Regulation of chloride transport in parotid secretory granules by membrane fluidity

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Abstract
Zymogen granule membranes contain Cl- conductance and Cl/anion exchange activities that become important for primary fluid production after fusion with the apical plasma membrane of the acinar cell. We have used steady-state fluorescence anisotropy of diphenylhexatriene derivatives and measurements of Cl- transport in isolated secretory granules to determine the contribution of membrane fluidity to the regulation of transport across the granule membrane. Secretory granules from several unstimulated glands (rat pancreas and parotid, rabbit gastric glands) were shown to have low membrane fluidity compared to plasma membranes. In addition, Cl- transport activity in different granule preparations showed a strong correlation to the membrane fluidity when measured with 1-[4-trimethylammonio)-phenyl]-6-phenyl-1,3,-hexatriene p-toluenesulfonate (TMA-DPH), but not with 3-[p-(6-phenyl)-1,3,5-hexatrienyl)-phenyl]propoionic acid (PA-DPH). These data suggest that TMA-DPH preferentially partitions into a specific lipid environment associated with, or which exerts an influence on, the Cl- transport proteins and that increases in the fluidity of this environment are associated with higher transport rates. Data from other types of plasma membranes indicate that TMA-DPH partitions much more than PA-DPH into the cytoplasmic leaflet, suggesting that this part of the granule membrane is involved in the observed fluidity changes. Furthermore, increasing the bulk membrane fluidity with the local anesthetics benzyl alcohol and n-alkanols increased the Cl- transport rates upto 10-fold. This increase was apparently through specific transporters as anion selectivity was maintained in spite of the higher absolute rates. Temperature also infuenced membrane fluidity and transport in a highly correlative manner. These observations are consistent with membrane acinar cells selectively regulating the fluidity of this environment.

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