Influence of beta-adrenoceptor tone on the cardioprotective efficacy of adenosine A(1) receptor activation in isolated working rat hearts

Abstract

This study investigated the role of beta-adrenoceptors in the cardioprotective and metabolic actions of adenosine A(1) receptor stimulation. Isolated paced (300 beats min(-1)) working rat hearts were perfused with Krebs-Henseleit solution containing 1.2 mM palmitate. Left ventricular minute work (LV work), O(2) consumption and rates of glycolysis and glucose oxidation were measured during reperfusion (30 min) following global ischaemia (30 min) as well as during aerobic conditions. Relative to untreated hearts, N(6)-cyclohexyladenosine (CHA, 0.5 microM) improved post-ischaemic LV work (158%) and reduced glycolysis and proton production (53 and 42%, respectively). CHA+propranolol (1 microM) had similar beneficial effects, while propranolol alone did not affect post-ischaemic LV work or glucose metabolism. Isoprenaline (10 nM) impaired post-ischaemic function and after 25 min ischaemia recovery was comparable with 30 min ischaemia in untreated hearts (41 and 53%, respectively). Relative to isoprenaline alone, CHA+isoprenaline improved recovery of LV work (181%) and reduced glycolysis and proton production (64 and 60%, respectively). In aerobic hearts, CHA, propranolol or CHA+propranolol had no effect on LV work or glucose oxidation. Glycolysis was inhibited by CHA, propranolol and CHA+propranolol (50, 53 and 52%, respectively). Isoprenaline-induced increases in heart rate, glycolysis and proton production were attenuated by CHA (85, 57 and 53%, respectively). The cardioprotective efficacy of CHA was unaffected by antagonism or activation of beta-adrenoceptors. Thus, the mechanism of protection by adenosine A(1) receptor activation does not involve functional antagonism of beta-adrenoceptors.

Figure 1

LV Work in working rat hearts during pre-ischaemic baseline perfusion, 30 min global ischaemia and 30 min reperfusion. Values are means±s.e.mean for n observations and are shown for hearts that were either untreated (n=12) or perfused in the presence of CHA (0.5 μM, n=9), propranolol (Prop, 1 μM, n=11), propranolol in combination with CHA (Prop+CHA, n=9). *Indicates a significant difference value during reperfusion in each of the drug-treated groups and the untreated group (ANOVA+Dunnetts).

Figure 2

Rates of glycolysis, glucose oxidation and proton production in working rat hearts during pre-ischaemic baseline perfusion and during reperfusion following global, no-flow ischaemia. Values are means±s.e.mean for n observations and are shown hearts that were either untreated (n=6) or subjected to ischaemia and reperfusion in the presence of CHA (0.5 μM, n=6), propranolol (Prop, 1 μM, n=6), propranolol in combination with CHA (Prop+CHA, n=7), isoprenaline (ISO, 10 nM, n=7) or isoprenaline in combination with CHA (ISO+CHA, n=7). *Indicates a significant difference between reperfusion and corresponding pre-ischaemia value (by Student's paired t-test), $ indicates a significant difference between values during reperfusion in each of the drug-treated groups and the untreated group (ANOVA+Dunnetts) and @ indicates a significant difference between Reperfusion values in the isoprenaline and isoprenaline+CHA groups (Student's unpaired t-test).

Figure 3

LV Work in working rat hearts during pre-ischaemic baseline perfusion, 25 min global ischaemia and 30 min reperfusion. Values are means±s.e.mean for n observations and are shown for isoprenaline-treated hearts that were perfused in the absence (ISO, 10 nM, n=11) or presence of 0.5 μM CHA (ISO+CHA, n=12). *Indicates a significant difference between reperfusion values in the isoprenaline and isoprenaline+CHA groups (Student's unpaired t-test).

Figure 4

Rates of glycolysis, glucose oxidation and proton production in working rat hearts during aerobic baseline perfusion and during a subsequent aerobic treatment period. Values are means±s.e.mean for n observations and are shown hearts that were either untreated (n=11) or perfused in the presence of CHA (0.5 μM, n=8), propranolol (Prop, 1 μM, n=5), propranolol in combination with CHA (Prop+CHA, n=5), isoprenaline (ISO, 10 nM, n=7) or isoprenaline in combination with CHA (ISO+CHA, n=7). *Indicates a significant difference between treatment and corresponding baseline value (by Student's paired t-test). $ Indicates a significant difference between baseline values in each of the drug-treated groups and the untreated group (ANOVA+Bonferroni). #Indicates a significant difference in treatment values between each of the drug-treated groups and the untreated group and @ indicates a significant difference between treatment values in the isoprenaline and isoprenaline+CHA groups (Student's unpaired t-test).