Perfusion of hearts with DETA/NO increases the calcium-retention capacity of subsequently isolated mitochondria. Rat hearts were perfused with Krebs-Henseleit solution only (control group) or with 50 μM DETA/NO for 3 min (DETA/NO group) or with 1 μM of PKG inhibitor KT 5823 for 5 min prior to perfusion with DETA/NO (DETA/NO + PKG inhibitor group). 30 min ischemia was induced as described in Methods. Isolated mitochondria (0.2 mg protein) were added to 3 ml incubation buffer C (200 mM sucrose, TrisHCl 10 mM, KH2PO4 1 mM, EGTA 10 μM, 0.3 mM pyruvate plus 0.3 mM malate, pH 7.4) and mitochondrial Ca2+ accumulation was measured fluorimetrically using Calcium Green-5N (excitation at 506 nm, emission at 535 nm) as described in Methods. Ca2+ retention capacity (amount required to be accumulated before large increase in fluorescence was observed due to opening of permeability transition pore) of control mitochondria was 124.6 ± 6.5 nmol/mg protein and was equated to 100%. * – statistically significant effect (p < 0.05, Tukey test) if compared to control, # – statistically significant effect of PKG inhibitor (p < 0.05, LSD test) if compared to DETA/NO group, ^ – statistically significant effect of DETA/NO if compared to ischemic group (p < 0.05, Tukey test). Means ± standard errors of 3–5 separate experiments are presented.