated rats.

Ion changes in spreading ischaemia induce rat middle cerebral artery constriction in the absence of NO

In rats, cortical spreading hyperaemia is coupled to a spreading neuroglial depolarization wave (spreading depression) under physiological conditions, whereas cortical spreading ischaemia is coupled to it if red blood cell products are present in the subarachnoid space. Spreading ischaemia has been proposed as the pathophysiological correlate of the widespread cortical infarcts abundantly found in autopsy studies of patients with subarachnoid haemorrhage. The purpose of the present study was to investigate whether the extracellular ion changes associated with the depolarization wave may cause the vasoconstriction underlying spreading ischaemia. We induced spreading ischaemia in vivo with the nitric oxide (NO) scavenger oxyhaemoglobin and an elevated K+ concentration in the subarachnoid space while slow potential, pH, extracellular volume and concentrations of K+, Na+, Ca2+ and Cl- were measured in the cortex with microelectrodes. We then extraluminally applied an ionic cocktail (cocktail(SI)) to the isolated middle cerebral artery in vitro, matching the ionic composition of the extracellular space as measured during spreading ischaemia in vivo. Extraluminal application of cocktail(SI) caused middle cerebral artery dilatation in the absence and constriction in the presence of NO synthase inhibition in vitro, corresponding with the occurrence of spreading hyperaemia in the presence and spreading ischaemia in the absence of NO in vivo. The L-type Ca2+ inhibitor nimodipine caused the cocktail(SI)-induced vasoconstriction to revert to vasodilatation in the absence of NO in vitro similar to the reversal of spreading ischaemia to spreading hyperaemia in response to nimodipine in vivo. We found that K+ was the predominant vasoconstrictor contained in cocktail(SI). Its vasoconstrictor action was augmented by NO synthase inhibition. Our results suggest that, under elevated baseline K+ as a hallmark of any condition of energy deficiency, the extracellular ion changes represent the essential mediator of the vascular response to spreading neuroglial depolarization. In the presence of NO they mediate vasodilatation and in its absence they mediate constriction.

Reduced molecular expression of K(+) channel proteins in vascular smooth muscle from rats made hypertensive with N{omega}-nitro-L-arginine

Smooth muscle membrane potential (E(m)) depends on K(+) channels, and arteries from rats made hypertensive with N(omega)-nitro-l-arginine (LHR) are depolarized compared with control. We hypothesized that decreased K(+) channel function, due to decreased K(+) channel protein expression, underlies E(m) depolarization. Furthermore, K(+) channel blockers should move control E(m) (-46 +/- 1 mV) toward that in LHR (-37 +/- 2 mV) and normalize contraction. The E(m) vs. K(+) relationship was less steep in LHR (23 +/- 2 vs. 28 +/- 1 mV/log K(+) concentration), and contractile sensitivity to K(+) was increased (EC(50) = 37 +/- 1 vs. 23 +/- 1 mM). Iberiotoxin (10 nM), an inhibitor of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels, depolarized control and LHR E(m) to -35 +/- 1 and -30 +/- 2 mV, respectively; however, effects on K(+) sensitivity were more profound in LHR (EC(50) = 25 +/- 2 vs. 15 +/- 3 mM). The voltage-dependent K(+) (K(V)) channel blocker 4-aminopyridine (3 mM) depolarized control E(m) to the level of LHR (-28 +/- 1 vs. -28 +/- 1 mV); however, effects on K(+) sensitivity were greater in LHR (EC(50) = 17 +/- 4 vs. 4 +/- 4 mM). Western blots revealed reduced BK(Ca) and K(V)1.5 channel expression in LHR arteries. The findings suggest that diminished expression of K(+) channels contributes to depolarization and enhanced contractile sensitivity. These conclusions are supported by direct electrophysiological assessment of BK(Ca) and K(V) channel function in control and LHR smooth muscle cells.

Inhibition of Rho-kinase stimulates nitric oxide-independent vasorelaxation

Vasoconstrictor factors, like urotensin, angiotensin and catecholamines, activate Rho-dependent serine-threonine kinase (Rho-kinase) and inhibition of this pathway represents a novel therapy for cardiovascular diseases with hypertensive syndrome. The disbalance of relaxing endothelial nitric oxide (NO)-producing and vasoconstrictive pathways can be especially important in diseases where hypertension is accompanied by endothelial dysfunction that compromises NO generation. However, a recent study reported that the efficacy of the Rho-kinase inhibitor (R)-(+)-trans-N-(4-Pyridyl)-4-(1-aminoethyl)cyclohexanecarboxamide (Y27632) is dramatically attenuated upon removal of endothelium or inhibition of endothelial NO synthase (eNOS). This raises the question whether Rho-kinase inhibition could be an effective treatment in case of hypertension associated with endothelial dysfunction. The purpose of the present study was to determine whether the vasorelaxing effect of Rho-kinase inhibition is mediated through eNOS-dependent mechanisms. We show here that in the models of genetically reduced endothelial NO production (eNOS-/- mice and spontaneous hypertensive rats (SHR)) or in models of pharmacologically reduced endogenous NO production (N(omega)-nitro-L-arginine methyl ester (LNAME) treatment), Rho-kinase inhibition induced a strong vasodilation and reduction of blood pressure indicating independence of Rho-kinase pathway from eNOS. An additional important finding of our study is that Rho-kinase inhibitors induce a strong vasorelaxation and blood pressure reduction upon intravenous injection not only in hypertensive but in normotensive animals, as well. Inhibition of Rho-kinase represents a promising possibility to treat hypertension that is accompanied by endothelial dysfunction.

17-β Estradiol Independently Regulates Erythropoietin Synthesis and NOS Activity during Hypoxia

We reported previously that 17-beta estradiol (E2-beta) attenuates hypoxic induction of erythropoietin (EPO) synthesis in rats. We hypothesized this attenuation is mediated by increased nitric oxide (NO) bio-availability. To investigate this hypothesis, ovariectomized estrogen-depleted rats were instrumented with arterial and venous catheters and treated with either E2-beta (20 microg/24 hrs) or vehicle (polypropylene glycol) for 7 days. Rats were placed in Plexiglas boxes and administered a bolus of either the NO synthase inhibitor, Nomega-nitro-L-arginine (l-NNA, 15 mg/kg) or saline. Following this bolus, saline or l-NNA was continuously infused (15 mg/kg/h) throughout the 8 hours of hypoxic exposure (12% O2). Hypoxia increased plasma NO metabolites (NOx) in both saline groups but more in E2-beta-treated rats. l-NNA prevented this increase in both groups. Renal endothelial NO synthase (NOS) expression was unaltered by hypoxia, l-NNA, or E2-beta. Despite preventing increases in plasma NOx during hypoxia, l-NNA did not affect E2-beta attenuation of EPO synthesis. We conclude that E2-beta independently attenuates hypoxic induction of EPO and augments hypoxic increases in NO synthesis.

Mechanism of enhanced calcium sensitivity and alpha 2-AR vasoreactivity in chronic NOS inhibition hypertension

PKC augments calcium sensitivity in spontaneously hypertensive rats and contributes to alpha(2)-adrenergic receptor (AR) contraction in rabbit saphenous vein. We showed previously that denuded aortic rings from N(omega)-nitro-l-arginine-treated hypertensive rats (LHR) contract more to CaCl(2) and to the alpha(2)-AR agonist UK-14304 than do rings from normotensive rats (NR). We hypothesized that enhanced PKC activity or a change in PKC isoform contributes to augmented calcium sensitivity and enhanced alpha(2)-AR contraction in LHR aorta. Current studies demonstrate that non-isoform-specific PKC inhibitors reduced UK-14304 contraction in both NR and LHR aorta. However, the calcium-dependent PKC inhibitor Gö-6976 only attenuated contraction in LHR aorta. Additionally, UK-14304 translocated PKC-delta to the membrane in NR aorta, whereas PKC-alpha was translocated to the membrane in LHR aorta. Finally, in ionomycin-permeabilized aorta Gö-6976 eliminated enhanced basal and augmented alpha(2)-AR-stimulated calcium sensitivity in LHR aorta but did not affect NR contraction. Together, these data suggest that PKC-alpha contributes to augmented calcium sensitivity and alpha(2)-AR reactivity after chronic nitric oxide synthase inhibition hypertension.

Tyrosine kinases regulate intracellular calcium during alpha(2)-adrenergic contraction in rat aorta

We have demonstrated enhanced contractile sensitivity to the alpha(2)-adrenoreceptor (alpha(2)-AR) agonist UK-14304 in arteries from rats made hypertensive with chronic nitric oxide synthase (NOS) inhibition (LHR) compared with arteries from normotensive rats (NR); additionally, this contraction requires Ca(2+) entry. We hypothesized that tyrosine kinases augment alpha(2)-AR contraction in LHR arteries by increasing Ca(2+). The tyrosine kinase inhibitor tyrphostin 23 significantly attenuated UK-14304 contraction of denuded thoracic aortic rings from NR and LHR. However, tyrphostin 23 did not alter UK-14304 contraction in ionomycin-permeabilized aorta, which indicates that tyrosine kinases regulate intracellular Ca(2+) concentration. The Src family inhibitor PP1 and the epidermal growth factor receptor kinase inhibitor AG-1478 did not alter alpha(2)-AR contraction, whereas the mitogen-activated protein kinase extracellular signal-regulated kinase kinase inhibitor PD-98059 attenuated the contraction. Contraction to CaCl(2) in ionomycin-permeabilized LHR rings was greater than in NR rings. UK-14304 augmented CaCl(2) contraction in ionomycin-permeabilized rings from both groups but to a greater extent in LHR aorta. Together, these data suggest that alpha(2)-AR stimulates contraction via two pathways. One, which is enhanced with NOS inhibition hypertension, activates Ca(2+) sensitivity and is independent of tyrosine kinases. The other is tyrosine kinase dependent and regulates intracellular Ca(2+) concentration.

Acute and chronic NOS inhibition enhances alpha(2)- adrenoreceptor-stimulated RhoA and Rho kinase in rat aorta

We demonstrated that arteries from rats made hypertensive with chronic nitric oxide (NO) synthase (NOS) inhibition (N(omega)-nitro-L-arginine in drinking water, LHR) have enhanced contractile sensitivity to alpha(2)-adrenergic receptors (alpha(2)-AR) agonist UK-14304 compared with arteries from normotensive rats (NR). NO may regulate vascular tone in part through suppression of RhoA and Rho kinase (ROK). We hypothesized that enhanced RhoA and ROK activity augments alpha(2)-AR contraction in LHR aortic rings. Y-27632 eliminated UK-14304 contraction in LHR and NR aortic rings. The order of increasing sensitivity to Y-27632 was the following: endothelium-intact NR, LHR, and endothelium-denuded NR. UK-14304 stimulated RhoA translocation to the membrane fraction in LHR and denuded NR but not in intact NR aorta. Basally, more RhoA was present in the membrane fraction in denuded NR than in intact NR or LHR aorta. Relaxation to S-nitroso-N-acetyl-penicillamine and Y-27632 in denuded ionomycin-permeabilized rings was greater in NR than in LHR. Together these studies indicate alpha(2)-AR contraction depends on ROK activity more in NR than LHR aorta. Additionally, endogenous NO may regulate RhoA activation, whereas chronic NOS inhibition appears to cause RhoA desensitization.