Abstract:
Numerous investigations have demonstrated large increases in
y-amino butyrate (GABA) levels in response to a variety of
stresses such as touch or cold shock (Wallace et ale 1984)
Circumstantial evidence indicating a role of Ca2
+ in these
increases includes elevated Ca2+ levels in response to touch and
cold shock (Knight et ale 1991), and the demonstration of a
calmodulin binding domain on glutamate decarboxylase (GAD), the
enzyme responsible for GABA synthesis (Baum et al 1993) In the
present study the possible role of Ca2+ and calmodulin in
stimulation of GAD and subsequent GABA accumulation was examined
using asparagus mesophyll cells. Images of cells loaded with the
Ca2+ indicator Fluo-3 revealed a rapid and transient increase in
cytosolic Ca2+ in response to cold shock. GABA levels increased
by 106% within 15 min. of cold shock. This increase was inhibited
70% by the calmodulin antagonist W7, and 42% by the Ca2+ channel
blocker La3+.. Artificial elevation of intracellular Ca2+ by the
Ca2+ionophore A23187 resulted in an 61% increase in GABA levels.
Stimulation of GABA synthesis by ABA resulted in an 83% increase
in GABA levels which was inhibited 55% by W7. These results
support the hypothesis that cold shock stimulates Ca2+ entry into
the cytosol of the cells which results in Ca2+/calmodulin
mediated activation of GAD and consequent GABA synthesis.
