Caffeine stimulates Ca2+ entry through store-operated channels to activate tyrosine hydroxylase in bovine chromaffin cells

The ability of caffeine-induced store Ca2+ mobilization to activate tyrosine hydroxylase was studied in bovine adrenal chromaffin cells. Caffeine increased tyrosine hydroxylase activity over 10 min with an EC50 of 3 mm and maximum effect at 20 mm. The maximum response to caffeine was substantial, being almost one third that of the strongest agonists acetylcholine and PACAP-27, about half that for K+ and similar to that for histamine. In contrast, catecholamine secretion evoked by caffeine was small, being less than 10% of the response to strong agonists. Caffeine-induced tyrosine hydroxylase activation was not mimicked or prevented by phosphodiesterase inhibition with isobutylmethylxanthine, nor was it mimicked by an equimolar concentration of sucrose. However, the effect of caffeine was prevented by depleting intracellular Ca2+ stores by thapsigargin pretreatment, and reduced substantially by removing extracellular Ca2+ , by blocking Ca2+ channels with Co2+ or Ni2+ , or by inhibiting store-operated channels with 2-aminoethyl diphenylborate. It was not affected by inhibiting Ca2+ entry through voltage-operated Ca2+ -channels or by tetrodotoxin. The effect of caffeine was mimicked by acute thapsigargin treatment or by depleting intracellular Ca2+ stores in Ca2+ -free buffer and then reintroducing extracellular Ca2+ . The results indicate that mobilizing store Ca2+ with caffeine is a very effective mechanism for activating tyrosine hydroxylase and that the majority of this response depends on extracellular Ca2+ entry through store-operated channels. They also suggest that extracellular Ca2+ entry through such channels regulates cellular responses differently to Ca2+ entry through voltage-operated Ca2+ channels.