RELEASE OF NITRIC-OXIDE FROM GLYCERYL TRINITRATE BY CAPTOPRIL BUT NOTENALAPRILAT - INVITRO AND INVIVO STUDIES

1 The hypotensive effects of glyceryl trinitrate (GTN, 0.5 mg kg-1) bu t not of 3-morpholinosydnonimine (SIN-1, 0.125 mg kg-1) in anaesthetiz ed rats were attenuated following a seven day (using a q.i.d. dosing s chedule) oral treatment with isosorbide-5-mononitrate (IS-5-MN; 5 mg k g-1) indicative of the induction of tolerance to GTN but not to SIN-1. The hypotensive effects of GTN did not decline when the sulphydryl (S H) containing angiotensin converting enzyme inhibitor (ACE-I), captopr il (CPT, 5 mg kg-1) or the structurally unrelated SH-containing, N-ace tylcysteine (NAC, 10 mg kg-1) but not the non-SH-containing ACE-I, ena laprilat (ENA, 5 mg kg-1) were given together with IS-5-MN for the sev en days treatment. 2 The attenuated hypotensive effects of GTN (0.5 mg kg-1) in rats treated with IS-5-MN were also restored when CPT (1 mg kg-1) or NAC (2.5 mg kg-1) but not ENA (1 mg kg-1) was administered in traperitoneally (i.p.) 30 min before GTN. Furthermore, in control rats , CPT or NAC but not ENA given i.p. 30 min before GTN, potentiated its haemodynamic effects. These effects were blocked by methylene blue (1 0 mg kg-1). At the same doses, CPT or NAC did not affect the hypotensi ve effects of SIN-1. 3 The reduced ability of cultured tolerant smooth muscle cells (SMC, 24 x 10(3) cells) or endothelial cells (EC, 40 x 1 0(3) cells) to potentiate the anti-platelet effects of GTN (44 muM) wa s restored by CPT or NAC but not by ENA or glutathione (all at 0.5 mm) . Potentiation of the anti-platelet effects of tolerant SMC or EC by C PT or NAC was abolished by co-incubation with oxyhaemoglobin (Oxy-Hb, 10 muM) indicative of nitric oxide (NO) formation. 4 When GTN (150-240 0 muM) was incubated with CPT, NAC or glutathione but not ENA (all at 0.1 mM) for 30 min in Krebs buffer at 37-degrees-C a concentration-dep endent increase in nitrite (NO2-) formation was observed. 5 The antipl atelet effects of GTN (5.5-352 muM) were potentiated by co-incubation with CPT or NAC but not with ENA or glutathione (all at 0.5 mM). The c oncentration of GTN required to inhibit platelet aggregation by 50% (I C50) was 110 +/- 2 muM for GTN alone, 14 +/- 2 muM for GTN in the pres ence of NAC and 30 +/- 2 muM for GTN in the presence of CPT. The poten tiation of the effects of GTN by CPT or NAC was inhibited by co-incuba tion with Oxy-Hb (10 muM). By themselves, CPT or NAC did not inhibit p latelet aggregation. 6 The ability of CPT to restore (a) the haemodyna mic effects of GTN in tolerant rats and (b) the reduced capacity of to lerant SMC or EC to potentiate the anti-platelet effects of GTN is not related to its ACE inhibitory activity. 7 CPT also potentiated the hy potensive effects of GTN in non-tolerant rats, and in vitro CPT releas ed NO from GTN in the absence of a GTN to NO converting cell, so that it is unlikely that reversal of tolerance by CPT is due to the repleni shment of intracellular thiols. Rather it can be explained by the abil ity of CPT to release NO from GTN in the extracellular space. This ext racellular formation of NO from GTN by CPT would then compensate for t he impaired enzymic biotransformation of GTN to NO that develops durin g tolerance as was originally proposed for NAC.