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Differential effects of chronic glyceryl trinitrate (GTN) exposure on endothelial and vascular smooth muscle cells
A major problem associated with chronic GTN administration is the rapid development of tolerance to its vasodilator and antianginal effects. It has been postulated that endothelial cell (EC) dysfunction, as a result of increased superoxide production secondary to angiotensin II-induced up-regulation of EC NAD(P)H oxidase, may mediate the reduced vasodilator response to GTN during the development of nitrate tolerance (Münzel et al., 2000). We investigated 1) whether a temporal relationship exists between chronic GTN exposure, EC dysfunction and reduced relaxation responses to GTN, and 2) whether the AT1-receptor antagonist, losartan, prevented GTN-induced EC dysfunction. Male Sprague-Dawley rats (250-275g) were exposed to a continuous source of GTN via the subdermal implantation of transdermal GTN patches; control animals were implanted with drug-free patches (Ratz et al., 2002). Three to eight rats were randomly assigned to each treatment group. In study one and two, rats were administered 0.4 mg/hr GTN for 6, 12, 24, 48, or 96 hr, or were administered GTN for 48 hr followed by a 12, 24, 48, or 96 hr nitrate-free period. In the third, rats were treated with losartan (30 mg/kg/day) for 7 days combined with GTN treatment (0.4mg/hr) for the final 2 days. Following treatment, ex vivo relaxation responses (EC50 and maximal relaxation (Emax)) of the isolated aorta were obtained for acetylcholine (ACh) and GTN to assess endothelium-dependent and -independent relaxation, respectively. No changes were observed after a 6 or 12hr exposure to GTN. After 24-hr GTN exposure, there was a significant 1.8-fold increase in the EC50 for GTN-induced relaxation and an 11% (p<0.01) decrease in Emax. In contrast, the relaxation responses to ACh remained unchanged, indicating that the reduced vasodilator response to GTN occurred prior to EC dysfunction. After 48 and 96 hr exposure to GTN, there were significant increases in the EC50 and significant decreases in the Emax for both GTN- and ACh-induced relaxation, indicating the development of GTN tolerance and EC dysfunction. However, there was a greater increment of impairment for ACh-induced relaxation between the two time points. In study 2, the full restoration of relaxation responses to ACh and GTN occurred after a 24hr nitrate-free period. In study three, losartan treatment had no effect on GTN tolerance. In contrast, losartan treatment prevented GTN-induced EC dysfunction. Thus the impaired vasodilator response to GTN in GTN-tolerant tissues was still apparent, despite preservation of EC-dependent relaxation responses by losartan treatment. These results suggest that the reduced vasodilator response to GTN in nitrate-tolerant animals is not a consequence of endothelial dysfunction. Rather, chronic GTN exposure results in alterations in VSM and endothelial function, and the time-course and extent of functional changes varies with cell type. Supported by the Heart and Stroke Foundation of Ontario and the Canadian Institutes of Health Research.
Munzel T, et al. (1995). J. Clin. Invest. 95:187-194. |
