Dominant Role of Mitochondria in Clearance of Large Ca2+ Loads from Rat Adrenal Chromaffin Cells

Abstract

Cytosolic Ca2+ (Ca2+c) clearance from adrenal chromaffin cells was studied by whole-cell patch clamp and indo-1 Ca2+ photometry after influx of Ca2+ through voltage-dependent Ca2+ channels. We isolated the rates of Ca2+c clearance by several mechanisms using combinations of the following agents (with their expected targets): Li+ or TEA substituted for Na+ (Na(+)-Ca2+ exchange), 1 mM La3+ applied after the depolarization (Na(+)-Ca2+ exchange and plasma membrane Ca(2+)-ATPase), 1 microM thapsigargin (pumping into reticular stores), and 2 microM carbonyl cyanide m-chlorophenylhydrazone (uptake into mitochondria). Remarkably, whenever [Ca2+]c rose above approximately 500 nM, Ca2+c clearance by mitochondria exceeded clearance by either Na(+)-Ca2+ exchange or the Ca2+ pumps of the plasma and reticular membranes. As [Ca2+]c fell again, Ca2+ reemerged from mitochondria, prolonging the final return to basal levels.

Mitochondrial involvement in neurodegenerative diseases

“…From the classical viewpoint, adequate levels of ATP are essential to maintain the neuronal plasma membrane potential via the sodium-potassium ATPase, which consumes about 40% of the energy (1). In addition, mitochondria are an important intracellular Ca 2þ sequestration system (2,3), and especially synaptic mitochondria are indispensible for neurotransmitter reserve pool mobilization in the presynaptic compartment (4). Due to these important features, mitochondria can modulate neuronal excitability and synaptic transmission (5,6).…”

Mitochondria and chromaffin cell function

“…κ S values 1,000 have been reported for myocytes [58] and various neuronal types [1,23,46,54]. On the other hand, values of 40-100 have been reported for rat chromaffin cells [11,29] and bovine chromaffin cells [40,60,63].…”

“…Coupling to transport by the endoplasmic reticulum Ca 2+ entry through VOCCs in chromaffin cells has been studied with great detail [29,60,63]. Membrane depolarisation to 0 mV elicits Ca 2+ currents peaking near 800 pA that deactivate with a half-time of 300-500 ms.…”

“…2a). The calciumbinding capacity for Ca 2+ (κ S ) inside the mitochondrial matrix seems to be very high, in the 10 3 range [11,17,29,30]. In addition, it has been proposed recently that precipitation of Ca 3 (PO 4 ) 2 into the matrix con account for additional, almost unlimited, buffering [16,57].…”

“…It has been repeatedly reported that mitochondria can clear cytosolic Ca 2+ loads in rat [11,29,57] and bovine chromaffin cells [36,55,60]. We have studied the kinetics of uptake by mitochondria in permeabilized bovine chromaffin cells infected with mitochondria-targeted aequorin probes [36,55].…”