A caffeine-sensitive Ca2+ store modulates K+-evoked secretion in chromaffin cells

Area of Science:

• Neuroscience
• Cell Biology
• Pharmacology

Background:

• Catecholamine release from adrenal chromaffin cells is crucial for physiological responses.
• The role of caffeine in modulating this release and its underlying mechanisms, particularly calcium dynamics, requires further elucidation.

Purpose of the Study:

• To investigate the effect of caffeine on catecholamine secretion from bovine adrenal medulla chromaffin cells.
• To determine whether caffeine inhibits secretion by blocking calcium channels or by affecting intracellular calcium handling.

Main Methods:

• On-line monitoring of catecholamine release using an electrochemical detector.
• Superfusion of cells with Krebs-N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid solution.
• Application of depolarizing pulses (70 mM K+/2 mM Ca2+) in the presence and absence of caffeine.
• Measurement of intracellular calcium signals using fura 2 in single cells.

Main Results:

• Caffeine (10 mM) progressively blocked K+-evoked catecholamine secretion, with blockade reaching 80% after repeated challenges.
• The inhibitory effect of caffeine was dependent on exposure time and recovery was gradual.
• Caffeine's effects on secretion did not correlate with changes in extracellular calcium entry.
• Secretion data aligned with the disappearance and recovery of intracellular calcium signals, suggesting an intracellular calcium store mechanism.

Conclusions:

• Caffeine depresses catecholamine secretion by impacting intracellular calcium stores, not by inhibiting calcium channel activity.
• The filling state of a caffeine-sensitive intracellular calcium store appears to regulate the extent of exocytosis.
• This finding challenges previous assumptions about caffeine's mechanism of action on chromaffin cell secretion.