Regulation of tetrahydrobiopterin and monoamine transport by cyclic GMP

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We found that the vesicular monoamine transport activity of neuronal cell line, PC12, was stimulated by cyclic GMP (cGMP) while it was inhibited by cyclic AMP (cAMP). We also found that both cGMP and cAMP down-regulated the transport of tetrahydrobiopterin (BH4), a cofactor for nitric oxide synthase and for aromatic amino acid hydroxylases, through plasma membranes of various cells. In this works, we examined the mechanisms of BH4 transport and the effects of cGMP on the BH4 transport and on the vesicular monoamine transport.1.BH4 transport : Outward flow of endogenous BH4 to extracellular fluid and inward flow of exogenous BH4 into the cells were mediated by different transport systems. Three pathways were found to be present for BH4 outward flow : 1)a pathway by transporter belonged to ion gradient-dependent toxin-extruding antiporter (TEXAN) family. This type of BH4 transporter was down-regulated by cGMP. 2)a pathway which was sensitive to osmotic pressure and involved a membrane protein belonged to aquaporin family for BH4 transport. This pathway was therefore regulated by Na+-K+-ATPase and ion transporters for K+ and Cl- ions. 3)a pathway mediated by a member of ABC transporter family such as multi-drug resistant protein (MDR).On the other hand, BH4 transporter which is responsible for exogenous BH4 uptake into cytoplasm was specifically present in intestinal epithelial cells and in kidney. This type of BH4 transport activity was also detected in erythrocytes.2.Vesicular monoamine transport : Vesicular monoamine transport was up-regulated by cGMP in a dose-dependent manner. Phosphorylation of the vesicular monoamine transporter (VMAT) molecules by cGMP was not detected. cGMP seemed to affect the targeting of VMAT molecules to the vesicular membranes. Studies is in progress to search proteins which might be involved in the VMAT targeting to the secretory vesicle membranes as a mediater of cGMP effect.