Tetrahydrobiopterin Synthesis in Rat Cardiac Myocytes; An Event Required for Cytokine-induced NO Generation

Abstract

Summary Cardiac myocytes are known to express the high-capacity inducible isoform of nitric oxide (NO) synthase (iNOS). Since tetrahydrobiopterin (BH4) is an essential cofactor for NO formation, we investigated whether BH4 synthesis is required for cytokine-induced NO production in cultutred rat cardiac myocytes. The total biopterin content of untreated cardiac myocytes was below our limit of detection. However, treatment with inter-leukin-1α. and interferon-γ (IL-1/IFN) caused a significant rise in biopterin levels and induced NO synthesis. iNOS mRNA and GTP cyclohydrolase I (GTPCH) mRNA were induced by IL-1/IFN in parallel. 2,4-Diamino-6-hydroxypyridine (DAHP), a selective inhibitor of GTPCH, inhibited both the increase in cellular levels of BH4 as well as the concomitant formation of NO caused by IL-1/IFN. This inhibition by DAHP was reversed by coaddition of sepiapterin which is a substrate for BH4 synthesis. Thus BH4 synthesis is an absolute requirement for induction of NO synthesis by cytokines in cardiac myocytes.