Metabolic and secretory response of parotid cells to cationic amino acids. Uptake and catabolism of L-arginine and L-ornithine

Abstract

L-Arginine and L-ornithine, which stimulate amylase release, are taken up by rat parotid cells. L-Arginine is converted, in an NADPH-dependent manner and to a limited extent to L-citrulline in parotid cell homogenates, despite the absence of ornithine transcarbamylase activity. L-Arginine is largely converted to urea and L-ornithine. The generation of putrescine and polyamines from L-ornithine occurs at a very low rate, relative to the cell content in preformed amines. The major fate of exogenous or arginine-derived ornithine consists in its conversion to L-glutamate, which is then further metabolized. These findings raise several hypotheses for the secretory response of the parotid cells to cationic amino acids, including their accumulation as positively charged molecules inside the cell and the generation of either NO, amines, substrates for a transglutaminase-catalyzed reaction, or ATP through oxidative catabolism. However, each of these hypotheses meets with objections, the modality for the stimulation of amylase release by cationic amino acids being eventually considered as an unsettled matter.