Effects of targeted deletion of A1 adenosine receptors on postischemic cardiac function and expression of adenosine receptor subtypes

Abstract

To examine ischemic tolerance in the absence of A₁ adenosine receptors (A₁ARs), isolated wild-type (WT) and A₁AR knockout (A₁KO) murine hearts underwent global ischemia-reperfusion, and injury was measured in terms of functional recovery and efflux of lactate dehydrogenase (LDH). Hearts were analyzed by real-time RT-PCR both at baseline and at intervals during ischemia-reperfusion to determine whether compensatory expression of other adenosine receptor subtypes occurs with either A₁AR deletion and/or ischemia-reperfusion. A₁KO hearts had higher baseline coronary flow (CF) and left ventricular developed pressure (LVDP) than WT hearts, whereas heart rate was unchanged by A₁AR deletion. After 20 min of ischemia, CF was attenuated in A₁KO compared with WT hearts, and this reduction persisted throughout reperfusion. Final recovery of LVDP was decreased in A₁KO hearts (54.4 ± 5.1 vs. WT 81.1 ± 3.4% preischemic baseline) and correlated with higher diastolic pressure during reperfusion. Postischemic efflux of LDH was greater in A₁KO compared with WT hearts. Real-time RT-PCR demonstrated the absence of A₁AR transcript in A₁KO hearts, and the message for A_2A, A_2B, and A₃ adenosine receptors was similar in uninstrumented A₁KO and WT hearts. Ischemia-reperfusion increased A_2B mRNA expression 2.5-fold in both WT and A₁KO hearts without changing A₁ or A₃ expression. In WT hearts, ischemia transiently doubled A_2A mRNA, which returned to preischemic level upon reperfusion, a pattern not observed in A₁KO hearts. Together, these data affirm the cardioprotective role of A₁ARs and suggest that induced expression of other adenosine receptor subtypes may participate in the response to ischemia-reperfusion in isolated murine hearts.