Measurement of Calcium Buffering by Intracellular Organelles in Brain

Abstract

Cells maintain low concentrations of intracellular free calcium ([Ca²⁺]_i) by the effective operation of Ca²⁺ pumps located in plasma membrane as well as intracellular organelles, such as mitochondria and endoplasmic reticulum (microsomes) (1,2). Under normal conditions, Ca²⁺ enters the cell by diffusion down an electrochemical gradient through voltage-dependent or receptor-mediated Ca²⁺-sensitive channels (2). Calcium can also be released from intra-cellular stores such as endoplasmic reticulum and mitochondria. As cytosolic free Ca²⁺ increases, Ca²⁺-binding proteins, mitochondria, and microsomes initially sequester the Ca²⁺ from cytosol. However, if there is a sustained influx of Ca²⁺, low cytoplasmic Ca²⁺ level is maintained by active extrusion through plasma membrane Ca²⁺-ATPase and by the Na⁺/Ca²⁺ exchanger (1,2). Mitochondria and microsomes differ in the mechanisms by which they sequester cytoplasmic Ca²⁺. Microsomal Ca²⁺-sequestration is an active process involving ATP hydrolysis by Ca²⁺-ATPase. On the other hand, mitochondrial Ca²⁺-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.