Processing and secretion in the neurohypophysis. Stability of isolated secretory vesicles and role of internal pH.

A possible role of low pH in secretory vesicles for processing and secretion in the neurohypophysis was investigated. Subcellular fractionation of guinea-pig neural lobes revealed that a proton present in the membranes from this tissue could not be ascribed to secretory vesicles. However, a proton pump was found in coated microvesicles. Secretory vesicles isolated from rats and guinea pigs were stable under conditions known to lyse secretory vesicles from the adrenal medulla owing to the generation of a proton gradient. These results suggest that the internal pH of secretory vesicles from the neurohypophysis is closer to neutral than is the pH in chromaffin secretory vesicles. Processing of a neurophysin-glycopeptide intermediate from the biosynthesis of vasopressin in intact secretory vesicles incubated in vitro was activated by the addition of NH4Cl, known to increase the intravesicular pH. This activation of neurohormone processing was also apparent in isolated nerve endings incubated in the presence of NH4Cl, suggesting that NH4Cl can also be used to increase the intravesicular pH in intact nerve endings. However, NH4Cl did not affect the secretion of neurohormones, indicating that a low intravesicular pH is not important for exocytosis in the neurohypophysis. Our results indicate that a low pH generated during processing by mechanisms other than ATP-dependent proton transport may inhibit the processing enzymes, thereby preventing extensive breakdown of neurohormone precursors.