Effect of nitrovasodilators and inhibitors of nitric oxide synthase on ischaemic and reperfusion function of rat isolated hearts

1. The functional role of the nitric oxide (NO)/guanosine 3′:5′-cyclic monophosphate (cyclic GMP) pathway in experimental myocardial ischaemia and reperfusion was studied in rat isolated hearts.
2. Rat isolated hearts were perfused at constant pressure with Krebs-Henseleit buffer for 25 min (baseline), then made ischaemic by reducing coronary flow to 0.2 ml min⁻¹ for 25 or 40 min, and reperfused at constant pressure for 25 min. Drugs inhibiting or stimulating the NO/cyclic GMP pathway were infused during the ischaemic phase only. Ischaemic contracture, myocardial cyclic GMP and cyclic AMP levels during ischaemia, and recovery of reperfusion mechanical function were monitored.
3. At baseline, heart rate was 287±12 beats min⁻¹, coronary flow was 12.8±0.6 ml min⁻¹, left ventricular developed pressure (LVDevP) was 105±4 mmHg and left ventricular end-diastolic pressure 4.6±0.2 mmHg in vehicle-treated hearts (control; n=12). Baseline values were similar in all treatment groups (P>0.05).
4. In normoxic perfused hearts, 1 μM N^G-nitro-L-arginine (L-NOARG) significantly reduced coronary flow from 13.5±0.2 to 12.1±0.1 ml min⁻¹ (10%) and LVDevP from 97±1 to 92±1 mmHg (5%; P<0.05, n=5).
5. Ischaemic contracture was 46±2 mmHg, i.e. 44% of LVDevP in control hearts (n=12), unaffected by low concentrations of nitroprusside (1 and 10 μM) but reduced to ∼30 mmHg (∼25%) at higher concentrations (100 or 1000 μM; P<0.05 vs control, n=6). Conversely, the NO synthase inhibitor L-NOARG reduced contracture at 1 μM to 26±3 mmHg (23%), but increased it to 63±4 mmHg (59%) at 1000 μM (n=6). Dobutamine (10 μM) exacerbated ischaemic contracture (81±3 mmHg; n=7) and the cyclic GMP analogue Sp-8-(4-p-chlorophenylthio)-3′,5′-monophosphorothioate (Sp-8-pCPT-cGMPS; 10 μM) blocked this effect (63±1 mmHg; P<0.05 vs dobutamine alone, n=5).
6. At the end of reperfusion, LVDevP was 58±5 mmHg, i.e. 55% of pre-ischaemic value in control hearts, significantly increased to ∼80% by high concentrations of nitroprusside (100 or 1000 μM) or L-NOARG at 1 μM, while a high concentration of L-NOARG (1000 μM) reduced LVDevP to ∼35% (P<0.05 vs control; n=6).
7. Ischaemia increased tissue cyclic GMP levels 1.8 fold in control hearts (P<0.05; n=12); nitroprusside at 1 μM had no sustained effect, but increased cyclic GMP ∼6 fold at 1000 μM; L-NOARG (1 or 1000 μM) was without effect (n=6). Nitroprusside (1 or 1000 μM) marginally increased cyclic AMP levels whereas NO synthase inhibitors had no effect (n=6).
8. In conclusion, the cardioprotective effect of NO donors, but not of low concentrations of NO synthase inhibitors may be due to their ability to elevate cyclic GMP levels. Because myocardial cyclic GMP levels were not affected by low concentrations of NO synthase inhibitors, their beneficial effect on ischaemic and reperfusion function is probably not accompanied by reduced formation of NO and peroxynitrite in this model.