Acetylcholine and potassium elicit different patterns of exocytosis in chromaffin cells when the intracellular calcium handling is disturbed.

Abstract

Listen

Translate article

During fast superfusion of bovine chromaffin cells with normal Krebs-HEPES solution containing 2 mM Ca2+, pulses of 100 microM ACh or 100 mM K+ of increasing duration (1-5 s) caused similar exocytosis of about 3-4 microC catecholamine. Depletion of endoplasmic reticulum (ER) Ca2+ by pretreatment with 1 microM thapsigargin, 10 mM caffeine and 10 microM ryanodine more than halved the responses to ACh but did not affect the responses to K+ responses. In these ER Ca2+-depleted cells the protonophore carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) (20 microM given during the 5 s preceding each pulse) augmented the responses to ACh responses fourfold for all pulse durations applied (1-5 s) whereas responses to K+ were potentiated twofold with 1 to 2 s pulses but were not affected with longer pulse durations. ACh pulses applied to fura-2-loaded cells evoked increases of bulk cytosolic [Ca2+] ([Ca2+](c)) that were substantially smaller than those elicited by K+ pulses. Confocal microscopy of fluo-3-loaded cells showed that ACh pulses elicited discrete and more localized [Ca2+](c) elevations, whereas K+ pulses produced higher [Ca2+](c) transients that spread out quickly throughout the cytosol. These results suggest that mitochondria sense the increase of local [Ca2+](c) elicited by ACh (that evokes firing of action potentials) much better than that induced by K+ (that produces sustained cell depolarisation). This implies that mitochondria are more sensitive to the local [Ca2+](c) changes resulting from the physiological triggering of action potentials by ACh.