Calcium entry, calcium redistribution, and exocytosis

Area of Science:

• Cellular Physiology
• Neuroendocrinology
• Calcium Signaling

Background:

• Cytosolic calcium concentration ([Ca(2+)](c)) in chromaffin cells is tightly regulated.
• Multiple mechanisms, including plasmalemmal Ca(2+) channels, endoplasmic reticulum (ER), and mitochondria, control [Ca(2+)](c).
• These regulatory systems influence catecholamine release, a key function of chromaffin cells.

Purpose of the Study:

• To investigate the intricate regulatory mechanisms governing cytosolic Ca(2+) concentration in chromaffin cells.
• To elucidate the roles of Ca(2+) channels, ER, and mitochondria in shaping [Ca(2+)](c) transients.
• To understand how these Ca(2+) dynamics impact catecholamine exocytosis.

Main Methods:

• Utilized targeted aequorin and confocal microscopy to monitor intracellular Ca(2+) levels.
• Investigated Ca(2+) dynamics under various stimulation conditions (ACh, high K(+), caffeine).
• Assessed the impact of mitochondrial function inhibition (protonophores) on Ca(2+) uptake and catecholamine release.

Main Results:

• Ca(2+) entry through channels refills the ER, triggering Ca(2+)-induced Ca(2+) release (CICR).
• The ER acts as a dynamic source or sink for Ca(2+), modulating catecholamine release.
• Mitochondria exhibit rapid, large Ca(2+) transients ([Ca(2+)](M)) upon stimulation.
• Physiological stimuli create localized [Ca(2+)](c) microdomains, driving CICR, mitochondrial uptake, and exocytosis.
• Inhibiting mitochondrial Ca(2+) uptake significantly enhances catecholamine release (3-5 fold).

Conclusions:

• Chromaffin cell Ca(2+) regulation is a multi-component process involving channels, ER, and mitochondria.
• Mitochondrial Ca(2+) uptake plays a critical role in modulating, rather than solely supporting, catecholamine release.
• Ca(2+) redistribution, potentially involving CICR and mitochondrial release, influences vesicle transport and exocytosis.