Tyramine produces interstitial adenosine-mediated activation of ecto-5′-nucleotidase in rat heart in vivo

Abstract

We examined the effect of tyramine on the production of adenosine in rat heart. A flexibly mounted microdialysis setup was used to measure the concentration of interstitial adenosine and to assess the activity of ecto-5′-nucleotidase in in vivo rat hearts. The microdialysis probe was implanted in the left ventricular myocardium of anesthetized rats and perfused with Tyrode solution containing adenosine 5′-monophosphate (AMP) at a rate of 1.0 μl/min. The concentration of adenosine in the effluent (dialysate) was measured by high-performance liquid chromatography (HPLC). Dialysate adenosine obtained during perfusion with the AMP-containing solution through the probe originated from the hydrolysis of AMP by endogenous ecto-5′-nucleotidase, and the level of adenosine reflected the activity of ecto-5′-nucleotidase in the tissue. Tyramine (0–4 mM) increased the adenosine concentration measured during the perfusion of AMP (100 μM) in a concentration-dependent manner. α,β-Methyleneadenosine 5′-diphosphate (α,β-meADP, 100 μM), an inhibitor of ecto-5′-nucleotidase, abolished the AMP-induced increase in dialysate adenosine. Tyramine (1 mM) increased the adenosine concentration measured in the presence of 100 μM AMP (i.e., the activity of ecto-5′-nucleotidase) by 65.8±19.9% ( n=6, P<0.05), an increase which was inhibited by an antagonist of the α 1-adrenoceptor (prazosin, 50 μM) or of protein kinase C (chelerythrine, 10 μM). These data provide the first evidence that α 1-adrenoceptor stimulation and the subsequent activation of protein kinase C can increase adenosine concentrations in the interstitial space of ventricular muscle in vivo, through activation of endogenous ecto-5′-nucleotidase. To examine the effect of tyramine on the production of adenosine by ischemia–reperfusion of the rat myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery. When the heart was reperfused, elevation of the level of adenosine in the ischemic zone was observed, but this change was not significant. However, when corresponding experiments were performed with a subsequent systemic administration of tyramine (1 mM), a marked elevation in the level of adenosine was observed. The results suggest that tyramine elevates adenosine via stimulation of α 1-adrenoceptors and protein kinase C-mediated activation of ecto-5′-nucleotidase in rat heart.