Two possible mechanisms underlying nitrate tolerance in monkey coronary arteries

Prevention of nitroglycerin tolerance in vitro by T0156, a selective phosphodiesterase type 5 inhibitor

The efficacy of nitroglycerin as a vasodilator is limited by tolerance, which develops shortly after treatment begins. The present study aims to examine whether T0156, a newly developed potent and selective inhibitor of phosphodiesterase type 5 (PDE5), could attenuate the tolerance to nitroglycerin on rat aortas. Rat aortic rings were suspended in organ bath for the measurement of changes in isometric tension and nitrate tolerance was acutely induced by preceding exposure for 90 min to 30 microM nitroglycerin. Concentration-response curves to nitroglycerin were obtained on aortic rings pre-contracted with phenylephrine. Pre-exposure of rings with or without endothelium to nitroglycerin reduced the relaxations to nitroglycerin. The tissue levels of cyclic GMP were measured by enzyme immunoassay kit. Treatment with T0156 inhibited and prevented the reduced relaxation and cyclic GMP levels in response to nitroglycerin in tolerant rings. In contrast, nitroglycerin-induced tolerance was unaffected by cilostazol (PDE3 inhibitor) and rolipram (PDE4 inhibitor). Finally, incubation of aortic rings with thromboxane prostanoid receptor antagonist, cyclooxygenase inhibitor, or endothelin ET(A) receptor antagonist did not inhibit the development of tolerance. The present results suggest that nitroglycerin tolerance may involve an increased activity of PDE5 but not PDE3 or PDE4 isoforms in vascular smooth muscle cells since T0156 prevents the development of tolerance. Thromboxane A(2), cyclooxygenase (COX)-dependent prostaglandins and endothelin 1 play little role in the acute induction of nitroglycerin tolerance.

Endothelin ETA receptor antagonist did not affect development of tolerance to glyceryl trinitrate in rat

Glyceryl trinitrate (GTN), extensively used for the treatment of cardiovascular disorders, is associated with rapid development of tolerance. The exact mechanism responsible for tolerance development to GTN is still not known. Recently, it has been demonstrated that GTN tolerance is associated with increased expression of endothelin (ET). This study was carried out to determine the effect of ET(A) receptor antagonist, BMS182874, on the development of tolerance to GTN in urethane-anaesthetized rats. Diastolic blood pressure (DBP), systolic blood pressure (SBP) and heart rate (HR) were continuously recorded in vehicle- and BMS182874 (3 mg kg(-1), i.v.)-treated rats. GTN was infused at the rate of 10 microg min(-1), intravenously for 4 h. Tolerance to GTN was determined using challenge doses of GTN (10, 30 and 90 microg, i.v.). GTN produced a fall in DBP, SBP and an increase in HR. In vehicle-treated rats, the fall in SBP before induction of GTN tolerance was 28 +/- 2, 43 +/- 4 and 52 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. However, following GTN infusion (10 microg min(-1), i.v. for 4 h) a rapid development of tolerance was observed and the fall in SBP was 1 +/- 1, 9 +/- 4 and 15 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. Similarly, in BMS182874-treated rats the fall in SBP in non-tolerant rats was 28 +/- 4, 42 +/- 4 and 48 +/- 5 mmHg with 10, 30 and 90 microg GTN, respectively. In BMS182874-treated rats following GTN infusion (10 microg min(-1), i.v. for 4 h) a rapid development of tolerance was observed and the fall in SBP was 4 +/- 3, 10 +/- 2 and 13 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. The decrease in DBP and SBP in vehicle- and BMS182874-treated GTN-tolerant rats was statistically similar. These results suggest that ET(A) receptor antagonist BMS182874 did not affect development of tolerance to GTN in rats.

Endothelial modulation and tolerance development in the vasorelaxant responses to nitrate of rabbit aorta

We investigated the endothelial modulations in nitrate tolerance in isolated rabbit aorta. Nitrate tolerance was induced by a 72-h treatment with transdermal nitroglycerin (NTG, 0.4 mg/h) in conscious rabbits, which was verified by a 20-fold increase in the EC50 values [NTG tolerance (6.1 +/- 0.8) x 10(-7) M vs control (3.0 +/- 0.6) x 10(-8) M]. The relaxations to NTG in tolerant and nontolerant aortic strips were enhanced when their endothelia were denuded [E(-)]. In the presence of endothelium [E(+)], NTG-tolerant vessels were not tolerant to acetylcholine (ACh), which can release endothelial nitric oxide (NO), exogenous NO or 8-bromo (Br)-cGMP. In NTG-tolerant and nontolerant vessels with endothelium, concentration-response curves for NO were the same as those in endothelium-absent tolerant vessels. In both NTG-tolerant and nontolerant vessels, treatment with superoxide dismutase (SOD, 20 units/ml), an O2-. scavenger, unaffected the responses to NTG reduced in the presence of endothelium, but treatment with NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), an NO synthase (NOS) inhibitor, reversed these reductions. Thus, our data did not indicate that an increased endothelial superoxide O2-. production contributes to nitrate tolerance. Our study suggested that (i) an impaired biotransformation process from NTG to NO is responsible for the occurrence of nitrate tolerance and (ii) vascular response to NTG enhanced by endothelial removal is related to blocked endothelial NO release.

Regulation of cGMP-specific phosphodiesterase (PDE5) phosphorylation in smooth muscle cells

Nitric oxide and endogenous nitrovasodilators regulate smooth muscle tone by elevation of cGMP and activation of cyclic GMP-dependent protein kinase (PKG). The amplitude and duration of the cGMP signal in smooth muscle is regulated in large part by cGMP-specific cyclic nucleotide phosphodiesterase (PDE5). Previous in vitro data have suggested that both cAMP-dependent protein kinase and PKG can regulate the activity of PDE5. To test if this type of regulation is important in the intact cell, we have generated phospho-PDE5-specific antisera and have utilized isolated smooth muscle cells from mice having a disruption in the PKG I gene as well as cells from normal human smooth muscle. The data show that in human smooth muscle cells, activation of PKG by 8-Br-cGMP led to phosphorylation and activation of PDE5. In the same cells, 8-Br-cAMP had no significant effect on PDE5 phosphorylation. Treatment of wild-type mouse aortic smooth muscle cells with 8-Br-cGMP also induced the phosphorylation of PDE5, whereas no phosphorylation was seen in smooth muscle cells isolated from mice in which the gene for PKG I had been disrupted. As with the human cells, no phosphorylation was seen in the mouse cells in response to 8-Br-cAMP. These results strongly suggest that a major regulatory pathway for control of PDE5 phosphorylation and activity in intact smooth muscle is via PKG-dependent phosphorylation of PDE5. Finally, experiments with calyculin A and okadaic acid suggest that PP1 phosphatase, the catalytic subunit of myosin phosphatase, can regulate PDE5 dephosphorylation. Together, the data suggest that phosphorylation and activation of PDE5 by PKG I and its subsequent dephosphorylation by myosin phosphatase may be key steps in the regulation of relaxation/contraction cycles of smooth muscle.