Role of the endothelium and cyclic GMP in renal vasodilator responses to cryptolepine in rats.

Abstract

Isolated perfused rat kidney was used to examine the possible mechanisms involved in the hypotensive/vasodilator actions of cryptolepine. In kidneys preconstricted by phenylephrine (PE 5-7.5 x 10(-7) M), cryptolepine at bolus doses of 2.5, 5, and 10 micrograms elicited dose-dependent reductions in perfusion pressure by 29.8 +/- 4.1, 43.3 +/- 3.9, and 54.3 +/- 4.9 mm Hg, respectively. In the presence of indomethacin, cryptolepine-induced reduction in perfusion pressure was not significantly changed, suggesting a lack of a cyclooxygenase-mediated component in its renal vasodilator response. Removal of the endothelium with p-bromophenacyl bromide (p-BPB 10 microM) inhibited the vasodilator response to cryptolepine 2.5, 5, and 10 micrograms to 10.2 +/- 1.8, 15.9 +/- 1.5, and 20.2 +/- 2.0 mm Hg, respectively (p < 0.01). The vasodilator response to acetylcholine (ACh 50 ng) was also reduced from a control value of 56.7 +/- 4.5 to 15.3 +/- 1.9 mm Hg (p < 0.01); responses to sodium nitroprusside (SNP 5 micrograms) and isoprenaline (1 microgram) were not affected. In kidneys treated with hydroquinone (10(-5) and 10(-4) M), a specific inhibitor of endothelium-dependent vasodilation, cryptolepine- and ACh-induced vasodilation were inhibited dose dependently (p < 0.01). N omega-nitro-L-arginine (L-NNA 10(-5)-10(-4) M), a specific inhibitor of the synthesis/release of endothelium-derived relaxing factor/nitric oxide (EDRF/NO), attenuated the vasodilator response to cryptolepine and ACh (50 ng) dose dependently. At 10(-4) M L-NNA, cryptolepine-induced vasodilation was reduced to 6.6 +/- 2.2 (2.5 micrograms), 10.9 +/- 2.2 (5 micrograms), and 13.3 +/- 1.4 mm Hg (10 micrograms). L-Arginine (10(-4) and 3 x 10(-4) M) but not D-arginine (10(-4) M) inhibited the effects of L-NNA, with vasodilatory effects of cryptolepine returning to control values, suggesting that the vasodilator material released by cryptolepine is EDRF, possibly NO. Methylene blue (MB 10(-4) M), the inhibitor of soluble guanylate cyclase which inhibited 50 ng ACh and 5 micrograms SNP-induced vasodilation also reduced the vasodilatory responses to cryptolepine to 0.8 +/- 0.8 (2.5 micrograms), 4.2 +/- 4.2 (5 micrograms), and 10.8 +/- 6.2 mm Hg (10 micrograms) suggesting that the effector pathway for cryptolepine-induced vasodilation is soluble guanylate cyclase-linked increase in cyclic GMP of vascular smooth muscle.