Reduced coronary reserve in response to short-term ischaemia and vasoactive drugs in ex vivo hearts from diabetic mice

Inotropic Effects of Prostacyclins on the Right Ventricle Are Abolished in Isolated Rat Hearts With Right-Ventricular Hypertrophy and Failure

BACKGROUND

Prostacyclin mimetics are vasodilatory agents used in the treatment of pulmonary arterial hypertension. The direct effects of prostanoids on right-ventricular (RV) function are unknown. We aimed to investigate the direct effects of prostacyclin mimetics on RV function in hearts with and without RV hypertrophy and failure.

METHODS

Wistar rats were subjected to pulmonary trunk banding to induce compensated RV hypertrophy (n = 32) or manifest RV failure (n = 32). Rats without banding served as healthy controls (n = 30). The hearts were excised and perfused in a Langendorff system and subjected to iloprost, treprostinil, epoprostenol, or MRE-269 in increasing concentrations. The effect on RV function was evaluated using a balloon-tipped catheter inserted into the right ventricle.

RESULTS

In control hearts, iloprost, treprostinil, and MRE-269 improved RV function. The effect was, however, absent in hearts with RV hypertrophy and failure. Treprostinil and MRE-269 even impaired RV function in hearts with manifest RV failure.

CONCLUSIONS

Iloprost, treprostinil, and MRE-269 improved RV function in the healthy rat heart. RV hypertrophy abolished the positive inotropic effect, and in the failing right ventricle, MRE-269 and treprostinil impaired RV function. This may be related to changes in prostanoid receptor expression and reduced coronary flow reserve in the hypertrophic and failing right ventricle.

Enhanced A2A adenosine receptor-mediated increase in coronary flow in type I diabetic mice

Adenosine A2A receptor (A2AAR) activation plays a major role in the regulation of coronary flow (CF). Recent studies from our laboratory and others have suggested that A2AAR expression and/or signaling is altered in disease conditions. However, the coronary response to AR activation, in particular A2AAR, in diabetes is not fully understood. In this study, we use an STZ mouse model of type 1 diabetes (T1D) to look at CF responses to the nonspecific AR agonist NECA and the A2AAR specific agonist CGS 21680 in-vivo and ex-vivo. Using immunofluorescence, we also explored the effect of diabetes on A2AAR expression in coronary arteries. NECA mediated increase in CF was significantly increased in hearts isolated from STZ-induced diabetic mice. In addition, both in in-vivo and ex-vivo responses to A2AAR activation using CGS 21680 were significantly higher in diabetic mice when compared to their controls. Immunohistochemistry showed an upregulation of A2AAR in both coronary smooth muscle and endothelial cells (~160% and ~140%, respectively). Our data suggest that diabetes resulted in an increased A2AAR expression in coronary arteries which resulted in enhanced A2AAR-mediated increase in CF observed in diabetic hearts. This is the first report implying that A2AAR has a role in the regulation of CF in diabetes, supporting recent studies suggesting that the use of adenosine and its A2A selective agonist (regadenoson, Lexiscan®) may not be appropriate for the detection of coronary artery diseases in T1D and the estimation of coronary reserve.

Inhibition of cyclooxygenase-2 causes a decrease in coronary flow in diabetic mice. The possible role of PGE2 and dysfunctional vasodilation mediated by prostacyclin receptor

Several lines of evidence suggest that cyclooxygenase-2 (COX-2) activity can have a beneficial role in the maintenance of vascular tone of the blood vessels in diabetes. Specifically, the increased production of prostacyclin (PGI2) and prostaglandin E2 (PGE2), mediated by COX-2, has been suggested to compensate for decreased synthesis of nitric oxide (NO). The study investigates whether inhibition of COX-2 may reduce the coronary flow in diabetic animals and may also lead to decreased synthesis of prostaglandins. Mice aged 18-20 weeks were used for the study: those with leptin receptor deficiency (db/db) served as a model of diabetes while heterozygous (db/+) mice served as controls. Coronary flow was measured by the Langendorff method, and prostaglandin synthesis by myocardia was assayed in heart perfusates. COX-2 inhibition was found to reduce basal coronary flow in db/db mice but had no effect in db/+ mice. Secretion of PGE2 was found to be higher in db/db mice, while prostacyclin synthesis did not differ. COX-2 inhibition decreased production of both prostaglandins to similar levels in both groups. The use of ONO-1301, a specific agonist for the prostacyclin receptor revealed that vasodilating responses mediated by the receptor were impaired in db/db mice. The expression levels of the receptor in cardiac tissue did not differ between the groups. It is concluded that the increased COX-2 contribution to vasodilation in diabetic animals appears to be partially a result of increased COX-2-dependent synthesis of PGE2 and also may be caused by impaired vasodilation mediated by the prostacyclin receptor.

Impaired cardiac functional reserve in type 2 diabetic db/db mice is associated with metabolic, but not structural, remodelling

AIM

To identify the initial alterations in myocardial tissue associated with the early signs of diabetic cardiac haemodynamic dysfunction, we monitored changes in cardiac function, structural remodelling and gene expression in hearts of type 2 diabetic db/db mice.

METHODS

Cardiac dimensions and function were determined echocardiographically at 8, 12, 16 and 18 weeks of age. Left ventricular pressure characteristics were measured at 18 weeks under baseline conditions and upon dobutamine infusion.

RESULTS

The db/db mice were severely diabetic already at 8 weeks after birth, showing elevated fasting blood glucose levels and albuminuria. Nevertheless, echocardiography revealed no significant changes in cardiac function up to 18 weeks of age. At 18 weeks of age, left ventricular pressure characteristics were not significantly different at baseline between diabetic and control mice. However, dobutamine stress test revealed significantly attenuated cardiac inotropic and lusitropic responses in db/db mice. Post-mortem cardiac tissue analyses showed minor structural remodelling and no significant changes in gene expression levels of the sarcoplasmic reticulum calcium ATPase (SERCA2a) or beta1-adrenoceptor (beta1-AR). Moreover, the phosphorylation state of known contractile protein targets of protein kinase A (PKA) was not altered, indicating unaffected cardiac beta-adrenergic signalling activity in diabetic animals. By contrast, the substantially increased expression of uncoupling protein-3 (UCP3) and angiopoietin-like-4 (Angptl4), along with decreased phosphorylation of AMP-activated protein kinase (AMPK) in the diabetic heart, is indicative of marked changes in cardiac metabolism.

CONCLUSION

db/db mice show impaired cardiac functional reserve capacity during maximal beta-adrenergic stimulation which is associated with unfavourable changes in cardiac energy metabolism.

Adenosine actions are preserved in corpus cavernosum from obese and type II diabetic db/db mouse

INTRODUCTION

Erectile dysfunction (ED) in diabetes is associated with autonomic neuropathy and endothelial dysfunction. Whereas the nonadrenergic-noncholinergic (NANC)/neurogenic nitric oxide pathway has received great attention in diabetes-associated ED, few studies have addressed sympathetic overactivity.

AIM

To test the hypothesis that adenosine-induced inhibition of adrenergic-mediated contractile responses in mouse corpus cavernosum is impaired in the presence of diabetes.

METHODS

The db/db (obesity and type II diabetes caused by a leptin receptor mutation) mouse strain was used as a model of obesity and type II diabetes, and standard procedures were performed to evaluate functional cavernosal responses.

MAIN OUTCOME MEASURES

Increased cavernosal responses to sympathetic stimulation in db/db mice are not associated with impaired prejunctional actions of adenosine.

RESULTS

Electrical field stimulation (EFS)-, but not phenylephrine (PE)-, induced contractions are enhanced in cavernosal strips from db/db mice in comparison with those from lean littermates. Direct effects of adenosine, 2-chloro-adenosine, A(1) receptor agonist C-8031 (N6 cyclopentyladenosine), and sodium nitroprusside are similar between the strips from lean and db/db mice, whereas relaxant responses to acetylcholine and NANC stimulation are significantly impaired in the cavernosal strips from db/db mice. 5'-Iodotubercidin (adenosine kinase inhibitor) and dipyridamole (inhibitor of adenosine transport), as well as the A(1) agonist C-8031, significantly and similarly inhibit contractions induced by stimulation of adrenergic nerves in the cavernosal strips from lean and db/db mice.

CONCLUSIONS

Results from this study suggest that corpora cavernosa from obese and diabetic db/db mice display altered neural-mediated responses that would favor penile detumescence, i.e., increased contractile response to adrenergic nerve stimulation and decreased relaxant responses upon activation of NANC nerves. However, increased cavernosal responses to adrenergic nerve stimulation are not due to impaired negative modulation of sympathetic neurotransmission by adenosine in this diabetic model.