Rapid and parallel secretion of lysosomal beta-N-acetylglucosaminidase and preloaded fluorescein-labelled dextran was initiated in macrophages by agents affecting intracellular pH (methylamine, chlorpromazine, and the ionophores monensin and nigericin). In order to evaluate the relative role of changes in lysosomal and cytosolic pH, these parameters were monitored by using pH-sensitive fluorescent probes [fluorescein-labelled dextran or 2′,7′-bis(carboxyethyl)-5(6)-carboxyfluorescein]. All agents except chlorpromazine caused large increases in lysosomal pH under conditions where they induced secretion. By varying extracellular pH and ion composition, the changes in lysosomal and cytosolic pH could be dissociated. Secretion was then found to be significantly modulated by changes in cytosolic pH, being enhanced by alkalinization and severely inhibited by cytosolic acidification. However, changes in cytosolic pH in the absence of stimulus were unable to initiate secretion. Dissociation of the effects on lysosomal and cytosolic pH was also achieved by combining stimuli with either nigericin or acetate. Further support for a role of intracellular pH in the control of lysosomal enzyme secretion was provided by experiments where bicarbonate was included in the medium. The present study demonstrates that an increase in lysosomal pH is sufficient to initiate lysosomal enzyme secretion in macrophages and provides evidence for a significant regulatory role of cytosolic pH.

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