Vasodilator effects of adenosine on retinal arterioles in streptozotocin-induced diabetic rats

Adenosine is a potent vasodilator of retinal blood vessels and is implicated to be a major regulator of retinal blood flow during metabolic stress, but little is known about the impact of diabetes on the role of adenosine in regulation of retinal hemodynamics. Therefore, we examined how diabetes affects adenosine-induced vasodilation of retinal arterioles. Male Wistar rats were treated with streptozotocin (80 mg/kg, intraperitoneally), and experiments were performed 6–8 weeks later. Rats were treated with tetrodotoxin (50 μg/kg, intravenously [i.v.]) to eliminate any nerve activity and prevent movement of the eye and infused with methoxamine continuously to maintain adequate systemic circulation. Fundus images were captured with a digital camera that was equipped with a special objective lens, and diameters of retinal arterioles were measured. Adenosine increased diameters of retinal arterioles and decreased systemic blood pressure. These responses were significantly attenuated by the nitric oxide synthase inhibitor N^G-nitro-l-arginine methyl ester (30 mg/kg, i.v.) and the adenosine triphosphate-dependent K⁺ (K_ATP) channel blocker glibenclamide (20 mg/kg, i.v.). The depressor responses to adenosine were reduced in diabetic rats, whereas diabetes did not alter vasodilation of retinal arterioles to adenosine. In contrast, both depressor response and vasodilation of retinal arteriole to acetylcholine were reduced in diabetic rats. The retinal vasodilator responses to adenosine and acetylcholine observed in diabetic rats were diminished by N^G-nitro-l-arginine methyl ester. There were no differences in the responses to pinacidil, a K_ATP channel opener, between the diabetic and nondiabetic rats. These results suggest that both the activation of nitric oxide synthase and opening of K_ATP channels contribute to the vasodilator effects of adenosine in rats in vivo. However, diabetes has no significant impact on the vasodilation mediated by these mechanisms in the retinal circulation.     

Vasodilation of retinal arteriole mediated by corticotropin-releasing factor receptor is impaired in streptozotocin-induced diabetic rats

We investigated the vasodilator responses of retinal arterioles induced by stimulating corticotropin-releasing factor receptors in non-diabetic and diabetic rats. Male Wistar rats were treated with streptozotocin (65 mg/kg, i.v.) and experiments were performed 6-8 weeks later. Rats were treated with tetrodotoxin (50 mug/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye and infused with a mixture of norepinephrine and epinephrine to maintain adequate systemic circulation under artificial ventilation. Fundus images were captured with an original high-resolution digital fundus camera system. The vasodilator responses of retinal arterioles were assessed by measuring changes in diameters of retinal arterioles in response to urocortin and urocortin 2. Both urocortin (0.03-1.0 micromol/kg, i.v.) and urocortin 2 (0.1-3.0 micromol/kg, i.v.) increased diameters of retinal arterioles and decreased systemic blood pressure in a dose-dependent manner. The responses to urocortins were reduced in diabetic rats. These results suggest that urocortin and urocortin 2 play as vasodilators in retinal and peripheral resistance arterioles. The impairment of vasodilation mediated by the corticotropin-releasing factor receptors may contribute to the alteration of retinal and systemic circulation in the diabetic state.     

Role of calcium-activated potassium channels in acetylcholine-induced vasodilation of rat retinal arterioles in vivo

The vascular endothelium plays an important role in regulating retinal blood flow via actions of several vasodilators, including nitric oxide (NO), prostaglandin I?, and an endothelium-derived hyperpolarizing factor (EDHF). Our previous in vivo studies demonstrated that acetylcholine (ACh) dilates the rat retinal arteriole partly through NO- and prostaglandin-independent pathway, possibly the EDHF-mediated pathway, but the underlying mechanism(s) remains to be elucidated. It has been suggested that activation of Ca2+-activated K+ (K(Ca)) channels contributes to the EDHF-mediated responses; therefore, the roles of K(Ca) channels in ACh-induced vasodilation of retinal arterioles were examined in rats. The retinal vascular responses were assessed by determining changes in diameters of retinal arterioles in ocular fundus images that were captured with an original fundus camera system. Intravitreal injection of charybdotoxin, an inhibitor of intermediate- and large-conductance K(Ca) (I/BK(Ca)) channels, or iberiotoxin, an inhibitor of large-conductance K(Ca) (BK(Ca)) channels, significantly reduced the ACh-induced vasodilation of retinal arterioles, whereas neither apamin, an inhibitor of small-conductance K(Ca) (SK(Ca)) channels, nor TRAM-34, an inhibitor of intermediate-conductance K(Ca) (IK(Ca)) channels, altered the response. The vasodilator response to ACh observed under the combined blockade of NO synthase and cyclooxygenase with N(G)-nitro-L-arginine methyl ester plus indomethacin was also diminished by iberiotoxin. Iberiotoxin did not affect the NO donor NOR3-induced vasodilation of retinal arterioles, whereas it significantly reduced the BK(Ca) channel opener BMS-191011-induced responses. These results suggest that activation of BK(Ca) channels is involved in the EDHF-mediated component of the vasodilator response to ACh in the rat retinal arterioles in vivo.     

Beta-adrenoceptor-mediated vasodilation of retinal blood vessels is reduced in streptozotocin-induced diabetic rats

We investigated the effects of epinephrine and dopamine on retinal blood vessels in streptozotocin (STZ, 80 mg/kg, i.p.)-treated rats and age-matched control rats to determine whether diabetes mellitus alters the retinal vascular responses to circulating catecholamines. Experiments were performed 6-8 weeks after treatment with STZ or the vehicle. The fundus images were captured with the digital fundus camera system for small animals we developed and diameters of retinal blood vessels contained in the digital images were measured. Epinephrine increased the diameters of retinal blood vessels, but the vasodilator responses were reduced in diabetic rats. Dopamine produced a biphasic retinal vascular response with an initial vasoconstriction followed by a vasodilation. The vasoconstrictor effects of dopamine on retinal arterioles were enhanced in diabetic rats, whereas the difference between the two groups was abolished by treatment with propranolol. The vasodilator effect of isoproterenol, but not of the activator of adenylyl cyclase colforsin, on retinal blood vessels was reduced in diabetic rats. No difference in vasoconstriction of retinal blood vessels to phenylephrine between non-diabetic and diabetic rats was observed. The vasodilator responses of retinal blood vessels to 1,1-dimethyl-4-phenylpiperazinium, a ganglionic nicotinic receptor agonist, were also attenuated in diabetic rats. These results suggest that diabetes mellitus alters the retinal vascular responses to circulating catecholamines and the impairment of vasodilator responses mediated by beta-adrenoceptors contributes to the alteration.     

Vasodilation of retinal arterioles induced by activation of BKCa channels is attenuated in diabetic rats

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate the retinal vascular tone, but question of whether the impairment of the channel function contributes to abnormalities of retinal circulation has not yet been completely elucidated. The purpose of this study was to examine effects of diabetes on the vasodilation induced by activation of BK(Ca) channels. Male Wistar rats were treated with streptozotocin and experiments were performed 2 weeks later. The streptozotocin-treated animals were given drinking water containing 5% d-glucose to shorten the term in the development of retinal vascular dysfunction. The retinal vascular responses were assessed by measuring diameter of retinal arterioles in the fundus images that were captured with an original fundus camera system. In non-diabetic rats, vasodilator effects of acetylcholine on retinal arterioles were significantly reduced by iberiotoxin, an inhibitor of BK(Ca) channels. However, the inhibitory effect of iberiotoxin was not observed in diabetic rats, and the responses to the BK(Ca) channel opener BMS-191011 were almost completely abolished. The retinal vasodilator response to acetylcholine, possibly an endothelium-derived hyperpolarizing factor-mediated response, observed after treatment with N(G)-nitro-l-arginine methyl ester and indomethacin was markedly reduced in diabetic rats. The responses to pinacidil, an opener of ATP-sensitive K(+) channels, were unchanged. These results suggest that the retinal vasodilator response mediated through mechanisms involving activation of BK(Ca) channels is diminished at the early stage of diabetes in rats. The impairment of BK(Ca) channel function may contribute to abnormal retinal hemodynamics in diabetes and consequently play an important role in the pathogenesis of diabetic retinopathy.     

Pharmacological evidence for the presence of functional β3-adrenoceptors in rat retinal blood vessels

The aim of this study was to examine whether stimulation of β₃-adrenoceptors dilates rat retinal blood vessels and how diabetes affects the vasodilator responses. Images of ocular fundus were captured with an original high-resolution digital fundus camera in vivo. The retinal vascular responses were evaluated by measuring diameter of retinal blood vessels contained in the digital images. Both systemic blood pressure and heart rate (HR) were continuously recorded. The β₃-adrenoceptor agonist CL316243 (0.3–10 μg/kg/min, i.v.) increased diameter of retinal arterioles (at 10 μg/kg/min, a 31% increase) and decreased mean blood pressure (at 10 μg/kg/min, a 21% decrease) in a dose-dependent manner. CL316243 produced a small but significant increase in HR (at 10 μg/kg/min, a 9% increase). Both SR59230A (1 mg/kg, i.v.) and L-748337 (50 μg/kg, i.v.), β₃-adrenoceptor antagonists, significantly prevented CL316243-induced retinal vasodilator responses. Similar observations were made with another β₃-adrenoceptor agonist, BRL37344. The β₂-adrenoceptor agonist salbutamol also increased diameter of retinal arterioles (at 10 μg/kg/min, a 43% increase), whereas the drug produced greater decrease in blood pressure (at 10 μg/kg/min, a 46% decrease) and increase in HR (at 10 μg/kg/min, a 16% increase), compared with β₃-adrenoceptor agonists. The retinal vasodilator responses to CL316243 and BRL37344 observed under blockade of β₁/β₂-adrenoceptors with propranolol (2 mg/kg, i.v. bolus followed by 100 μg/kg/min infusion) were unaffected 2 weeks after induction of diabetes by the combination of streptozotocin treatment and d-glucose feeding. On the other hand, the vasodilator responses to salbutamol of retinal arterioles were significantly reduced in diabetic rats. These results suggest that stimulation of β₃-adrenoceptors causes the vasodilation of retinal arterioles in vivo and the vasodilator responses are unaffected at the early stage of diabetes.     

Noradrenaline contracts rat retinal arterioles via stimulation of α(1A)- and α(1D)-adrenoceptors

The aim of this study was to characterize the α?-adrenoceptor subtype(s) involved in the noradrenaline-induced contraction of retinal arterioles in rats. In vivo ocular fundus images were captured with a digital camera equipped with a special objective lens. By measuring changes in diameter of retinal arterioles in the fundus images, retinal vascular response was assessed. The systemic blood pressure and heart rate in the animals were also continuously recorded. Following blockade of β?/β?-adrenoceptors with propranolol, noradrenaline (0.03-3 μg/kg/min, i.v.) decreased the diameter of retinal arterioles and increased the mean blood pressure in a dose-dependent manner. The highest dose (3 μg/kg/min, i.v.) of noradrenaline caused a small increase in heart rate. The α(1A)-adrenoceptor antagonist RS100329 (0.1 mg/kg, i.v.) and the α(1D)-adrenoceptor antagonist BMY 7378 (1 mg/kg, i.v.) significantly prevented noradrenaline-induced contraction of retinal arterioles and pressor responses whereas the α(1B)-adrenoceptor antagonist L-765314 (1 mg/kg, i.v.) did not. The α(1A)-adrenoceptor agonist, A 61603 (0.03-0.3 μg/kg/min, i.v.), also caused contractile responses of retinal arterioles and pressor responses. These responses were almost completely prevented by RS100329 (0.1 mg/kg, i.v.), but not by BMY 7378 (1 mg/kg, i.v.). These results suggest that the contractile effects of noradrenaline on retinal arterioles and peripheral resistance vessels are, at least in part, mediated by stimulation of α(1A)- and α(1D)-adrenoceptors. Furthermore, it is likely that the α?-adrenoceptor subtype(s) involved in rat vascular responses are similar in both retinal and peripheral circulation.     

Probucol prevents the attenuation of β2-adrenoceptor-mediated vasodilation of retinal arterioles in diabetic rats

Probucol is an antihyperlipidemic drug with potent antioxidant properties. Oxidative stress plays an important role in the pathogenesis of diabetic retinopathy. In this study, we aimed to investigate the protective effects of probucol against diabetes-induced retinal vascular dysfunction in a rat model of diabetes. Diabetes was induced by a combination of streptozotocin treatment and D-glucose feeding, and retinal vasodilator responses were assessed by measuring the diameter of retinal arterioles. The vasodilator effect of salbutamol, a β₂-adrenoceptor agonist, on retinal arterioles was significantly diminished 2 weeks after the induction of diabetes. In non-diabetic rats, vasodilator responses to salbutamol were significantly reduced after an intravitreal injection of iberiotoxin, a blocker of large-conductance K_Ca (BK_Ca) channels. However, this effect was not observed in diabetic rats. Probucol had no significant effect on salbutamol-induced changes in diameter of retinal arterioles in non-diabetic rats, whereas it could prevent the attenuation of retinal vasodilator response to salbutamol in diabetic rats. These results suggest that the reduced function of BK_Ca channels is involved in the attenuation of β₂-adrenoceptor-mediated retinal vasodilation in diabetic rats. Probucol preserves the BK_Ca channel function in retinal arterioles under diabetic conditions; therefore, it may show beneficial effects on diabetic retinopathy by preventing or slowing the impairment of the retinal circulation in patients with diabetes mellitus.

     

BMS-191011, an opener of large-conductance Ca2+-activated potassium channels, dilates rat retinal arterioles in vivo

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate vascular smooth muscle tone but the role of BK(Ca) channels in regulation of retinal circulation remains unclear. In the present study, we examined the effects of BMS-191011 and NS 1619, openers of BK(Ca) channels, on rat retinal blood vessels in vivo. Male Wistar rats (8- to 10-week-old) were anesthetized with pentobarbital sodium (50 mg/kg, intraperitoneally (i.p.)) and treated with tetrodotoxin (50 μg/kg, intravenously (i.v.)) to eliminate any nerve activity and prevent movement of the eye under artificial ventilation. A mixture solution of adrenaline and noradrenaline (9:1) was infused to maintain adequate systemic circulation. BMS-191011 (10-100 μg/kg, i.v.) and NS 1619 (0.1-1.0 μg/kg, i.v.) increased the diameter of retinal arterioles without altering systemic blood pressure and heart rate significantly. The vasodilator responses to BMS-191011, but not to NS 1619, were significantly diminished by intravitreal injection of iberiotoxin (an inhibitor of BK(Ca) channels, 20 pmol/eye). These results suggest that BMS-191011 dilates rat retinal arterioles through activation of iberiotoxin-sensitive BK(Ca) channels in vivo. The BK(Ca) channel opener could be considered as a candidate for improving retinal circulation without severe cardiovascular side-effects.     

Quercetin antagonism of Bay K 8644 effects on rat tail artery L-type Ca(2+) channels

Macroangiopathy is a major complication of diabetes mellitus in which dysfunction of vascular endothelium induced by excessive oxidative stress is an early and key determinant. As an endogenous antioxidant, taurine possesses endothelial protective effect in vitro. LOX-1 is an endothelial receptor for oxidized low-density lipoprotein (oxLDL) which might mediate endothelial dysfunction and subsequent atherogenesis in diabetes. We used streptozotocin-induced rats as models of type 1 diabetes to evaluate the protective effect of taurine against vascular endothelial dysfunction in type 1 diabetes and the possibly involved molecule mechanism. Eight male Wistar rats were used as normal control group. Sixteen diabetic rats induced by one single injection of streptozocin (60 mg/kg, i.p.) were randomly divided into two groups after the diabetes onset: diabetes mellitus group and taurine-treated diabetes group. 6 weeks afterward, endothelium-dependent vasodilation of isolated thoracic aorta, serum oxLDL and soluble intercellular adhesion molecule (sICAM-l) levels, LOX-1 and intercellular adhesion molecule (ICAM-1) expression on aortas were determined respectively. Streptozocin-induced diabetic rats were complicated with excessive oxidative stress and endothelial dysfunction: increased serum oxLDL and sICAM-1, inhibited endothelium-dependent vasodilator responses to acetylcholine (1 nM-0.1 microM). Simultaneously, LOX-1 and ICAM-1 expression were enhanced in aortas of diabetic rats; whereas blunted endothelium-dependent vasodilator responses to acetylcholine, increased serum oxLDL and sICAM-1 level as well as overexpression of LOX-1 and ICAM-1 were all attenuated significantly by taurine treatment. In conclusion, taurine improves vascular endothelial dysfunction induced by experimental type 1 diabetes and this effect might be associated with downregulation of LOX-1 and ICAM-1 expression on aortic vascular endothelium via its antioxidative property.     

Role of β<Subscript>3</Subscript>-adrenoceptors in regulation of retinal vascular tone in rats

The aim of this study was to determine the role of β₃-adrenoceptors in the action of endogenous catecholamines (adrenaline and noradrenaline) on rat retinal arterioles in vivo. Using an original high-resolution digital fundus camera, the rat ocular fundus images were captured. The diameter of retinal arterioles contained in the images was measured. Both systemic blood pressure and heart rate were recorded continuously. Adrenaline (0.3–5.0 μg/kg/min, i.v.) increased the diameter of retinal arterioles, mean blood pressure and heart rate in a dose-dependent manner. Under blockade of β₁/β₂-adrenoceptors with propranolol (2 mg/kg, i.v. bolus followed by 100 μg/kg/min infusion), adrenaline decreased the diameter of retinal arterioles. Similar observation was made under treatment with the β₃-adrenoceptor antagonist L-748337 (50 μg/kg, i.v.). The pressor response to adrenaline was enhanced by propranolol, but not by L-748337. The positive chronotropic action of adrenaline was markedly prevented by propranolol, whereas it was unaffected by L-748337. Noradrenaline (0.03–1.0 μg/kg/min, i.v.) decreased the diameter of retinal arterioles but increased the mean blood pressure and heart rate. The effects of noradrenaline on retinal arteriolar diameter and blood pressure were unaffected by propranolol or L-748337. The positive chronotropic action of noradrenaline was almost completely abolished by propranolol. These results suggest that β₃-adrenoceptors play crucial roles in vasodilator responses to adrenaline of retinal arterioles but have minor or no effect on noradrenaline-induced responses. The results also indicate that the functional role of β₃-adrenoceptors may be more important than that in peripheral resistance vessels.     

Intravenously administered phosphodiesterase 4 inhibitors dilate retinal blood vessels in rats

In the present study, we examined effects of intravenously administered inhibitors of phosphodiesterase 4 (rolipram and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro-20-1724)) and non-selective inhibitor of phosphodiesterases (theophylline) on diameter of retinal blood vessel and fundus (retinal/choroidal) blood flow in rats. Male Wistar rats (8- to 10-week-old) were treated with tetrodotoxin (50 μg/kg, i.v.) to eliminate any nerve activity and prevent the eye movement under artificial ventilation. Methoxamine was used to maintain adequate systemic circulation. Ocular fundus images were captured with an original high-resolution digital fundus camera for small animals. Diameters of retinal blood vessels contained in the digital images were measured using image-processing softwares on a personal computer. Fundus blood flow was measured using a laser Doppler flow meter. Both rolipram (0.01-10 μg/kg/min, i.v.) and Ro-20-1724 (0.01-10 μg/kg/min, i.v.) increased diameters of retinal blood vessels in a dose-dependent manner without significant effect on systemic blood pressure, heart rate and fundus blood flow. The effects of phosphodiesterase 4 inhibitors on retinal arterioles were greater than those on retinal venules. Similarly, theophylline (0.1-10 mg/kg/min, i.v.) dilated retinal blood vessels, whereas it decreased blood pressure and increased heart rate markedly. These results suggest that phosphodiesterase 4 contributes to maintenance of retinal vascular tone. Inhibitors of phosphodiesterase 4 could be considered as a candidate for therapeutic drugs to treat diseases associated with disorders of retinal circulation without severe cardiovascular side-effects.     

In vivo evidence that L-S-nitrosocysteine may exert its vasodilator effects by interaction with thiol residues in the vasculature

This study examined the effects of the lipophobic thiol chelator, para-hydroxymercurobenzoic acid (25 and 50 micromol/kg, i.v.) on the falls in mean arterial blood pressure and regional vascular resistances produced by L-S-nitrosocysteine (400 nmol/kg, i.v.) and the nitric oxide (NO)-donors, (Z)-1-&z. sfnc;N-methyl-N-[6(N-methylammoniohexyl)amino]&z.sfnc; diazen-1-ium-1, 2-diolate (MAHMA NONOate, 25 nmol/kg, i.v.) and sodium nitroprusside (10 microg/kg, i.v.), in urethane-anesthetized rats. The L-S-nitrosocysteine-induced responses were markedly diminished whereas the MAHMA NONOate- and sodium nitroprusside-induced responses were minimally affected by para-hydroxymercurobenzoic acid. These results suggest that the vasodilator actions of L-S-nitrosocysteine involves the interaction with membrane thiols in vascular smooth muscle of resistance arteries and that para-hydroxymercurobenzoic acid does not markedly affect NO-mediated vasodilation.     

Vasodilator effect of nicorandil on retinal blood vessels in rats

We examined the effect of nicorandil on retinal blood vessels in rats in vivo. Male Wistar rats (8 to 10 weeks old) were anaesthetised with thiobutabarbital (120 mg/kg, intraperitoneal). Fundus images were captured with a digital camera that was equipped with a special objective lens. Diameters of retinal blood vessels were measured with a personal computer. Nicorandil (1–300 μg kg⁻¹ min⁻¹, intravenous [i.v.]) increased diameters of retinal blood vessels and decreased systemic blood pressure in a dose-dependent manner. Both responses to nicorandil were attenuated by glibenclamide (20 mg/kg, i.v.), an adenosine triphosphate (ATP)-dependent K⁺ (K_ATP) channel blocker. On the other hand, indomethacin (5 mg/kg, i.v.), a cyclooxygenase inhibitor, attenuated the vasodilation of retinal blood vessels, but not depressor response, to nicorandil and sodium nitroprusside. Pinacidil (1–300 μg kg⁻¹ min⁻¹, i.v.), a K_ATP channel opener, also dilated retinal blood vessels and decreased systemic blood pressure. The responses to pinacidil were prevented by glibenclamide, but not by indomethacin. The vasodilation of retinal arteriole, but not depressor response, to sodium nitroprusside (1–30 μg kg⁻¹ min⁻¹, i.v.), a nitric oxide donor, was attenuated by indomethacin. These results suggest that nicorandil dilates retinal blood vessels through opening of K_ATP channels and production of prostaglandins that are probably generated by nitric oxide.     

L-beta,beta-dimethylcysteine attenuates the haemodynamic responses elicited by systemic injections of peroxynitrite in anaesthetized rats

1 There is direct chemical evidence that L-beta,beta-dimethylcysteine (L-penicillamine (L-PEN)) is a scavenger of peroxynitrite. The aim of this study was to determine whether L-PEN attenuates the haemodynamic responses elicited by peroxynitrite in pentobarbital-anaesthetized rats. 2 Peroxynitrite (1-20 micromol kg(-1), i.v.) elicited dose-dependent reductions in mean arterial blood pressure (MAP) and mesenteric and hindquarter vascular resistances. 3 L-PEN (2 mmol kg(-1), i.v.) elicited relatively minor but significant increases in MAP and vascular resistances. The initial reductions in MAP and vascular resistances elicited by peroxynitrite were not diminished after administration of L-PEN whereas they were much shorter in duration. As such, the total reductions in MAP and vascular resistances were markedly reduced by L-PEN. 4 The finding that L-PEN (2 mmol kg(-1), i.v.) did not affect the hypotensive or vasodilator responses elicited of the ATP-dependent potassium-channel agonist, cromakalim (3-18 microg kg(-1), i.v.), suggests that this dose of L-PEN is not a nonselective inhibitor of vasodilation. 5 These findings suggest that L-PEN may effectively scavenge peroxynitrite in vivo and/or interfere with the mechanisms by which peroxynitrite elicits its vasodilator responses.     

Downregulation of propranolol-sensitive beta-adrenoceptor signaling after inhibition of nitric oxide synthesis

The beta-adrenoceptor agonist, isoprenaline, elicits vasodilation and tachycardia in anesthetized rats via activation of propranolol-sensitive beta1- and beta2-adrenoceptors and also by propranolol-insensitive beta1- and beta3-adrenoceptors. The aim of this study was to determine whether the relative contribution of propranolol-sensitive and -insensitive beta-adrenoceptors to the changes in heart rate (HR) and vascular resistances elicited by isoprenaline is altered after blockade of nitric oxide (NO) synthase, in pentobarbital-anesthetized rats. The hemodynamic responses elicited by isoprenaline (0.1 and 0.5 microg kg(-1), i.v.) were determined before and after injection of saline or the NO synthase inhibitor, N(G)-nitro-L-arginine methylester (L-NAME, 50 micromol kg(-1), i.v.), and again after injection of the beta1- and beta2-adrenoceptor antagonist, propranolol (1 mg kg(-1), i.v.). The responses elicited by the above doses of isoprenaline were also determined before and during infusion of the alpha1-adrenoceptor agonist, phenylephrine (3 microg kg(-1) min(-1), i.v.), and again 15-20 min after injection of propranolol (1.0 mg kg(-1), i.v.). Both doses of isoprenaline elicited tachycardia and reductions in vascular resistances. Propranolol eliminated the responses elicited by the lower dose of isoprenaline and substantially diminished the responses elicited by the higher dose of the beta1-, beta2- and beta3-adrenoceptor agonist. The maximal vasodilator responses elicited by both doses of isoprenaline were not diminished whereas the maximal increases in HR were higher after injection of L-NAME. The ability of propranolol to diminish the hemodynamic actions of isoprenaline was substantially diminished in L-NAME-treated rats, whereas propranolol retained its potency in rats that received an equi-pressor infusion of the alpha1-adrenoceptor agonist, phenylephrine. The finding that the maximal vasodilator responses elicited by isoprenaline were not diminished by L-NAME suggests that the vasodilation elicited by this drug was due to direct activation of beta-adrenoceptors on vascular smooth muscle and that the full compliment of isoprenaline-sensitive receptors was not changed after inhibition of NO synthesis. However, these results suggest that the activities of propranolol-sensitive beta-adrenoceptors are downregulated, whereas propranolol-insensitive beta-adrenoceptors are upregulated upon the loss of exposure to endothelial nitrosyl factors.     

Mechanisms of the increased pressor response to vasopressors in the mesenteric bed of nitric oxide-deficient hypertensive rats

In the present study we analyzed mesenteric vascular reactivity of chronic nitric oxide (NO)-deficient hypertensive rats (NW-nitro-L-Arginine Methyl Ester, L-NAME, 50 mg/kg/day, oral, 3 weeks). Perfusion pressure changes in response to cumulative additions of methoxamine and KCl were significantly increased in the mesenteric vessels of the L-NAME-treated as compared with vessels of the controls. Verapamil reduced the responses to methoxamine, but those of the hypertensive rats were still enhanced. In contrast, responses to KCl were almost completely abolished by verapamil. In mesenteric vessels perfused with zero calcium and high-potassium Krebs, pressor responses to the re-addition of calcium were also significantly enhanced in the hypertensive rats compared to the controls. Vasodilator responses to acetylcholine in KCl-preconstricted vessels, while still significant, were reduced in the L-NAME-treated rats. In this case, acute inhibition of NO blocked the vasodilator responses to acetylcholine and abolished the differences between the two groups. In methoxamine-preconstricted vessels and in the presence of acute inhibition of NO and prostaglandins, vasodilator responses to acetylcholine were significantly greater in the hypertensive vessels than in controls. In conclusion, the mesenteric vessels of L-NAME hypertensive rats show an enhanced response to vasopressors which is related to calcium entry. These data also reveal the existence of an enhanced role of a NO and prostaglandin-independent vasodilator factor, probably endothelium-derived hyperpolarizing factor that may play a compensatory role in the deficiency of NO.     

Chronic administration of genistein improves aortic reactivity of streptozotocin-diabetic rats: mode of action

Diabetes mellitus is associated with major cardiovascular risk factors which are responsible for excess morbidity and mortality. Soy isoflavones like genistein are beneficial for correcting the hyperglycemia and preventing some diabetic complications. Thus, the effect of chronic administration of genistein was studied on aortic reactivity of streptozotocin (STZ)-diabetic rats. Male diabetic rats received genistein 1 mg/kg/day (i.p.) for 4 weeks 3 days after diabetes induction. Contractile responses to KCl and phenylephrine (PE) and relaxation responses to acetylcholine (ACh) and isosorbide dinitrate (ISD) were obtained from aortic rings. Maximum contractile response of endothelium-intact rings to KCL and PE was significantly lower in genistein-treated diabetic rats relative to untreated diabetic ones. Endothelium removal abolished the significant difference between genistein-treated and untreated diabetic groups regarding contractile response to KCl and PE. Meanwhile, endothelium-dependent relaxation to ACh was significantly higher in genistein-treated diabetic rats as compared to diabetic ones. Pretreatment of rings with N(omega)-L-arginine methyl ester (L-NAME) and indomethacin (INDO) significantly attenuated the observed responses. Meanwhile, one-month diabetes resulted in an elevation of malondialdehyde (MDA) and decreased superoxide dismutase (SOD) activity in aortic tissue and genistein treatment attenuated the increased MDA content and reduced activity of SOD. Therefore, chronic treatment of diabetic rats with genistein could prevent the abnormal functional changes in vascular reactivity in diabetic rats through nitric oxide- and prostaglandin-dependent pathways and via attenuating oxidative stress in the wall of aortic tissue.     

Stimulation of prostanoid IP and EP(2) receptors dilates retinal arterioles and increases retinal and choroidal blood flow in rats

We examined the effects of vasodilatory prostaglandins (prostacyclin and prostaglandin E₂) and selective agonists for prostanoid EP₂ and EP₄ receptor on the diameters of retinal blood vessels and fundus (retinal/choroidal) blood flow in rats. Male Wistar rats (8- to 10-week-old) were treated with tetrodotoxin (50 μg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye and infused with a mixture solution of norepinephrine and epinephrine (1:9) to maintain adequate systemic circulation under artificial ventilation. Fundus images were captured with a digital camera that was equipped with the special objective lens for small animals, and the diameters of retinal arterioles and venules were measured on a personal computer. Fundus blood flow was estimated using a laser Doppler flowmetry. Intravenous infusions of prostacyclin and prostaglandin E₂ dilated retinal blood vessels, increased fundus blood flow and decreased systemic blood pressure in a dose-dependent manner. The effects of vasodilatory prostaglandins on retinal arterioles were greater than those on retinal venules. Similarly, a prostanoid EP₂ receptor agonist (ONO-AE1-259-01) dilated retinal blood vessels, and increased fundus blood flow and decreased systemic blood pressure. However, a prostanoid EP₄ receptor agonist (ONO-AE1-329) failed to increase fundus blood flow, despite its comparable depressor response with those to vasodilatory prostaglandins and the prostanoid EP₂ receptor agonist. The responses to forskolin, an activator of adenylyl cyclase, were very similar to those to prostacyclin and the prostanoid EP₂ receptor agonist.

These results suggest that prostacyclin and prostaglandin E₂ act as vasodilators in retinal and choroidal circulation, and prostanoid IP and EP₂ receptors play an important role in the regulation of ocular hemodynamics in rats.