Norepinephrine stimulation of G proteins results in an increased intracellular concentration of calcium. This increase is not reversed by the extracellular calcium chelator EGTA. Which of the following is the most likely source of the increased intracellular calcium concentration?
Option Endoplasmic reticulum
Golgi complex
Mitochondria
Nucleus
Plasma membrane
Correct answer
Option Endoplasmic reticulum
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correct answer: A
To answer this question, you must know which organelles are sources of calcium available for use WITHIN the cell.
The main sources of calcium in eukaryotic cells are the Smooth Endoplasmic Reticulum (A, SER) and the mitochondria ©.
Release of intracellular calcium stores upon G protein activation follows a well established pathway: the beta-gamma subunit of a G- protein is cleaved and proceeds to activate Phospholipase C (PLC). PLC, in turn, cleaves the membrane bound phospholipid [phosphatidylinositol 4,5-bisphosphate] (PIP2) molecule into 2 components: Diacylglycerol (DAG) and Inositoltriphosphate (IP3).
IP3 then binds to IP3 receptors present on the surface of the SER, which opens calcium channels and releases Ca2++ into the cytosol. Calcium release by the mitochondria is not the direct target of the G-protein mediated calcium release.
The other answer choices are mere distractors and are NOT sources of calcium within animal cells.