Adenosine induced direct negative inotropic effect is abolished during global ischemia: role of protein kinase C and prostacyclin

Adenosine acts as a cardioprotective agent by producing coronary vasodilation, decreasing heart rate and by antagonizing the cardiostimulatory effect of catecholamines; adenosine also exerts a direct negative inotropic effect. Myocardial ischemia is known to be associated with enhanced levels of adenosine, increased protein kinase C (PKC) activity and prostacyclin (PGI2) release. The present study was conducted to determine if myocardial ischemia alters the cardioprotective effect of adenosine by increasing PKC activity and PGI2 release in the isolated rat heart perfused at 10 ml/min with Krebs-Henseleit buffer (KHB; 95% O2+5% CO2). Adenosine (10 mmol/min) reduced myocardial contractility as indicated by a decrease in contractility (dp/dtmax), heart rate (HR) and coronary perfusion pressure (PP). In hearts subjected to 30 min of ischemia (without perfusion) and then reperfused with KHB, adenosine failed to decrease dp/dtmax, HR or PP. However, during infusion of PKC inhibitor H-7 (1-(5-Isoquinolinesulfonyl)-2-methylpiperazine hydrochloride) (H-7; 6 mmol/min), which commenced 10 min before ischemia and continued throughout reperfusion, adenosine produced a decrease in dp/dtmax, HR and PP, similar to that before ischemia. Infusion of the PKC activator phorbol 12,13-dibutyrate (PDBu; 2 nmol/min) but not an inactive analogue in non-ischemic hearts prevented the adenosine induced decrease in dp/dtmax. During infusion of H-7, PDBu failed to block the direct negative inotropic effect of adenosine in non-ischemic hearts. In addition, pretreatment with H-7 or indomethacin (cyclooxygenase inhibitor) significantly reduced the PGI2 release following ischemia. This data suggest that PKC and PGI2 regulate the direct negative inotropic effect of adenosine, which is abolished during ischemia.     

Alterations in adenylate cyclase activity due to streptozotocin-induced diabetic cardiomyopathy

The effect of chronic experimental diabetes on the adenylate cyclase system (AC) in the rat heart was investigated. Rats were made diabetic by an intravenous injection of streptozotocin (65 mg/kg), hearts were removed 8 weeks later and washed cell particles were isolated. AC activity was measured in the absence and presence of different concentrations of forskolin, NaF, GTP analogue [Gpp(NH)p] or epinephrine. A significant depression in the epinephrine stimulated AC activity was observed in diabetic hearts. Basal AC activity and stimulation of AC with forskolin, NaF and Gpp(NH)p were not significantly different between control and diabetic preparations. These results indicate no apparent alterations in the regulatory or catalytic properties of AC in hearts from chronic diabetic rats. The observed depression in epinephrine stimulated AC activity may account for the depressed inotropic action of catecholamines in the diabetic cardiomyopathy.     

Exaggerated coronary reactivity to endothelin-1 in diabetes: reversal with bosentan

We previously demonstrated that chronic endothelin receptor blockade (with bosentan) improved functional cardiac performance in streptozotocin-diabetic rats, suggesting a novel role of endothelin-1 (ET-1) in modulating diabetic heart dysfunction. To gain insight into the mechanism(s) underlying this effect, we examined the coronary vascular responses to ET-1 in hearts from diabetic and control rats treated with or without bosentan. Rats were divided into control, control-treated, diabetic, and diabetic-treated groups. The control-treated and diabetic-treated groups received bosentan (100 mg x kg(-1) x d(-1)) for 8 weeks. Following treatment, hearts were isolated and perfused, and coronary reactivity to ET-1 was assessed by measuring the changes in coronary perfusion pressure in response to ET-1 (50 and 100 pM). Additionally, maximal coronary blood flow (assessed with 10(-5) M adenosine) was measured in isolated perfused hearts. The key observation is that coronary reactivity to ET-1 was significantly higher in the diabetic than the control rats. This effect was normalized in diabetic rats chronically receiving bosentan. Maximal coronary vasodilation did not differ between the four groups. In conclusion, the reactivity of ET-1 is altered in the isolated perfused coronary vascular bed from diabetic rats, and chronic ET receptor blockade restores this reactivity to control values. These observations provide a possible mechanism for the improvement in diabetic heart function observed after chronic bosentan treatment.     

Transcoronary gene transfer of SERCA2a increases coronary blood flow and decreases cardiomyocyte size in a type 2 diabetic rat model

The Otsuka Long-Evans Tokushima fatty rat is an animal model of Type 2 diabetes mellitus (DM), which is characterized by diastolic dysfunction associated with decreased sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a). The aim of this study was to examine whether gene transfer of SERCA2a can influence coronary blood flow and cardiomyocyte diameter in this model. DM rats were injected with adenovirus carrying SERCA2a (DM+SERCA) or beta-galactosidase gene (DM+betaGal). Coronary blood flow was measured in cross-circulated excised hearts 3 days after infection. Although in all groups coronary blood flow remained unchanged even if left ventricular (LV) volume or intracoronary Ca(2+) infusion was increased, the DM+SERCA group showed a sustained increase in coronary blood flow compared with the other groups. This result suggests that the sustained high coronary blood flow is a specific response in SERCA2a-overexpressed hearts. Although the LV weight-to-body weight ratio (LV/BW) and cardiomyocyte diameter were higher in the DM and DM+betaGal groups than in the non-DM group, in the DM+SERCA group, these measurements were restored to non-DM size. The percentages of collagen area in the three DM groups was significantly higher than results shown in non-DM rats, and there were no significant differences in collagen area percentage among the three DM groups. These results suggest that a lowered LV/BW by SERCA2a overexpression is due mainly to reduced size of cardiomyocytes without any changes in collagen area percentage. In conclusion, in DM failing hearts, SERCA2a gene transfer can increase coronary blood flow and reduce cardiomyocyte size without reduction in collagen production.     

Vascular reactivity of the coronary artery in steelhead trout (Oncorhynchus mykiss

1. The responses of vascular rings from the coronary artery in steelhead trout, Oncorhynchus mykiss, were measured using isometric force transducers.2. Coronary rings contracted with acetylcholine.3. Nitroglycerine and sodium nitroprusside relaxed coronary rings.4. Adenosine, ADP and ATP caused contractions.5. A comparison is made with the reactivity of the coronary artery in rainbow trout.     

Impaired capsaicin-induced decrease in heart rate and coronary flow in isolated heart of diabetic rats

The effect of capsaicin (0.1 microM) on heart rate and coronary flow was studied in Langendorff-perfused heart from streptozotocin-induced (50 mg/kg i.v.) diabetic rats where sensory neuropathy developed. In hearts from animals 4- and 8-week diabetes baseline heart rate and coronary flow decreased from 317.9 +/- 2.9 b.p.m. and 13.4 +/- 0.7 m/min to 255.1 +/- 12.7 and 219.8 +/- 2.8 b.p.m. and 8.9 +/- 0.6 and 10.0 +/- 0.1 ml/min (P<0.05), respectively. Capsaicin significantly decreased both variables in either normal or 4-week diabetic animals its effects, however, on coronary flow or heart rate were missing in preparations from 8-week diabetic rats. Endothelin-1 (0.1 nM), the putative mediator of the capsaicin effect, significantly decreased heart rate and coronary flow irrespective of the presence or absence of diabetes. In the femoral nerve of streptozotocin-treated animals conduction velocity involving both fast conducting A- and slow-conducting C-fibres was decreased proportional to the duration of the pre-existing diabetic state. It is concluded that in insulin deficient diabetes the diminished responses evoked by capsaicin on heart rate and coronary flow are signs of sensory neuropathy. This is related to a feeble endothelin release from sensory nerve endings without changes in post-receptor mechanisms mediating the endothelin effects.     

Mechanical responses of developing Fisher rat heart. Effects of steroid hormone

Age-dependent changes in the mechanical responses of developing Fisher rat heart during the first three postnatal weeks were studied in relation to the hypothesis that the abnormality observed in the mechanical responses of the rat heart might be calcium related. Therefore the effect of frequency of stimulation as well as the response to calcium, epinephrine and ouabain on hearts of untreated and cortisol-treated rats was compared. The positive force-frequency response observed in fetal rat heart reverted to a highly negative response by the 12th to 14th postnatal day. The biphasic mechanical responses directly paralleled reported changes in circulating glucocorticoid levels in developing rat. The force-frequency response was maximally negative when the circulating levels of glucocorticoids were lowest. The reversion of the negative force-frequency responses coincided with a gradual increase reported in the circulating levels of glucocorticoids. The negative force-frequency response was absent in the cortisol-treated developing rat heart and a definite positive pattern was observed as the rats developed. A high sensitivity to free calcium concentration, seen in control fetal and and newborn hearts, diminished after the second postnatal week. By the third postnatal week, the sensitivity to high extracellular calcium concentrations was significantly reduced. The sensitivity to calcium persisted in the cortisol-treated hearts during the 3 postnatal weeks. Cortisol-treated hearts were more responsive to epinephrine than controls. The abbreviation of time to peak tension, a hallmark of the catecholamine effect, was observed at a younger age in the cortisol-treated hearts. Cortisol-treated hearts were more responsive to the inotropic effects of ouabain than controls. The possible involvement of glucocorticoids in the control of calcium handling elements of the myocardium is discussed.     

Diabetes-induced changes of nitric oxide influence on the cardiovascular action of secretin

The modulation by condition of the lack or the excess of nitric oxide (NO) on cardiovascular action of secretin in diabetic rats was investigated. In vitro the isolated heart function and in vivo, the systolic (SBP), diastolic (DSP) blood pressure and heart rate (HR) were measured. Secretin evoked inotropic positive effect and increased coronary outflow (CO), in vivo did not increase systemic pressure and the highest dose of the peptide increased the heart rate. NO synthase inhibitor, N(G) nitro-L-arginine methyl ester (L-NAME) deeply increased the systemic pressure and in vitro decreased coronary outflow. L-arginine and sodium nitroprusside (SNP) did not influence the isolated heart function and in vivo decreased the systemic pressure. L-NAME preserved the inotropic positive effect of secretin and the increase of the coronary outflow. In vivo co-administration of L-NAME+secretin evoked hypotensive effect and abolished the increase of the heart rate after the highest dose of the peptide. L-arginine abolished inotropic positive effect of the peptide and the increase of coronary outflow. In vivo co-administration of these substances caused hypotension and attenuated the increase of the heart rate after the highest dose of secretin. Co-injection of SNP and secretin preserved the inotropic effect of secretin and abolished the increase of the coronary outflow. In vivo infusion of SNP+secretin evoked hypotension and similarly to L-arginine, SNP abolished tachycardia induced by the highest dose of secretin. Both the lack and the excess of nitric oxide changed the cardiovascular action of secretin in diabetic rats.     

Cardiac kinin level in experimental diabetes mellitus: role of kininases

Diabetes mellitus impairs the cardiac kallikrein-kinin system by reducing cardiac kallikrein (KLK) and kininogen levels, a mechanism that may contribute to the deleterious outcome of cardiac ischemia in this disease. We studied left ventricular (LV) function and bradykinin (BK) coronary outflow in buffer-perfused, isolated working hearts (n = 7) of controls and streptozotocin (STZ)-induced diabetic rats before and after global ischemia. With the use of selective kininase inhibitors, the activities of angiotensin I-converting enzyme, aminopeptidase P, and neutral endopeptidase were determined by analyzing the degradation kinetics of exogenously administered BK during sequential coronary passages. Basal LV function and coronary flow were impaired in STZ-induced diabetic rats. Neither basal nor postischemic coronary BK outflow differed between control and diabetic hearts. Reperfusion after 15 min of ischemia induced a peak in coronary BK outflow that was of the same extent and duration in both groups. In diabetic hearts, total cardiac kininase activity was reduced by 41.4% with an unchanged relative kininase contribution compared with controls. In conclusion, despite reduced cardiac KLK synthesis, STZ-induced diabetic hearts are able to maintain kinin liberation under basal and ischemic conditions because of a primary impairment or a secondary downregulation of kinin-degrading enzymes.     

Negative inotropic effect of insulin in papillary muscles from control and diabetic rats

The inotropic effects of insulin in the rat heart are still incompletely understood. In this study, the effects of insulin on cardiac contraction were studied in right ventricular papillary muscles from both control rats and rats with chronic diabetes (lasting 16 weeks). Diabetes was induced by the application of streptozotocin (STZ) and the development of diabetes was documented by increased levels of blood glucose, by reduction in body weight and by decreased plasma concentrations of insulin. The contraction was significantly smaller in diabetic rats. Insulin (80 IU/l) reduced the contraction force in both control and diabetic groups. The post-rest potentiation of contraction was not influenced by insulin in control rats, but insulin increased it in diabetic rats. The negative inotropic effect of insulin was preserved in the presence of cyclopiazonic acid (3 micromol/l), a blocker of sarcoplasmic reticulum (SR) Ca2+ pump, in both control and diabetic groups. In contrast, the negative inotropic effect of insulin was completely prevented in the presence of nifedipine (3 micromol/l), a blocker of L-type Ca2+ current. We conclude that insulin exerts a significant negative inotropic effect in rat myocardium, both control and diabetic. This effect is probably related to processes of SR Ca2+ release triggering, whereas SR Ca2+ loading is not involved.     

Influence of arachidonic acid metabolites on cardiac alpha 1-adrenoceptor responses in diabetic rats.

There is evidence that one or more metabolites of arachidonic acid can produce positive inotropic effects and may also be implicated in the enhanced alpha 1-adrenoceptor responses in hearts from diabetic rats. We therefore carried out a study to investigate the possibility that arachidonic acid metabolites could be involved in the altered cardiac alpha 1-effect of norepinephrine (in the presence of propranolol) in chronic streptozotocin-diabetic rats. Our results have shown that in the presence of the cyclooxygenase inhibitor indomethacin or the thromboxane synthetase inhibitor imidazole, the norepinephrine-stimulated positive inotropic effect and the formation of inositol 1,4,5-trisphosphate were significantly increased in control hearts but were unaltered in hearts from diabetic rats. The addition of the prostacyclin synthetase inhibitor tranylcypromine reduced the norepinephrine-stimulated positive inotropic effect and inositol 1,4,5-trisphosphate formation only in diabetic hearts and had no effect in the controls. The nature and physiological significance of the enhanced positive inotropic effect and inositol 1,4,5-trisphosphate formation in the control heart with the addition of indomethacin and imidazole are still unclear. The effect of tranylcypromine may indicate the participation of prostacyclin in mediating the enhanced alpha 1-inotropic effect of norepinephrine in the chronic diabetic heart.     

Augmentation of the inotropic response to insulin in diabetic rat hearts.

Insulin participates in the modulation of myocardial function, but its inotropic action in diabetes mellitus is not fully clear. In the present study, we examined contractile responses to insulin in left-ventricular papillary muscles and ventricular myocytes isolated from hearts of normal or short-term (5-7 days) streptozotocin-induced (65 mg/kg) diabetic rats. Mechanical properties of papillary muscles and ventricular myocytes were evaluated using a force transducer and an edge-detector, respectively. Contractile properties of papillary muscles or cardiac myocytes, electrically stimulated at 0.5 Hz, were analyzed in terms of peak tension development (PTD) or peak twitch amplitude (PTA), time-to-peak contraction (TPT) and time-to-90% relaxation (RT90). Intracellular Ca2+ transients were measured as fura-2 fluorescence intensity change (deltaFFI). Insulin (1-500 nM) had no effect on PTD in normal myocardium, whereas it produced a positive inotropic response in preparations from diabetic animals, with a maximal increase of 11%. Insulin did not modify TPT or RT90 in either group. Further studies revealed that insulin enhanced cell shortening in diabetic but not normal myocytes, with a maximal increase of 21%. Consistent with its action on the mechanical properties of papillary muscles and cardiac myocytes, insulin also induced a dose-dependent increase in the intracellular Ca2+ transient in diabetic but not normal myocytes. Collectively, these data suggest that the myocardial contractile response to insulin may be altered in diabetes.     

Effect of experimental diabetes on isolated rat heart responsiveness to isoproterenol

The isolated perfused working rat heart was used to study experimental diabetes-induced alterations in the sensitivity and responsiveness of the myocardium to the effects of isoproterenol. Experimental diabetes was induced by intravenous administration of either 65 mg/kg alloxan or 60 mg/kg streptozotocin. The positive inotropic and cardiac relaxant effects of isoproterenol were studied at various time points after the induction of diabetes. There were no changes either in the sensitivity or in the maximum responses of diabetic rat hearts to the positive inotropic effect of isoproterenol at any time point studied. However, the cardiac relaxant effect of isoproterenol was depressed in acute as well as chronic diabetic rat hearts when compared with age-matched controls. Ventricular noradrenaline content was unchanged in 180-day diabetic rat hearts indicating the absence of a diabetes-induced sympathetic neuropathy in the heart. The depressed relaxing effect of isoproterenol may have resulted from alterations in energy utilization and sarcoplasmic reticular function in diabetic rat hearts.     

Adenosine A(2a)-receptor activation increases contractility in isolated perfused hearts

Adenosine A(2a)-receptor activation enhances shortening of isolated cardiomyocytes. In the present study the effect of A(2a)-receptor activation on the contractile performance of isolated rat hearts was investigated by recording left ventricular pressure (LVP) and the maximal rate of LVP development (+dP/dt(max)). With constant-pressure perfusion, adenosine caused concentration-dependent increases in LVP and +dP/dt(max), with detectable increases of 4.1 and 4.8% at 10(-6) M and maximal increases of 12.0 and 11.1% at 10(-4) M, respectively. The contractile responses were prevented by the A(2a)-receptor antagonists chlorostyryl-caffeine and aminofuryltriazolotriazinyl-aminoethylphenol (ZM-241385) but were not affected by the beta(1)-adrenergic antagonist atenolol. The adenosine A(1)-receptor antagonist dipropylcyclopentylxanthine and pertussis toxin potentiated the positive inotropic effects of adenosine. The A(2a)-receptor agonists ethylcarboxamidoadenosine and dimethoxyphenyl-methylphenylethyl-adenosine also enhanced contractility. With constant-flow perfusion, 10(-5) M adenosine increased LVP and +dP/dt(max) by 5.5 and 6.0%, respectively. In the presence of the coronary vasodilator hydralazine, adenosine increased LVP and +dP/dt(max) by 7.5 and 7.4%, respectively. Dipropylcyclopentylxanthine potentiated the adenosine contractile responses with constant-flow perfusion in the absence and presence of hydralazine. These increases in contractile performance were also antagonized by chlorostyryl-caffeine and ZM-241385. The results indicate that adenosine increases contractile performance via activation of A(2a) receptors in the intact heart independent of beta(1)-adrenergic receptor activation or changes in coronary flow.     

Recent advances in understanding the relationship between adenosine metabolism and the function of T and B lymphocytes in diabetes

Adenosine plays an important role in physiology of several organs. Its turnover inside and outside of the cell is controlled by several enzymes and transport processes. The action of extracellular adenosine is mediated via at least four receptors named A(1), A(2A), A(2B), and A(3). Recent studies have reported that adenosine is a significant mediator of regulatory lymphocyte function. Numerous data indicates that adenosine affects T lymphocyte activation, proliferation and lymphocyte-mediated cytolysis. Impaired lymphocyte functioning and enhanced susceptibility to infections is a common feature of human diabetes. This review collects data bringing us closer to understanding the disturbances in lymphocytes adenosine homeostasis in diabetes. Adenosine receptors and nucleoside transporters are targets for potential drugs in many pathophysiological situations. Therefore, action of adenosine on lymphocyte function in diabetes may be important target for modulation of immune responses and understanding of mechanisms leading to several pathologies of immune cells observed in diabetes.     

Altered coronary and cardiac adrenergic response in the failing hamster heart: role of cyclooxygenase derivatives

Although the influence of the adrenergic system has been studied in the presence of heart failure, controversies still exist. Since cyclooxygenase derivatives appear to modulate coronary and cardiac adaptation in the failing heart, we hypothesized that cyclooxygenase derivatives may participate in the altered adrenergic responses in this situation. Isolated hearts from cardiomyopathic (UM-X7.1 subline) and normal hamsters, aged > 240 days, were utilized. Coronary and cardiac response to alpha1-, beta1-, and beta2-adrenergic stimulations was observed before and after pretreatment with indomethacin, a cyclooxygenase inhibitor. Reduction of coronary flow elicited by alpha1-adrenergic stimulation was unchanged in the presence of heart failure, while beta1- and beta2-induced vasodilatations were reduced. Inotropic response to alpha1 and beta1 stimulations were also reduced in failing hearts, while beta2-adrenergic action was unchanged. Pretreatment with indomethacin exacerbated coronary flow reduction observed with alpha1 stimulation in failing hearts only. Beta2-induced coronary vasodilatation and inotropic response to alpha1 and beta2 stimulations were impaired similarly in the presence of indomethacin in normal and failing hearts. The results suggest a complex interaction between adrenergic and cyclooxygenase activation.     

Transmural differences in respiratory capacity across the rat left ventricle in health, aging, and streptozotocin-induced diabetes mellitus: evidence that mitochondrial dysfunction begins in the subepicardium

In diabetic cardiomyopathy, ventricular dysfunction occurs in the absence of hypertension or atherosclerosis and is accompanied by altered myocardial substrate utilization and depressed mitochondrial respiration. It is not known if mitochondrial function differs across the left ventricular (LV) wall in diabetes. In the healthy heart, the inner subendocardial region demonstrates higher rates of blood flow, oxygen consumption, and ATP turnover compared with the outer subepicardial region, but published transmural respirometric measurements have not demonstrated differences. We aim to measure mitochondrial function in Wistar rat LV to determine the effects of age, streptozotocin-diabetes, and LV layer. High-resolution respirometry measured indexes of respiration in saponin-skinned fibers dissected from the LV subendocardium and subepicardium of 3-mo-old rats after 1 mo of streptozotocin-induced diabetes and 4-mo-old rats following 2 mo of diabetes. Heart rate and heartbeat duration were measured under isoflurane-anesthesia using a fetal-Doppler, and transmission electron microscopy was employed to observe ultrastructural differences. Heart rate decreased with age and diabetes, whereas heartbeat duration increased with diabetes. While there were no transmural respirational differences in young healthy rat hearts, both myocardial layers showed a respiratory depression with age (30-40%). In 1-mo diabetic rat hearts only subepicardial respiration was depressed, whereas after 2 mo diabetes, respiration in subendocardial and subepicardial layers was depressed and showed elevated leak (state 2) respiration. These data provide evidence that mitochondrial dysfunction is first detectable in the subepicardium of diabetic rat LV, whereas there are measureable changes in LV mitochondria after only 4 mo of aging.     

Cardiovascular effects of endomorphins in alloxan-induced diabetic rats

Endomorphins, the endogenous, potent and selective mu-opioid receptor agonists, have been shown to decrease systemic arterial pressure (SAP) in rats. In the present study, responses to endomorphins were investigated in systemic vascular bed of alloxan-induced diabetic rats and in non-diabetic rats. Diabetes was induced by alloxan (220 mg/kg, i.p.) in male Wistar rats. At 4-5 weeks after the onset of diabetes, intravenous injections of endomorphins (1-30 nmol/kg) led to an increase of SAP and heart rate (HR) consistently and dosed-dependently. SAP increased 7.68+/-3.73, 11.19+/-4.55, 21.19+/-2.94 and 27.48+/-6.21% from the baseline at the 1, 3, 10 and 30 nmol/kg dose, respectively, of endomorphin 1 (n=4; p<0.05), and similar changes were observed in response to endomorphin 2. The hypertension could be antagonized markedly by i.p. 2 mg/kg of naloxone. On the other hand, bilateral vagotomy would attenuate the effects of hypertension and diminished the changes of HR in response to endomorphins. With diabetic rats, 6-10 weeks after the induction of diabetes, intravenous injections of endomorphins produced non-dose-related various changes in SAP, such as a single decrease, or a single increase, or biphasic changes characterized by an initial decrease followed by a secondary increase, or no change at all. These results suggest that diabetes may lead to the dysfunction of the cardiovascular system in response to endomorphins. Furthermore, the diabetic rats of 4-5 weeks after alloxan-treatment, the increase in SAP and HR caused by i.v. endomorphins might be explained by a changed effect of vagus and by a naloxone-sensitive mechanism.     

Exercise restores coronary vascular function independent of myogenic tone or hyperglycemic status in db/db mice

Regulation of coronary function in diabetic hearts is an important component in preventing ischemic cardiac events but remains poorly studied. Exercise is recommended in the management of diabetes, but its effects on diabetic coronary function are relatively unknown. We investigated coronary artery myogenic tone and endothelial function, essential elements in maintaining vascular fluid dynamics in the myocardium. We hypothesized that exercise reduces pressure-induced myogenic constriction of coronary arteries while improving endothelial function in db/db mice, a model of type 2 diabetes. We used pressurized mouse coronary arteries isolated from hearts of control and db/db mice that were sedentary or exercised for 1 h/day on a motorized exercise-wheel system (set at 5.2 m/day, 5 days/wk). Exercise caused a approximately 10% weight loss in db/db mice and decreased whole body oxidative stress, as measured by plasma 8-isoprostane levels, but failed to improve hyperglycemia or plasma insulin levels. Exercise did not alter myogenic regulation of arterial diameter stimulated by increased transmural pressure, nor did it alter smooth muscle responses to U-46619 (a thromboxane agonist) or sodium nitroprusside (an endothelium-independent dilator). Moderate levels of exercise restored ACh-simulated, endothelium-dependent coronary artery vasodilation in db/db mice and increased expression of Mn SOD and decreased nitrotyrosine levels in hearts of db/db mice. We conclude that the vascular benefits of moderate levels of exercise were independent of changes in myogenic tone or hyperglycemic status and primarily involved increased nitric oxide bioavailability in the coronary microcirculation.     

Pharmacology of the Adenosine A3 Receptor in the Vasculature and Essential Hypertension

Background

Essential hypertension is considered to be a multifactorial disorder and its aetiology has yet to be clearly identified. As the adenosine receptors have a significant role in mediating vasodilation, alterations in their structures or signalling pathways may be involved in the development of hypertension. This study aimed to measure the expression of adenosine A₃ receptors in a range of cardiovascular tissues and determine whether they could be altered with essential hypertension, and to functionally test responses to adenosine A₃ receptor agonists in coronary blood vessels using the isolated perfused heart preparation.

Methods

mRNA samples from cardiovascular tissues and a range of blood vessels were collected from 10 week old male spontaneously hypertensive rats and age-gender matched Wistar rats (n = 8). The Langendorff heart perfusion preparation was used to characterise adenosine A₃ receptor mediated coronary vasodilation in the rat heart.

Results

Adenosine A₃ receptor agonists induced coronary vasodilation. The expression of adenosine A₃ receptors in cardiovascular tissues was altered in a tissue-specific pattern. Specifically, down-regulation of adenosine A₃ receptor expression occurred in hypertensive hearts, which might be associated with attenuated vasodilator responses observed in coronary vessels to adenosine A₃ receptor agonists.

Conclusions

This study demonstrated alterations in the expression of adenosine A₃ receptors occurred in a tissue specific mode, and reduced adenosine A₃ receptor mediated coronary vasodilation in hearts from spontaneously hypertensive rats. Our findings with regard to changes in the adenosine A₃ receptor in hypertensive hearts suggest that adenosine A₃ receptor might play a role in the physiopathology of essential hypertension and potentially open the way to pharmacologic manipulation of vasomotor activity by the use of adenosine A₃ receptor agonists.