Vol. 295, Issue 2, 578-585, November 2000

Endothelin Receptor Antagonism Does Not Prevent the Development of In Vivo Glyceryl Trinitrate Tolerance in the Rat¹

Jodan D. Ratz², Amy B. Fraser³, Karen J. Rees-Milton, Michael A. Adams and Brian M. Bennett⁴

Department of Pharmacology and Toxicology, Faculty of Health Sciences, Queen's University, Kingston, Ontario, Canada

There is evidence that increased endothelial production of endothelin-1 (ET-1) may contribute to glyceryl trinitrate (GTN) tolerance. We used the competitive ET_A receptor antagonist ZD2574 to determine whether chronic ET_A receptor blockade affected the biochemical and functional responses to GTN during the development of GTN tolerance in vivo. Tolerance induced using transdermal GTN patches resulted in a 5.3 ± 1.2-fold increase in the EC₅₀ value for GTN relaxation in isolated aorta from GTN-tolerant rats. Coadministration of ZD2574 (100 mg kg¹ t.i.d. for 3 days) during tolerance induction had no effect on GTN-induced relaxation. This dose of ZD2574 markedly blunted the pressor response to ET-1, indicating effective blockade of ET_A receptors, and also abolished the initial transient depressor response to ET-1, indicating that blockade of endothelial ET_B receptors also occurred using this dosage regimen for ZD2574. Consistent with the relaxation data, coadministration of ZD2574 had no effect on the decrease in GTN-induced cGMP accumulation or on the decrease in GTN biotransformation that occurred in aortae from GTN-tolerant animals. Radioimmunoassay data indicated that the GTN tolerance induction protocol caused a 2.3 ± 0.4-fold and a 2.2 ± 0.5-fold increase in total tissue ET-1 levels in tolerant aorta and vena cava, respectively. These data suggest that chronic inhibition of ET receptors by ZD2574 was not sufficient to prevent or diminish the tolerance-inducing effects of GTN, and that the increase in ET-1 levels observed in tolerant tissues may occur as a consequence of the vascular changes that occur during chronic GTN exposure.