Granules as Calcium Stores

Abstract

Dense core granules of exocrine cells are a potential source of calcium, and results to date have been controversial regarding whether the calcium in these organelles is involved in controlling the cytosolic calcium concentration. Mitchell et al. developed a fusion protein between the dense core transmembrane protein VAMP and the calcium-sensitive bioluminescent protein aequorin in order to visualize calcium in the dense core granules of a pancreatic β cell line. Vesicular free [Ca 2+ ] was approximately 50 μM, significantly lower than endoplasmic reticulum (ER) or Golgi free [Ca 2+ ]. Vesicular [Ca 2+ ] increased when calcium was reintroduced to the cells following calcium depletion. Uptake required adenosine triphosphate (ATP) and was inhibited by high concentrations of orthovanadate, but not by the ER Ca 2+ ATPase inhibitor thapsigargin or by agents that altered intravesicular pH or elevations in Na + concentration, suggesting that the uptake does not occur through a Ca 2+ /H + exchanger or through a Na + /Ca 2+ exchanger. Activation of inositol trisphosphate receptors did not produce a decrease in vesicular [Ca 2+ ], but did decrease ER [Ca 2+ ]. Activation of the ryanodine receptor (RyR) did stimulate a decrease in both ER and vesicular [Ca 2+ ]. Under conditions in which the ER calcium pool was depleted, stimulation of the RyRs produced an increase in cytosolic [Ca 2+ ] presumably from the vesicular calcium pool. Treatment of the cells with glucose or nutrients that stimulate secretion in these cells by activation of voltage-gated calcium channels also increased ER and vesicular [Ca 2+ ]. Thus, the dense core vesicles do represent a dynamic pool of calcium that is affected by cytosolic [Ca 2+ ] and that can be mobilized by activation of RyRs. K. J. Mitchell, P. Pinton, A. Varadi, C. Tacchetti, E. K. Ainscow, T. Pozzan, R. Rizzuto, G. A. Rutter, Dense core secretory vesicles revealed as a dynamic Ca 2+ store in neuroendocrine cells with a vesicle-associated membrane protein aequorin chimaera. J. Cell Biol. 155 , 41-51 (2001). [Abstract] [Full Text]