Synaptotagmin I increases the probability of vesicle fusion at low [Ca2+] in pituitary cells |
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Authors: | Kreft M Kuster V Grilc S Rupnik M Milisav I Zorec R |
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Institution: | Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Medical Faculty, 1000 Ljubljana, Slovenia. |
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Abstract: | Synaptotagmin I (Syt I),a low-affinity Ca2+-binding protein, is thought to serve asthe Ca2+ sensor in the release of neurotransmitter.However, functional studies on the calyx of Held synapse revealed thatthe rapid release of neurotransmitter requires only approximatelymicromolar Ca2+], suggesting that Syt I may play a morecomplex role in determining the high-affinity Ca2+dependence of exocytosis. Here we tested this hypothesis by studying pituitary cells, which possess high- and low-affinityCa2+-dependent exocytic pathways and express Syt I. Usingpatch-clamp capacitance measurements to monitor secretion and the acuteantisense deletion of Syt I from differentiated cells, we have shownthat the rapid and the most Ca2+-sensitive pathway ofexocytosis in rat melanotrophs requires Syt I. Furthermore, stimulationof the Ca2+-dependent exocytosis by cytosol dialysis withsolutions containing 1 µM Ca2+] was completelyabolished in the absence of Syt I. Similar results were obtained by thepreinjection of antibodies against the CAPS (Ca2+-dependentactivator protein for secretion) protein. These results indicate thatsynaptotagmin I and CAPS proteins increase the probability of vesiclefusion at low cytosolic Ca2+]. |
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