Internal pH of isolated newly formed and aged neurohypophysial granules.

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Abstract
The pH gradient (.DELTA.pH) across the membrane of bovine neurohypophyseal granules was estimated by using the [14C]methylamine partition technique. The granule membrane showed high permeability to sugars such as sucrose or sorbitol and to cations. Granules suspended in sucrose medium showed an acidic internal .DELTA.pH that decreased with external pH from 1.0 pH U at pH 7.5 to 0 at pH 4.5. High ionic strength of the external medium destroyed the .DELTA.pH, indicating that it originated from a Donnan equilibrium. In a medium constituted to simulate cytoplasm, the .DELTA.pH was .apprx. 0.5 pH U. Aged neurosecretory granules were more acidic inside than were newly formed granules. The results are discussed in relationship to the nature and the duration of maturation and degradation processes of the granule matrix.