COLD-SHOCK-STIMULATED GAMMA-AMINOBUTYRIC-ACID SYNTHESIS IS MEDIATED BY AN INCREASE IN CYTOSOLIC CA2+, NOT BY AN INCREASE IN CYTOSOLIC H+

Synthesis of the nonprotein amino acid gamma-aminobutyric acid is stim ulated within minutes by diverse environmental factors. Synthesis (L-G lu + H+ --> gamma-aminobutyric acid + CO2) is catalysed by L-Glu decar boxylase, a cytosolic enzyme having an acidic pH optimum. This study u ses isolated Asparagus sprengeri (Regel) mesophyll cells to investigat e the possible role of Ca2+ in stimulated gamma-aminobutyric acid synt hesis. Abrupt cold shock (20 degrees C to 1 degrees C) stimulated gamm a-aminobutyric acid levels from 2.7 to 5.6 nmol/10(6) cells within 15 min. This 100% increase was reduced to 28% in the presence of the Ca2 channel blocker lanthanum, and was significantly reduced by incubatio n with 1 mM of the calmodulin antagonist (6-aminohexyl)-5-chloro-1-nap hthalene-sulfonamide. Incubation at 20 degrees C with 25 mu M calcimyc in, a Ca2+ ionophore, increased levels by 61% within 15 min. A fluores cent Ca2+ indicator demonstrated that cytosolic Ca2+ increased within 2 s of cold shock, followed by a return to initial levels within 25 s. In contrast, comparable experiments indicate a rapid and prolonged de crease in cytosolic H+. L-Glu decarboxylase isolated from asparagus cl adophylls was stimulated 100% by addition of 500 mu M Ca2+ and 200 nM calmodulin. This activity was reduced to control values by the calmodu lin antagonist. Collectively, the data indicate that cold shock initia tes a signal transduction pathway in which increased cytosolic Ca2+ st imulates calmodulin-dependent L-Glu decarboxylase activity and gamma-a minobutyric acid synthesis. This mechanism appears independent of incr eased H+.