BOVINE ADRENAL CHROMAFFIN CELLS CONTAIN AN INOSITOL 1,4,5-TRISPHOSPHATE-INSENSITIVE BUT CAFFEINE-SENSITIVE CA2+ STORE THAT CAN BE REGULATED BY INTRALUMINAL FREE CA2+

We have characterized some propertiess of the caffeine-sensitive Ca2+ store in bovine chromaffin cells. Addition of 10 mM-caffeine to permeabilized cells that were allowed to sequester Ca2+ in the presence of the precipitating anion pyrophosphate induced a transient rise in free Ca2+ concentration that was blocked by 10-mu-M-Ruthenium Red. Caffeine was able to release Ca2+ after the InsP3-sensitive Ca2+ pool had been completely emptied, and 10 mu-M-InsP3 still released Ca2+ in the presence of a high dose (50 mM) of caffeine, indicating that there are selectively sensitive Ca2+ pools in these cells. The progressive hydrolysis of pyrophosphate by a cytosolic pyrophosphatase induced a spontaneous Ca2+ release after a latency. Caffeine prevented this spontaneous Ca2+ release, indicating that the pyrophosphate-sensitive Ca2+ pool was caffeine-sensitive. On varying the free Ca2+ concentration within the caffeine-sensitive pool (by using methylenediphosphonic acid, pyrophosphate or no precipitating anion), we observed that the Ca2+-releasing effect of caffeine was dependent on an elevated intraluminal free Ca2+ concentration. In conclusion, the caffeine-sensitive Ca2+ store in bovine chromaffin cells is largely distinct from the InsP3-sensitive Ca2+ store, and its release mechanism shares characteristics with the ryanodine receptor of muscle cells.