Na+/Ca2+ exchange in coated microvesicles

Coated microvesicles isolated from bovine neurohypophyses could be
loaded with Ca2+ in two different ways, either by incubation in the
presence of ATP or by imposition of an outwardly directed Na+
gradient. Na+, but not K+, was able to release Ca2+ accumulated by
the coated microvesicles. These results suggest the existence of an
ATP-dependent Ca2+-transport system as well as of a Na+/Ca2+
carrier in the membrane of coated microvesicles similar to that
present in the membranes of secretory vesicles from the
neurohypophysis. A kinetic analysis of transport indicates that the
apparent Km for free Ca2+ of the ATP-dependent uptake was 0.8
microM. The average Vmax. was 2 nmol of Ca2+/5 min per mg of
protein. The total capacity of microvesicles for Ca2+ uptake was
3.7 nmol/mg of protein. Both nifedipine (10 microM) and NH4Cl (50
mM) inhibited Ca2+ uptake. The ATPase activity in purified
coated-microvesicles fractions from brain and neurohypophysis was
characterized. Micromolar concentrations of Ca2+ in the presence of
millimolar concentrations of Mg2+ did not change enzyme activity.
Ionophores increasing the proton permeability across membranes
activated the ATPase activity in preparations of coated
microvesicles from brain as well as from the neurohypophysis. Thus
the enzyme exhibits properties of a proton-transporting ATPase.
This enzyme seems to be linked to the ion accumulation by coated
microvesicles, although the precise coupling of the proton
transport to Ca2+ and Na+ fluxes remains to be determined.