Alterations in the β-adrenergic receptor system after hypothermic ischemia in hearts with preischemic β-receptor desensitization

Although hypothermic cardioplegic arrest is a basic method of myocardial protection in cardiac surgery, the β-adrenergic receptor (BAR) system has been little investigated in the heart subjected to hypothermic ischemia. Additionally, although the hypothermic arrest is often induced in hearts with preischemic desensitization of the BAR system by preceding congestive heart failure, the functional state of the BAR system after ischemia has not been studied in these hearts. We investigated alterations in the BAR system after hypothermic ischemia in normal rat hearts and in those with preischemic desensitization of the BAR system produced with isoproterenol (ISP: 400 μg/kg/hr for 24 hr). Both normal and BAR-desensitized hearts were isolated and subjected either to 40 min of hypothermic (10°C) global ischemia followed by 40 min of reperfusion or subjected to time-matched aerobic perfusion with modified Krebs-Henseleit solution. At the end of perfusion (1) BAR binding properties with [3H]CGP-12177 and adenylate cyclase activity were measured in crude membrane fraction and (2) the inotropic response to ISP (ΔLV + dP/dtmax) was evaluated in an isovolumetric contracting heart preparation. Following reperfusion, normal hearts without desensitized BAR showed a higher Bmax value than those of nonischemic time-matched hearts (41.8 ± 3.1 vs 35.4 ± 2.4 fmole/mg protein, P <0.05), whereas the Kd value was in a similar range in the two groups. Basal adenylate cyclase activity and activities after stimulation by 10 mM sodium fluoride (NaF) and 100 μM forskolin were in a comparable degree between reperfused and nonischemic time-matched hearts, while the maximal ISP-stimulated adenylate cyclase activity obtained by 10–100 μM of ISP was higher in the former (51.2 ± 3.1 vs 40.3 ± 4.7 pmole cAMP/min/mg protein, P < 0.05). The maximal inotropic response to ISP (maximal ΔLV + dP/dtmax) was higher in reperfused hearts than nonischemic time-matched hearts (1650 ± 59 vs 1490 ± 83 mm Hg/sec, P < 0.05). On the other hand, in BAR-desensitized hearts where BAR density, maximal ISP-stimulated adenylate cyclase activity, and maximal inotropic response to ISP were all depressed, Bmax value was not different between the reperfused and nonischemic time-matched hearts (23.6 ± 2.9 vs 23.9 ± 2.8 fmole/mg protein), as was the Kd value. Adenylate cyclase activities under basal and stimulated conditions were in a similar range between the two groups. The ISP-stimulated maximal enzyme activity was also similar between these two groups (31.2 ± 5.3 vs 30.2 ± 4.1 pmole cAMP/min/mg protein), while maximal ΔLV + dP/dtmax was smaller in reperfused hearts than nonischemic hearts (726 ± 123 vs 1092 ± 105 mm Hg/sec, P <0.05).