Modulation of gastrin processing by vesicular monoamine transporter type 1(VMAT1) in rat gastrin cells

1. Gastrointestinal endocrine cells produce biogenic amines which are trans ported into secretory vesicles by one of two proton-amine exchangers, vesic ular monoamine transporters type 1 and 2 (VMAT1 and 2). We report here the presence of VMAT1 in rat gastrin (G) cells and the relevance of VMAT1 funct ion for the modulation of progastrin processing by biogenic and dietary ami nes. 2. In immunocytochemical studies VMAT1, but not VMAT2, was localized to sub populations of G cells and enterochromaffin (EC) cells; neither was found i n antral D cells. The expression of VMAT1 in antral mucosa was confirmed by Northern blot analysis, which revealed an mRNA band of approximately 3.2 k b, and by Western blot analysis, which revealed a major protein of 55 kDa. 3. In pulse-chase labelling experiments, the conversion of the amidated gas trin G34 to G17 was inhibited by biogenic amine precursors (L-DOPA and 5-hy droxytryptophan). This inhibition was stereospecific and sensitive to reser pine (50 nM), which blocks VMAT1 and VMAT2, but resistant to tetrabenazine, which is a selective inhibitor of VMAT2. 4. Dietary amines such as tyramine and tryptamine also inhibited G34 cleava ge. This effect was associated with a loss of the electron-dense core of G cell secretory vesicles. It was not stereospecific or reserpine sensitive, but was correlated with hydrophobicity. 5. Thus rat antral G cells can express VMAT1; transport of biogenic amines into secretory vesicles by VMAT1. is associated with inhibition of G34 clea vage, perhaps by raising intravesicular pH. Dietary amines also modulate cl eavage of progastrin-derived peptides, but do so by a VMAT1-independent mec hanism; they may act as weak bases that passively permeate secretory vesicl e membranes and raise intravesicular pH.