Internal pH of isolated chromaffin vesicles

Area of Science:

• Biochemistry
• Cell Biology
• Neuroscience

Background:

• Chromaffin vesicles store and release catecholamines.
• Understanding vesicle membrane properties is key to catecholamine regulation.

Purpose of the Study:

• To measure the passive permeability of chromaffin vesicles to H+.
• To determine the internal pH (pH) of chromaffin vesicles.
• To investigate the factors affecting the pH gradient across the vesicle membrane.

Main Methods:

• Potentiometric measurements of ion fluxes (K+, H+).
• Use of ionophores and uncouplers to assess membrane permeability.
• [14C]methylamine distribution assay for measuring pH gradient (ΔpH).

Main Results:

• Chromaffin granule membranes show high impermeability to protons (H+) and potassium (K+).
• The intravesicular space was found to be acidic (pH ≈ 5.5) at an external pH of 6.85, with a ΔpH of 1.16.
• The pH gradient was modulated by ionophores and uncouplers, and was independent of external ionic conditions, ruling out Donnan equilibrium effects.

Conclusions:

• The proton impermeability and acidic intravesicular pH are significant for regulating catecholamine uptake and storage.
• These properties are essential for maintaining the acidic environment required for efficient catecholamine packaging within chromaffin granules.