Impaired relaxing response to isoprenaline in isolated thoracic aorta of nephrotic rats: decrease in release of EDRF...

Isoprenaline-induced relaxation was investigated in aortas from control and daunomycin-induced nephrotic rats. In the endothelium-intact aortas precontracted with phenylephrine, the isoprenaline-induced relaxation and cyclic adenosine monophosphate (AMP) accumulation were significantly less in nephrotic rats than in control animals. Removal of the endothelium, pretreatment with methylene blue (MB), a guanylate cyclase inhibitor, or NW-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, markedly reduced the relaxation induced by isoprenaline in nephrotic and control animals. The increase in cyclic AMP content induced by isoprenaline also was inhibited by these treatments. In addition, the difference in the isoprenaline-induced relaxation and cyclic AMP accumulation between nephrotic and control preparations was abolished by these treatments. The tissue cyclic guanosine monophosphate (GMP) level was not affected by isoprenaline. In the presence of zaprinast (Zap), a cyclic GMP phosphodiesterase inhibitor, the cyclic GMP level in the endothelium-intact tissues was significantly lower in nephrotic rats than in control animals. Removal of endothelium or pretreatment with MB or L-NAME markedly reduced cyclic GMP content in nephrotic and control animals. In the endothelium-denuded tissues, the isoprenaline-induced relaxation and cyclic AMP accumulation were markedly potentiated by a low concentration of nitroprusside (NP). In the endothelium-intact aortas precontracted with phenylephrine, relaxations induced by dobutamine, salbutamol, and forskolin in nephrotic rats were not significantly different from those in control animals. In the endothelium-intact aortas precontracted with KCl, the isoprenaline-induced relaxation also was significantly less in nephrotic rats than in control animals. Pretreatment with prazosin, but not yohimbine, abolished this difference. These results indicate that nephrosis decreases the relaxing response of the endothelium-intact aortas to isoprenaline. In addition, these results suggest that the endothelium-derived relaxing factor (EDRF) released from the endothelial cells markedly enhances isoprenaline-induced increase in the tissue level of cyclic AMP. The decreased relaxing response to isoprenaline in nephrotic rats may be caused by the decrease in the endothelial-dependent cyclic GMP release caused by alpha 1-adrenoceptor activation by isoprenaline.