trans

244 INTRACELLULAR SIGNALING
FIGURE 11.1 (See also Color Plate 4) Regulation of intracellular free Ca 2 concentrations during the signal
process. Cells utilize redundant energy-dependent processes to store and release Ca 2 during the signal process. In
various protozoan parasites Ca 2 sequestration has been detected in the acidocalcisome (Ac), mitochondrion (Mt)
and endoplasmic reticulum (ER). Whether the nuclear envelope stores Ca 2 in protozoan parasites as occurs in
mammalian cells is not known. When a Ca 2 signal is terminated (Ca 2 OFF) the mitochondrial electrochemical
gradient () is used to sequester Ca 2 in the matrix space. Additionally, energy-dependent P-ATPase pumps
transport Ca 2 out of the cytoplasm. Vacuolar type Ca 2 -ATPase pumps (PMCA) have been identified in association
with acidocalcisomes and the plasma membrane. SERCA type pumps of the ER have also been identified. During
the signal process (Ca 2 ON), InsP 3 -dependent Ca 2 release from the ER and acidocalcisome has been reported
from some organisms, but not others. Ca 2 influx across the plasma membrane can be initiated with arachidonic
acid (AA) in some organisms.
Ca 2 and its gradual release may also extend
the Ca 2 signal, as has been proposed for
mammalian cells. In mammalian cells, Ca 2
within mitochondria stimulates a variety of
dehydrogenases and primes the mitochondria
to increase their energy output. However, Ca 2 -
dependent dehydrogenases have not yet been
identified in kinetoplastid or apicomplexan
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA