title: The electrochemical H+ gradient of platelet secretory alpha-granules. Contribution of a H+ pump and a Donnan potential.
abstract: Experiments were performed to determine the internal pH and membrane potential of platelet alpha-granules. Fluorescence microscopy showed accumulation of weak bases, indicative of an acidic interior, inside secretory vesicles in intact platelets and in isolated alpha-granules. Weak base uptake was pH-dependent and NH+4-sensitive. In isolated alpha-granules suspended in medium buffered at pH 7.2, a delta pH, the difference between internal and external pH, of 1.2 (inside acidic) was measured by [14C]methylamine distribution. Uptake of isotopic or fluorescent amines was reduced by H+/cation exchange via ionophores and by addition of NH+4, but also by increasing the ionic strength suggesting that delta pH is partly due to a Donnan potential. Transmembrane potential measurements by fluorescent or radioactive ion distribution indicated that in the absence of ATP, granules are internally negative. When measured with 86Rb+, this potential could be entirely collapsed by increasing the ionic strength. Addition of ATP X Mg in the absence of permeating anions made the intragranular space more positive, as expected from inward electrogenic H+ pumping. The results are compatible with the coexistence of sealed and leaky subpopulations of alpha-granules. Internal acidity was generated in sealed granules in vivo by a H+-pumping ATPase, whereas in leaky granules acidity is a consequence of an internally negative Donnan potential.
mesh_terms: D000641:Ammonia / Q000494:pharmacology; D000818:Animals; D001792:Blood Platelets / Q000502:physiology*; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D003594:Cytoplasmic Granules / Q000187:drug effects / Q000502:physiology*; D007425:Intracellular Membranes / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D008856:Microscopy, Fluorescence; D009550:Nigericin / Q000494:pharmacology; D009994:Osmolar Concentration; D012413:Rubidium / Q000097:blood; D013552:Swine; D014634:Valinomycin / Q000494:pharmacology
chemical_list: D014634:Valinomycin; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D000641:Ammonia; D012413:Rubidium; D009550:Nigericin