It is well known, that reperfusion injury is characterized by development of cytosolic Ca²⁺-oscialltions and hypercontracture at the start of reperfusion. The aim of present study was to investigate, that if the activation of protein kinase A (PKA) influences the above-mentioned parameters and to analyze, whether this influence is due to affecting the mitochondrial permeability transition pores (MPTP) or the sarcoplasmic reticulum (SR). Isolated adult rat cardiomyocytes were superfused anoxically (60 min; 100% N₂; no glucose; pH 6.4) and then reperfused with a normoxic buffer (20 min; 21% O₂; 2.5 mM glucose; pH 7.4). In addition to cell length measurement, cytosolic Ca²⁺ and MPTP-opening were measured by the fluorescence indicators Fura-2 and Calcein. The PKA-Activator Isoproterenol (10 nM, 100 nM, or 1 mM) or the direct inhibitor of the MPTP, Cyclosporine A (Csp A; 500 nM) were applied during reperfusion. Before the end of reperfusion, a Ca²⁺-release from the SR was induced by application of Caffeine (100 mM). Application of Isoproterenol in all three concentrations during reperfusion reduced the frequency of the Ca²⁺-oscillations (Oscillations in the 5^th min of reperfusion in presence of 10 mM Isoproterenol: 19.03 ± 4.36 vs. 35.41 ± 4.27 in control; n >= 14, p < 0.05 for all further measurements). The extent of hypercontracture was also significantly decreased by Isoproterenol (Cell length at the end of reperfusion in presence of 10 mM Isoproterenol: 70.05 ± 2.16 % of end-anoxic length vs. 62.26 ± 1.50 % in control).

The Csp A-sensitive loss of mitochondrial Calcein, which indicates the MPTP-opening, was not reduced in presence of Isoproterenol. The application of Isoproterenol during reperfusion led to enhanced Ca²⁺-uptake by the SR (Increase of Ca²⁺-concentration after Caffeine-induced Ca²⁺-release from the SR in presence of 10 nM Isoproterenol: 33.76 ± 9.79 % vs. 4.63 ± 1.04 % in control). The data of present study show, that activation of PKA markedly reduces the reperfusion-induced cytosolic Ca²⁺-oscialltions and hypercontracture. This protection against reperfusion injury was mediated by enhanced Ca²⁺-uptake by the SR and not by interaction with the MPTP.