Inhibition of Phosphodiesterase 5 Selectively Reverses Nitrate Tolerance in the Venous Circulation

An important component of the antianginal efficacy of glyceryl trinitrate (GTN) is attributable to its selective venodilator effect, resulting in decreased cardiac preload and myocardial oxygen demand. Tolerance to nitrates occurs during chronic exposure, and the current study assessed whether this was due to increased phosphodiesterase (PDE) activity in the venous circulation. Tolerance was induced in rats by continuous exposure to 0.4 mg/h GTN for 48 h. Tension recordings of isolated femoral artery and vein indicated that tolerance was more pronounced in femoral vein. 4-[[3,4-(Methylenedioxy)benzyl]amino]-6-chloroquinazoline (MBCQ), a selective PDE5 inhibitor, significantly decreased the EC(50) values for GTN-induced relaxation in both tolerant and nontolerant tissues, but with the greatest relative shift occurring in tolerant veins. MBCQ also increased the vasodilator potency of 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA/NO), a nitric oxide donor; however, cross-tolerance between DEA/NO and GTN was not observed. A significant increase in cGMP PDE activity was observed in tolerant femoral vein, whereas PDE activity was unchanged in femoral artery. Conscious rats treated with hexamethonium (30 mg/kg) to induce ganglionic blockade exhibited blunted central venous pressure (CVP) and mean arterial pressure (MAP) responses to bolus i.v. doses of GTN in GTN-tolerant animals. The cGMP PDE inhibitor zaprinast (1 mg/kg) selectively reversed the blunted CVP response to GTN in tolerant animals but had no effect on the CVP response to GTN in nontolerant animals or on the MAP response in either group. These results suggest that increased PDE5 activity in the venous circulation contributes to the altered hemodynamic response to GTN following chronic GTN exposure.