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Am J Physiol Cell Physiol 284: C547-C554, 2003. First published October 16, 2002; doi:10.1152/ajpcell.00333.2002
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Vol. 284, Issue 2, C547-C554, February 2003

Synaptotagmin I increases the probability of vesicle fusion at low [Ca2+] in pituitary cells

M. Kreft, V. Kuster, S. Grilc, M. Rupnik, I. Milisav, and R. Zorec

Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Medical Faculty, 1000 Ljubljana; and Celica Biomedical Science Center, 1000 Ljubljana, Slovenia

Synaptotagmin I (Syt I), a low-affinity Ca2+-binding protein, is thought to serve as the Ca2+ sensor in the release of neurotransmitter. However, functional studies on the calyx of Held synapse revealed that the rapid release of neurotransmitter requires only approximately micromolar [Ca2+], suggesting that Syt I may play a more complex role in determining the high-affinity Ca2+ dependence of exocytosis. Here we tested this hypothesis by studying pituitary cells, which possess high- and low-affinity Ca2+-dependent exocytic pathways and express Syt I. Using patch-clamp capacitance measurements to monitor secretion and the acute antisense deletion of Syt I from differentiated cells, we have shown that the rapid and the most Ca2+-sensitive pathway of exocytosis in rat melanotrophs requires Syt I. Furthermore, stimulation of the Ca2+-dependent exocytosis by cytosol dialysis with solutions containing 1 µM [Ca2+] was completely abolished in the absence of Syt I. Similar results were obtained by the preinjection of antibodies against the CAPS (Ca2+-dependent activator protein for secretion) protein. These results indicate that synaptotagmin I and CAPS proteins increase the probability of vesicle fusion at low cytosolic [Ca2+].

rat melanotrophs; exocytic module; membrane capacitance; calcium sensor


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