D. Salvemini et al., RELEASE OF NITRIC-OXIDE FROM GLYCERYL TRINITRATE BY CAPTOPRIL BUT NOTENALAPRILAT - INVITRO AND INVIVO STUDIES, British Journal of Pharmacology, 109(2), 1993, pp. 430-436
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.