Reversal of nitroglycerin tolerance in vitro by the cGMP-phosphodiesterase inhibitor zaprinast

Following in vitro exposure of rat aortic rings to 550 μM nitroglycerin for 1 h, tolerance was demonstrated by a significant increase in EC 50 values for nitroglycerin-induced relaxation. However, cross-tolerance to sodium nitroprusside was not observed. Co-incubation of aortic rings with the cGMP-phosphodiesterase (cGMP-PDE) inhibitor zaprinast (10 μM), during incubation with 550 μM nitroglycerin, did not prevent the development of tolerance. However, the addition of 0.30 or 10 μM zaprinast to tolerant aortic rings did restore responsiveness to nitroglycerin. The increase in cGMP in tolerant aortic rings in response to 300 nM nitroglycerin (2–4 fmol/μg) was significantly less than that observed for non-tolerant rings (6.6–12 fmol/μg), but cGMP levels were restored in tolerant rings by zaprinas (7–12 fmol/μg). These data suggest that inhibition of vascular cGMP-PDE acitivity does not prevent the development of tolerance in vitro, but does reverse the loss of vasorelaxant potency to nitroglycerin via restoration of intracellular cGMP levels.