DIFFERENT REGULATION OF VASCULAR TONE BY ANGIOTENSIN-II AND ENDOTHELIN-1 IN RAT AORTA

The effects of moderate cooling and of phenylarsine oxide on the contraction induced by two vasoactive peptides angiotensin II (Ali) and endothelin (ET-1), were investigated on endothelium-free rings of rat aortas. At 37-degrees-C, the contraction induced by AII (0.1-mu-M) was transient. This decline in tension is unlikely to be due to rapid degradation of AII. In contrast, ET-1 (10 nM) induced a slowly developing and sustained contraction similar to the one observed with phorbol 12-13 dibutyrate (PDB, 22 nM). Moderate cooling (25-degrees-C) significantly potentiated and prolonged the effect of AII but reduced the velocity of the ET-1 and PDB contraction, although the rate of the phenylephrine (1-mu-M) response remained unchanged. Phenylarsine oxide (100-mu-M) reduced the decline in tension in response to AII but inhibited the contraction elicited by ET-1 and PDB. In rings incubated in calcium-free medium (37-degrees-C), AII induced a phasic contraction. This was followed by a second phasic contraction after calcium (2.5 mM) had been restored to the bath. The intensity of this second contraction decreased as the time between AII and calcium injection increased. This method, using regression analysis, permitted us to determine the time taken to reduce the contraction by half (4.8 min; r: 0.96), which may reflect the half-time of receptor sequestration. In calcium-free medium, the contractions induced by ET-1 and PDB were slow and sustained. Thus, rapid AII-receptor internalization leads to a short-term regulation of vascular tone whereas activation of protein kinase C by ET-1 may induce a long-term regulation.