Background : Interleukin-1-α (IL-1) is a cytokine with potentially therapeutic immunoproliferative and tumori-cidal activities. Preliminary clinical studies suggest that use of IL-1 may be restricted by dose-limiting hypotension. Purpose: The purpose of this study was to investigate the role of nitric oxide (NO) as a possible mediator of this hypotension. Methods : Cytokine-treated rat aortic smooth muscle cells were assayed for nitrite production, a stable breakdown product of nitric oxide. Nitric oxide synthase from smooth muscle cells was partially characterized in cytosol preparations using a novel Fe 2+ -myoglobin method to test for nitric oxide production. To determine the role of NO on the immunorestorative and antineoplastic activity of IL-1, N ω -amino-L-arginine (NAA) or N ω -monomethyl-L-arginine (NMA), inhibitors of nitric oxide synthase, were added to either cultures of IL-1-dependent T cells or A375 melanoma cells exposed to IL-1. To investigate the effects of NAA in vivo, pentobarbital anesthetized dogs, which were made hypotensive by administration of IL-1, received a single intravenous bolus dose of NAA. The effects of NAA were then reversed by the administration of L-arginine. Results: Our results show that cultured IL-1-activated rat aortic smooth muscle cells synthesize nitric oxide, a potent vasodilator. Induction of nitric oxide synthase is augmented by interferon-7 and blocked by IL-1 receptor antagonist and by inhibitors of RNA or protein synthesis. Nitric oxide synthesis by IL-1-activated smooth muscle cells is inhibited by NAA, NMA, and N ω -nitro-L-arginine (NNA) with ED 50 (i.e., effective dose for 50% inhibition) values of 20, 60, and 1000 μM, respectively; this rank order of inhibition is characteristic of an agonist-unregulated, inducible isoform of nitric oxide synthase. In smooth muscle cells, inhibition of NO synthesis by NAA is reversed by excess L-arginine. Consistent with the induction of unregulated NO- synthesis in vascular smooth muscle in vivo, administration of IL-1 (50 μg/kg) to dogs caused a 33.5% decrease in systemic vascular resistance and a 28% decrease in blood pressure within 3 hours. Subsequent administration of NAA (20 mg/ kg) rapidly and completely reversed the hypotension and increased systemic vascular resistance; these effects of NAA were reversed by L-arginine. Neither the immunoproliferative nor the tumoricidal activity of IL-1 was diminished by NAA. Conclusions: Our results indicate that (a) vascular smooth muscle is a likely source as well as a target of IL-1-induced NO- synthesis, causing vasodilatation and hypotension, (b) nitric oxide synthase inhibitors can fully reverse this hypotension, and (c) the therapeutically useful properties of IL-1 are not diminished by nitric oxide synthase inhibitors. Implications: Administration of inhibitors of nitric oxide synthase can reverse the pathological cardiovascular effects of IL-1 at concentrations that do not interfere with the potentially useful immunoproliferative or tumoricidal effects of this cytokine. In the context of the current clicinal trials of IL-1, this finding would represent a very significant advantage. [J Natl Cancer Inst 84:1008–1016, 1992]

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