Fig. 4. IPC reduces renin and NE overflow, and shortens arrhythmias caused by I/R in murine hearts ex vivo. H4R activation mimics the cardioprotective anti-RAS effect of IPC, whereas H4R blockade or H4R gene deletion prevents IPC-induced cardioprotection. Coronary overflow of renin and NE, and duration of reperfusion arrhythmias (VT/VF) in control ischemia (i.e., 30-minute perfusion with glucose- and pyruvic acid-free KH buffer, 95% N2 + 5% CO2) followed by 30-minute reoxygenation with normal KH buffer (WT, n = 11; H4R−/−, n = 7). Other hearts were subjected to I/R preceded by IPC (i.e., 2 × 5-minute cycles of ischemia, each followed by 5-minute reoxygenation; WT, n = 9; H4R−/−, n = 9). Other hearts were pretreated with the selective H4R antagonist A943931 (300 nM, 20 minutes), and then subjected to IPC (i.e., 2 × 5-minute cycles of ischemia each followed by 5-minute perfusion with A943931), followed by a 15-minute drug-free washout before I/R (n = 7). Other hearts were perfused with the selective H4R agonist 4MeH (1 µM) for 2 × 5-minute cycles, each followed by a 5-minute washout before I/R (WT, n = 7; H4R−/−, n = 6). Other hearts were perfused with the general PKC activator PMA (0.05 nM) for 2 × 5-minute cycles, each followed by a 5-minute washout before I/R (WT, n = 7; H4R−/−, n = 5).*P < 0.05; **P < 0.01; and ***P < 0.001 versus control I/R. #P < 0.05; ##P < 0.01 versus IPC, by unpaired t test.