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How far does phospholipase C activity depend on the cell calcium concentration? A study in intact cells
The Biochemical Journal
|April 15, 1987
Summary
Phospholipase C (PLC) activity in liver cells is minimally affected by high cytosolic calcium (Ca2+) but partially dependent on it at lower concentrations. Vasopressin stimulates PLC, but InsP3 formation is not solely driven by increased Ca2+.
Area of Science:
- Biochemistry
- Cellular Biology
- Hormone Signaling
Background:
- Phospholipase C (PLC) plays a crucial role in cellular signaling pathways.
- Cytosolic calcium concentration ([Ca2+]i) is a key regulator of many cellular processes.
- Vasopressin is a hormone known to mobilize intracellular calcium and activate PLC.
Purpose of the Study:
- To investigate the dependence of phospholipase C activity on cytosolic Ca2+ concentration in intact liver cells.
- To determine the role of [Ca2+]i in vasopressin-mediated inositol trisphosphate (InsP3) formation.
- To elucidate the relationship between [Ca2+]i, PLC activity, and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) levels.
Main Methods:
- Studied intact liver cells (hepatocytes) under varying [Ca2+]i conditions.
- Clamped [Ca2+]i using quin2 loading and ionomycin.
- Measured InsP3 formation and PtdInsP2 degradation as indicators of PLC activity.
- Administered vasopressin to stimulate PLC and observed cellular responses.
Main Results:
- PLC activity is largely independent of [Ca2+]i above 193 nM but shows partial dependence at concentrations below this threshold.
- Vasopressin stimulation of InsP3 formation is partially reduced at low [Ca2+]i, indicating Ca2+ influences maximal response.
- At low [Ca2+]i, vasopressin increased basal PtdInsP2 levels, suggesting a potential stimulation of PtdInsP2 synthesis.
- InsP3 formation is not solely a consequence of increased [Ca2+]i, as vasopressin increased InsP3 even when [Ca2+]i was clamped.
Conclusions:
- In liver cells, phospholipase C activity is primarily regulated by factors other than cytosolic Ca2+ concentration, especially at physiological levels.
- While Ca2+ influences the maximal response, vasopressin-mediated InsP3 production is not strictly dependent on a rise in [Ca2+]i.
- Vasopressin may also modulate PtdInsP2 synthesis, a pathway that could become more significant under conditions of low intracellular Ca2+.