AFFERENT ARTERIOLAR RESPONSES TO ANG-II INVOLVE ACTIVATION OF PLA(2) AND MODULATION BY LIPOXYGENASE AND P-450 PATHWAYS

Activation of angiotensin receptors activates phospholipase A(2) (PLA( 2)) in various tissues, resulting in the release of arachidonic acid a nd formation of vasoactive metabolites. The present study examined the role of the lipoxygenase and cytochrome P-450 pathways by evaluating the effects of PLA(2), cyclooxygenase, lipoxygenase, and epoxygenase i nhibition on the afferent arteriolar responses to angiotensin II (ANG II) and norepinephrine in the in vitro perfused rat juxtamedullary nep hron preparation. ANG II (0.01-100 nM) resulted in a dose-dependent af ferent arteriolar vasoconstriction ranging from 3 +/- 1 to 32 +/- 2% ( n = 47). Norepinephrine at 0.01, 0.1, and 1.0 mu M also decreased affe rent arteriolar diameter by 5 +/- 1, 17 +/- 1, and 34 +/- 2%, respecti vely (n = 43). In the presence of arachidonyl trifluoromethyl ketone ( AACOCF(3), 20 mu M), a PLA(2) inhibitor, afferent arteriolar vasoconst riction to ANG II (100 nM) was attenuated, and the diameter decreased by 23 +/- 4% (n = 7). The cyclooxygenase inhibitor, indomethacin (100 mu M), and the cyclooxygenase-2 inhibitor, NS-398 (10 mu M), did not a ffect the afferent arteriolar response to ANG II. The lipoxygenase inh ibitor baicalein (1 mu M) attenuated the afferent arteriolar response to ANG II, and vessel diameter decreased by 11 +/- 5% (n = 6) in respo nse to 100 nM ANG II. On the other hand, miconazole (1 mu M), a select ive epoxygenase inhibitor, enhanced the afferent arteriolar vasoconstr iction to 100 nM ANG II. 17-Octadecynoic acid (17-ODYA, 1 mu M), an in hibitor of hydroxylase and epoxygenase metabolism of arachidonic acid, also increased the responsiveness of the afferent arteriole. PLA(2), lipoxygenase, or cytochrome P-450 inhibition had no effect on the affe rent arteriolar vasoconstriction to norepinephrine. The afferent arter iolar vasoconstrictor response to norepinephrine (0.1 mu M) was enhanc ed by indomethacin or NS-398, and diameter decreased by 25 +/- 3% and 28 +/- 4%, respectively. Results of this study suggest that metabolite s of the cyclooxygenase pathway attenuate the afferent arteriolar vaso constrictor effect of norepinephrine. Furthermore, these data suggest that activation of PLA(2) is involved in part of the afferent arteriol ar response to ANG II and that metabolites of the lipoxygenase pathway augment and metabolites of the epoxygenase pathway attenuate the affe rent arteriolar vasoconstrictor effect of ANG II.