On the role of intravesicular calcium in the motion and exocytosis of secretory organelles

Figures & data

Figure 1 Mechanism used for Ca²⁺ (and catecholamines, CA) turnover in chromaffin secretory organelles. The relative sizes for the granule matrix (1) and the free compartment (2) have been change for clarity. The H⁺ are pumped towards the vesicle lumen by an ATP dependent (V-ATPase, 3). Protons maintain the pH and the potential gradients with the help of Cl⁻ channels which acts as counter ions (4) to keep the Ψ ≈ −80 mV. Catecholamines (5) and Ca²⁺ (6) use H⁺ as antiporters to be accumulated inside vesicles, both carriers can work also in the reverse mode. The IP₃ receptors (7) release Ca²⁺ as a response to intracellular IP₃ whereas CICR (8) amplifies the Ca²⁺ signaling by releasing Ca²⁺ a response that is modulated by ryanodine and caffeine. The SERCA (9), not described yet in chromaffin granules will be the Ca²⁺ pump; this pump is blocked by thapsigargin. In these studies the luminal terminal of VAMP (10) (synaptobrevin) has been modified to place a Ca²⁺ sensor (low Ca²⁺-affinity aequorine) or pH sensor (EGFP).