Reserpine Attenuates Interstitial Adenosine-Mediated Activation of Ecto-5′-nucleotidase in Rat Hearts in Vivo

We examined whether reserpine-induced norepinephrine (NE) depletion attenuated the products of adenosine in rat heart. A flexibly mounted microdialysis technique was used to measure the concentration of interstitial adenosine and to assess the activity of ecto-5′-nucleotidase in rat hearts in situ. The microdialysis probe was implanted in the left ventricular myocardium of anesthetized rats and perfused with Tyrode solution containing adenosine 5′-monophosphate (AMP) at rate of 1.0 μl/min. The baseline level of dialysate adenosine was 0.51 ± 0.09 μM. The introduction of AMP (100 μM) through the probe increased markedly the dialysate adenosine to 8.95 ± 0.86 μM, and this increase was inhibited by ecto-5′-nucleotidase inhibitor, α,β-methyleneadenosine 5′-diphosphate (AOPCP, 100 μM), to 0.66 ± 0.38 μM. Thus, the level of dialysate adenosine is a measure of the ecto-5′-nucleotidase activity in the tissue in situ. AMP concentration for the half-maximal effect of adenosine release (EC50) was 107.3 μM. The maximum attainable concentration of dialysate adenosine (Emax) by AMP was 21.1 μM. However, the EC50 and Emax values with reserpinized animals were 106.9 and 7.1 μM, respectively. Electrical stimulation of the left stellate ganglion increased significantly dialysate adenosine concentration, from the control level of 8.66 ± 0.96 μM to 12.38 ± 1.11 μM. After stimulation, dialysate adenosine returned to near the prestimulation level. When corresponding experiments were performed with reserpinized animals, the effect of electrical stimulation was abolished. Tyramine (endogenous catecholamine trigger) increased the adenosine concentration in a concentration-dependent manner. However, the elevation of adenosine concentration with reserpinized animals was not observed. These results suggest that reserpine attenuates NE-induced adenosine via stimulation of α1-adrenoceptor and protein kinase C mediated activation of ecto-5′-nucleotidase in rat heart.