Relationship between vascular adrenergic receptors and prostaglandin biosyntheses in canine diabetic coronary arteries

In vitro effects of calcium dobesilate on the responsiveness of spontaneously diabetic rat aorta

We tested the effect of calcium dobesilate (DOBE) in aorta from spontaneously diabetic (BB/wor) rats. The contraction induced by 10(-6) M noradrenaline (NA) in BB/wor rats was smaller than that induced in control rats (1.21+/-0.11 vs 0.82+/-0.02 g, P<0.01, n=8, respectively) in arteries with intact endothelium. Incubation with DOBE (10(-4) M) impaired the contractions induced by NA in BB/wor rats (1.21+/-0.11 vs 0.67+/-0.01 g, P<0.01, n=8). The effect of DOBE was reversed by 10(-6) M propranolol (0.67+/-0.01 vs 1.20+/-0.60g, P<0.001, n=8, with 10(-4) M DOBE and 10(-4)M DOBE plus 10(-6) M propranolol, respectively). DOBE increased the endothelium-dependent relaxation in arteries from diabetic rats. These findings suggest that DOBE might improve vascular reactivity in BB/wor rats.

Role of vascular adrenergic mechanisms in the haemodynamic and PGI2 stimulating effects of angiotensin in diabetic dogs

In aware of the well-known altered vascular responsiveness in the diabetic vasculature, this study aimed to compare the haemodynamic and PGI2 releasing effects of angiotensin in metabolically healthy (12) and alloxan-(560 umol/kg) diabetic (12) dogs as well as to analyze the role of vascular adrenoceptors in this. In vivo the effect of intracoronarially administered angiotensin (63-125-250-500-1000 pmol/kg/min) on coronary blood flow, mean arterial blood pressure, myocardial contractile force and heart rate was investigated without and with pretreatment of 2 umol/kg phentolamine. In vitro PGI2 release by isolated coronary rings was induced by 50 nmol/l angiotensin before and after pretreatment with 5 umol/l phentolamine and measured by radioimmunoassay. Angiotensin enhances dose-dependently both the mean arterial blood pressure and coronary blood flow, while it provokes a considerable (p < 0.05) increase of PGI2 formation by isolated coronary arterial rings. These alterations could be prevented by phentolamine administration both in vivo and in vitro, while this drug did not affect the angiotensin-induced enhancement of diabetic coronary blood flow. On the other hand the increase of blood pressure by angiotensin was found to be more (p < 0.05) expressed in diabetes and it could be further potentiated by phentolamine. PGI2 synthesis by isolated diabetic coronary rings could not be modified either by angiotensin alone or in combination with phentolamine. On the basis of above data, the lack of stimulated vascular PGI2 formation mediated by alpha-adrenergic mechanisms is supposed to causatively contribute to the diminished sensitivity of diabetic coronary arteries to vasodilation.

Fosinopril improves regulation of vascular tone in mesenteric bed of diabetic rats

Because diabetes mellitus leads to vascular dysfunction, we examined the microvascular endothelial and smooth muscle function in long-term diabetes and a possible influence of fosinopril treatment (10 mg/kg). We investigated isolated perfused mesenteric beds of diabetic rats (4 groups: control, control + fosinopril, diabetes, diabetes + fosinopril; diabetes of 6-month duration, induced by streptozotocin, STC) were investigated using computer-assisted microvideoangiometry. Vascular diameter of four different vascular regions [classified as conductive (G1, 303 +/- 6.5 mu m and G2, 239 +/- 6.3 mu m) and resistance (G3, 192 +/- 4.5 mu m and G4, 124 +/- 2.6 mu m) vessel generations; resting conditions, control group] were increased in diabetes by approximately 20%. However, the endothelium-dependent relaxation in response to 1 mu M acetylcholine (ACh) was reduced from 38-44% to 20-25% (diabetes mellitus) with maximal impairment in G4 vessels. This could be significantly antagonized by fosinopril treatment. Similarly, vasodilation in response to 1 mu M glyceroltrinitrate (GTN) was reduced from 50-58 to 20-30%, but was partially prevented by fosinopril (32-38%), whereas potassium chloride (KCl)-induced vasoconstriction did not show differences between the groups. Inhibition of nitric oxide (NO) synthesis by 3 mu M L-NG-nitro arginine (L-NNA) resulted in a slight vasoconstriction of all vessels (12-25%), with maximum response in G3/G4. This was not altered by disease or treatment. We conclude that (a) long-term diabetes leads to endothelial and smooth muscle dysfunction with reduced capability of vasodilation and either an impairment of NO release or a reduced smooth muscle responsiveness to and (b) a predominant impairment of NO-dependent regulation in small resistance vessels, and (c) that fosinopril treatment can at least partially prevent this vascular dysfunction.

Evaluation of the mechanism of endothelial dysfunction in the genetically-diabetic BB rat

Endothelial dysfunction is known to occur in chemically-induced animal models of diabetes. The BB diabetic rat is a genetic diabetes-prone model which more closely resembles Type I diabetes mellitus. In this study, we examined the role of superoxide anion radical and cyclooxygenase activity on endothelial dysfunction in aorta of the spontaneous diabetic BB rat. Vascular endothelial function was studied in vitro in aortic rings from 8-wk diabetic rats and age-matched nondiabetic littermates. There was no alteration in reactivity to norepinephrine as a result of diabetes. Relaxation to acetylcholine (but not nitroglycerin) was impaired in diabetic rings. Relaxation to acetylcholine was abolished by 100 microM L-nitroarginine but unaltered by an equimolar concentration of aminoguanidine (an inducible nitric oxide synthase inhibitor) in both control and diabetic rings. Incubation with 10 microM indomethacin did not alter relaxation to acetylcholine in either control or diabetic rings. In contrast, addition of 20 U/ml superoxide dismutase enhanced relaxation to acetylcholine in diabetic rings but had no effect on relaxation to acetylcholine in control rings. Thus, nitric oxide-mediated, endothelium-dependent relaxation is diminished in aortic rings of the genetic diabetic BB rat. Furthermore, superoxide anion radicals but not cyclooxygenase products play an important role in endothelial dysfunction in this genetic diabetic model.

Maximal coronary flow reserve and metabolic coronary vasodilation in patients with diabetes mellitus

BACKGROUND

Structural and functional abnormalities of the coronary microcirculation have been reported in experimental diabetes mellitus. The purpose of this study was to evaluate coronary microvascular function in human diabetes.

METHODS AND RESULTS

Twenty-four diabetic and 31 nondiabetic patients were studied during cardiac catheterization. A Doppler catheter or guidewire was used to measure changes in coronary blood flow velocity in a nonstenotic artery. Maximal coronary blood flow reserve was determined by using intracoronary adenosine or papaverine. Coronary dilation in response to an increase in myocardial metabolic demand was assessed by using rapid atrial pacing. Maximal vasodilator responses to papaverine and adenosine were compared in 12 diabetic patients. Maximal pharmacologic coronary flow reserve was depressed in diabetic (2.8 +/- 0.2, n = 19) compared with nondiabetic (3.7 +/- 0.2, n = 21, P < .001) patients. During atrial pacing, the decrease in coronary vascular resistance was attenuated in the diabetic (-14 +/- 3%) compared with the nondiabetic (-24 +/- 2%, P < .05) patients. Differences in coronary microvascular function between diabetic and nondiabetic patients were not attributable to differences in drug therapy, resting hemodynamics, or incidence of hypertension. In 12 diabetic patients the maximal coronary vasodilator responses to papaverine and adenosine were similar.

CONCLUSIONS

This study demonstrates both reduced maximal coronary vasodilation and impairment in the regulation of coronary flow in response to submaximal increases in myocardial demand in patients with diabetes mellitus. These microvascular abnormalities may lead to myocardial ischemia in the absence of epicardial coronary atherosclerosis in some circumstances, and thus contribute to adverse cardiovascular events in diabetic patients.

Thromboxane A2/prostaglandin H2 receptors in streptozotocin-induced diabetes: effects of insulin therapy in the rat

Thromboxane A2 (TXA2) and prostaglandin H2 (PGH2) are potent vasoactive and proaggregatory agents whose synthesis has been shown to be elevated in diabetes mellitus. In the present study the effects of streptozotocin (STZ)-induced uncontrolled diabetes (Severe) and insulin-treated STZ diabetes (Moderate) on TXA2/PGH2 receptor density and affinity in platelets, glomerular membranes and aortic membranes were determined using [125I]-BOP, a TXA2/PGH2 receptor agonist. The affinity and density of platelet TXA2/PGH2 receptors in Control, Moderate and Severe groups and glomerular membranes were not significantly different. However, daily insulin therapy caused significant changes in both TXA2/PGH2 receptor affinity and density of aortic membranes: Kd (nM) = 0.67 +/- 0.09, (n = 5), for Control; 0.27 +/- 0.05*, (n = 6), Moderate; and 0.74 +/- 0.16, (n = 5), Severe; Bmax (fmoles/mg protein) = 38.6 +/- 3.1, Control; 20.2 +/- 4.2*, Moderate; and 37.1 +/- 4.1, Severe: (*p < 0.05 compared to Control and Severe). Contractile responses of aortic segments to the TXA2/PGH2 receptor agonist U46619 were determined. Untreated diabetes mellitus (Severe) was associated with a decreased responsiveness of aortic segments without affecting maximum contractile responses (EC50 = 24.6 +/- 5.9* nM, (n = 10); *p < 0.05) compared to Control rats (EC50 = 11.8 +/- 1.6 nM, (n = 13)). Insulin therapy reversed the decrease seen in the Severe group to a value not different from Control (EC50 = 11.4 +/- 1.2* nM, (n = 10); *p < 0.05 compared to Severe). These results suggest that insulin therapy in the diabetic state significantly influences aortic TXA2/PGH2 receptors, as well as vascular responsiveness to TXA2/PGH2 mimetics.

Disturbed lipid metabolism in diabetic coronary vessels

The aim of this study was to clarify whether or not arachidonic acid metabolic disorders are caused by a substrate inavailability and whether such disorders might contribute to circulatory disturbances in the diabetic myocardium. Norepinephrine induced a decrease in the conductivity of both coronary arterial bed and myocardial microcirculation in alloxan-diabetic dogs. It was markedly (p less than 0.05) attenuated both by indomethacin and acetylsalicylic acid pretreatments indicating an imbalance among the vasoactive prostanoids in diabetes. TXA2 release from the diabetic coronary rings was found to be elevated and could be normalized after the blockade of vascular adrenoceptors by phentolamine (p less than 0.05). PGI2 synthesis was also enhanced by adrenergic blockade in the diabetic arterial rings. After pretreatment with 14C arachidonic acid, in order to measure substrate availability, the arachidonic acid metabolic rate was less in the diabetic coronary arteries than in healty vessels (p less than 0.05). Ten mumol/l norepinephrine decreased arachidonic acid metabolism in the presence of prelabelled substrate in the diabetic animals, compared to an increase observed in metabolically healthy dogs. Therefore diabetes appears to diminish arachidonic acid metabolism and uptake independent of adrenoceptors and to induce an imbalance between vasoconstrictor and vasodilator cyclooxygenase products, resulting in elevated TXA2 release controlled by adrenergic mechanisms which may contribute to an impairment in myocardial microcirculation.