THE MECHANISM OF CARDIOPROTECTION BY S-NITROSOGLUTATHIONE MONOETHYL ESTER IN RAT ISOLATED HEART DURING CARDIOPLEGIC ISCHEMIC ARREST

1 This study was designed (i) to assess the effect of S-nitrosoglutath ione monoethyl ester (GSNO-MEE), a membrane-permeable analogue of S-ni trosoglutathione (GSNO), on rat isolated heart during cardioplegic isc haemia, and (ii) to monitor the release of nitric oxide (. NO) from GS NO-MEE in intact hearts using endogenous myoglobin as an intracellular . NO trap and the hydrophilic N-methyl glucamine dithiocarbamate-iron (MGD-Fe2+) complex as an extracellular . NO trap. 2 During aerobic pe rfusion of rat isolated heart with GSNO-MEE (20 mu mol l(-1)), there w as an increase in cyclic GMP from 105 +/- 11 to 955 +/- 193 pmol g(-1) dry wt. (P<0.05), and a decrease in glycogen content from 119 +/- 3 t o 96 +/- 2 mu mol g(-1) dry wt. (P<0.05), and glucose-6-phosphate conc entration from 258 +/- 22 in control to 185 +/- 17 nmol g(-1) dry wt. (P<0.05). During induction of cardioplegia, GSNO-MEE caused the accumu lation of cyclic GMP (100 +/- 6 in control vs. 929 +/- 168 pmol g(-1) dry wt. in GSNO-MEE-treated group, P<0.05), and depletion of glycogen from 117 +/- 3 to 103 +/- 2 mu mol g(-1) dry wt. (P<0.05) in myocardia l tissue. 3 Inclusion of GSNO-MEE (20 mu mol l(-1)) in the cardioplegi c solution improved the recovery of developed pressure (46 +/- 8 vs. 7 1 +/- 3% of baseline, P<0.05), and rate-pressure product from 34 +/- 6 to 63 +/- 5% of baseline (P<0.05), and reduced the diastolic pressure during reperfusion from 61 +/- 7 in control to 35 +/- 5 mmHg (P<0.05) after 35 min ischaemic arrest. GSH-MEE (20 mu mol l(-1)) in the cardi oplegic solution did not elicit the protective effect. 4 During cardio plegic ischaemia, GSNO-MEE (20-200 mu mol l(-1)) induced the formation of nitrosylmyoglobin (MbNO), which was detected by electron spin reso nance (ESR) spectroscopy. Inclusion of MGD-Fe2+ (50 mu mol l(-1) Fe2and 500 mu mol l(-1) MGD) in the cardioplegic solution along with GSNO -MEE yielded an ESR signal characteristic of the MGD-Fe2+-NO adduct. H owever, the MGD-Fe2+ trap did not prevent the formation of the intrace llular MbNO complex in myocardial tissue. During aerobic reperfusion, denitrosylation of the MbNO complex slowly occurred as shown by the de crease in ESR spectral intensity. GSNO-MEE treatment did not affect ub isemiquinone radical formation during reperfusion. 5 GSNO-MEE (20 mu l l(-1)) treatment elevated the myocardial cyclic GMP during ischaemia (47 +/- 3 in control vs. 153 +/- 34 pmol g(-1) dry wt. after 35 min is chaemia, P<0.05). The cyclic GMP levels decreased in the control group during ischaemia from 100 +/- 6 after induction of cardioplegia to 47 +/- 3 pmol g(-1) dry wt. at the end of ischaemic duration. 6 Glycogen levels were lower in GSNO-MEE (20 mu mol l(-1))-treated hearts throug hout the ischaemic duration (26.7 +/- 3.1 in control vs. 19.7 +/- 2.4 mu mol g dry(-1) wt. in GSNO-MEE-treated group at the end of ischaemic duration), because of rapid depletion of glycogen during induction of cardioplegia. During ischaemia, the amounts of glycogen consumed in b oth groups were similar. Equivalent amounts of lactate were produced i n both groups (148 +/- 4 in control vs. 141 +/- 4 mu mol g(-1) dry wt. in GSNO-MEE-treated group after 35 min in ischaemia). 7 The mechanism (s) of myocardial protection by GSNO-MEE against ischaemic injury may involve preischaemic glycogen reduction and/or elevated cyclic GMP lev els in myocardial tissue during ischaemia.