Measurement of calcium buffering by intracellular organelles in brain

Cells maintain low concentrations of intracellular free calcium ([Ca(2+)](i)) by the effective operation of Ca(2+) pumps located in plasma membrane as well as intracellular organelles, such as mitochondria and endoplasmic reticulum (microsomes) (1,2). Under normal conditions, Ca(2+) enters the cell by diffusion down an electrochemical gradient through voltage-dependent or receptor-mediated Ca(2+)-sensitive channels (2). Calcium can also be released from intra-cellular stores such as endoplasmic reticulum and mitochondria. As cytosolic free Ca(2+) increases, Ca(2+)-binding proteins, mitochondria, and microsomes initially sequester the Ca(2+) from cytosol. However, if there is a sustained influx of Ca(2+), low cytoplasmic Ca(2+) level is maintained by active extrusion through plasma membrane Ca(2+)-ATPase and by the Na(+)/Ca(2+) exchanger (1,2). Mitochondria and microsomes differ in the mechanisms by which they sequester cytoplasmic Ca(2+). Microsomal Ca(2+)-sequestration is an active process involving ATP hydrolysis by Ca(2+)-ATPase. On the other hand, mitochondrial Ca(2+)-sequestration is an electrophoretic uniport process driven by the potential difference established across the mitochondrial inner membrane by an ATP-energized proton pump (1). These calcium-buffering processes within the neuron are illustrated in Fig. 1. The efficient operation of calcium sequestration and extrusion mechanisms within the cell is crucial for the maintenance of normal calcium homeostasis. Fig. 1. Different calcium-buffering processes involved in the maintenance of normal cellular Ca(2+) homeostasis. The intracellular free Ca(2+) ranges from 0.1-0.3 µM, in where as extracellular calcium is in millimolar concentrations. There is about 10,000-fold concentration gradient across the plasma membrane. This gradient is maintained by the effective operation of calcium pumps located in mitochondria, endoplasmic reticulum, and plasma membrane. All of these processes are energy-dependent and require the hydrolysis of adenosine triphosphate (ATP).