AJP - Cell Physiology, Vol 252, Issue 1 C68-C76, Copyright © 1987 by American Physiological Society
High cytosolic pH inhibits Ca uptake by Myxicola axon mitochondria
R. F. Abercrombie and K. Gammeltoft
Microliter samples of cytoplasm containing mitochondria were aspirated from
giant axons of the marine annelid Myxicola infundibulum into polyethylene
tubes. The small molecular constituents within these cytoplasmic samples
were controlled by a dialysis capillary with a 6,000 molecular weight cut
off. The negative log of the calcium ion activity (pCa) (6.72 +/- 0.03, n =
40) and, in some cases, the pH (7.51 +/- 0.01, n = 7) of the samples were
monitored with ion-sensitive microelectrodes. Adding 5 mM succinate or 5 mM
ATP at pH 7.5 caused the Ca activity in the cytoplasm to drop from an
experimentally elevated value of approximately 10 microM to below 1 microM.
This decrease could be inhibited with ruthenium red, suggesting a
mitochondrial mechanism. Ca uptake, following the addition of either
succinate or ATP, was reversibly slowed when the cytoplasmic pH was
elevated to approximately 8.3. When ruthenium red was added after
mitochondria had taken up Ca, the Ca activity in the extramitochondrial
cytoplasm gradually increased suggesting ongoing release of Ca from storage
sites. Increasing the cytoplasmic pH to approximately 8.5 in the presence
of ruthenium red did not increase the ongoing release over that found with
ruthenium red alone. The apparent washout of Ca from the
energy-independent, nonmitochondrial Ca buffers was only slightly affected
by pH (pH 7.5-8.5). It is concluded that elevating intracellular pH to 8.3
slows the Ca uptake by mitochondria. Thus cytoplasmic pH may have a
function in regulating mitochondrial Ca metabolism and/or
extramitochondrial calcium activity.
