Ca2+-independent release

In neurons, the Ca2+-independent secretion is restricted to small synaptic vesicles, as demonstrated by synaptosomal and NMJ experiments, where glutamate, GABA, and acetylcholine are released in the absence of Ca2+, while catecholamines or peptides are not (Matteoli et al. 1988 Davletov et al. 1998 Khvotchev et al. 2000). Ca2+-independent release does not normally occur in endocrine cells (Grasso et al. 1980 Michelena et al. 1997 Silva et al. 2005), although in some cultured cells it does (Meldolesi et al. 1983 Lang et al. 1998 Tse and Tse 1999). 

The characteristics of Ca2+-independent release are peculiar it requires the presence of divalent cations, such as Mg2+, which can be added or removed in succession, causing respective bouts of secretion or its cessation (Misler and Hurlbut 1979). In the absence of Mg2+, this release can be supported by slightly hypertonic sucrose, by itself insufficient to cause secretion (Misler and Hurlbut 1979). The Ca2+-independent release can be blocked by millimolar La3+ (Rosenthal et al. 1990) or concanavalin A (Grasso et al. 1978 Boehm and Huck 1998). It may involve release of Ca2+ from mitochondria, as observed in peripheral (Tsang et al. 2000) and not central synapses (Adam-Vizi et al. 1993), but it is unclear if stored Ca2+i itself can trigger release. 

In the absence of Ca2+e, a-LTX only binds to LPH1 and PTPc. Ca2+-independent exocytosis requires the presence of Mg2+ and toxin insertion into the plasma membrane, but these conditions also induce formation of a-LTX channels. Influx of Na+ and efflux of K+ through these channels and associated efflux of small molecules and influx/efflux of water may cause secretion. In addition, transmitter release can be caused by membrane perturbation or direct interaction with secretory machinery. Some secretion may be nonvesicular. Receptor-mediated signaling can cause the activation of PKC in some cells. However, Ca2+-independent release is blocked by La3+, indicating that toxin pores play a crucial role in this release. 

Adam-Vizi V, Deri Z, Bors P et al (1993) Lack of involvement of [Ca2+]i in the external Ca2+-independent release of acetylcholine evoked by veratridine, ouabain and a-latrotoxin possible role of [Na+]i. J Physiol Paris 87 43-50... 

P2Y receptors that are found on endothelial cells elicit a Ca2+-dependent release of endothelium-dependent relaxing factor (EDRF) and vasodilation. A secondary activation of a Ca2+-sensitive phospholipase A2 increases the synthesis of endothelial prostacyclin, which limits the extent of intravascular platelet aggregation following vascular damage and platelet stimulation. The P2Y-mediated vasodilation opposes a vasoconstriction evoked by P2X receptors located on vascular smooth muscle cells. The latter elicit an endothelial-independent excitation (i.e. constriction). P2Y receptors are also found on adrenal chromaffin cells and platelets, where they modulate catecholamine release and aggregation respectively. 

Patel MN, Ardelt BK, Yim GKW, et al. 1991. Cyanide induces Ca2+-dependent and independent release of glutamate from mouse brain slices. Neurosci Lett 131 42-44. 

The ability of a-LTX to trigger neurotransmitter exocytosis in the absence of extracellular Ca2+ remains particularly interesting and inexplicable to the field (Longenecker et al. 1970 Ceccarelli et al. 1979 see also Siidhof (2001) and Ushkaryov et al. (2004) for review). This is clearly different from depolarization-induced exocytosis, which is Ca2+-dependent, but not unlike the effect of hypertonic sucrose. The possibility that a-LTX-induced release involves an unknown, Ca2+-independent mechanism which may also occur during normal synaptic activity has provided the casus belli for many a quest for a-LTX structure and receptors that could trigger neurotransmission via intracellular mechanisms. 

Scholze T, Moskvina E, Mayer M et al (2002) Sympathoexcitation by bradykinin involves Ca2+-independent protein kinase C. J Neurosci 22 5823-32 Schwartz EJ, Blackmer T, Gerachshenko T et al (2007) Presynaptic G-protein-coupled receptors regulate synaptic cleft glutamate via transient vesicle fusion. J Neurosci 27 5857-68 Searl TJ, Silinsky EM (1998) Increases in acetylcholine release produced by phorbol esters are not mediated by protein kinase C at motor nerve endings. J Pharmacol Exp Ther 285 247-51 Seino S, Shibasaki T (2005) PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis. Physiol Rev 85 1303 12... 

Shigetomi E, Kato F (2004) Action potential-independent release of glutamate by Ca2+ entry through presynaptic P2X receptors elicits postsynaptic firing in the brainstem autonomic network. J Neurosci 24 3125-35... 

Another messenger substance, nicotinic acid adenine dinucleotide phosphate, NAADP, can be also generated by the action of ADP-ribosyl cyclase (Fig. 6.9). NAADP has been discovered in brain and other tissues. NAADP releases Ca2+ independently of InsP3 and cyclic ADP ribose signals from intracellular stores (review Patel et al., 2001). The NAADP receptor however is not yet characterized. 

Kotlikoff This is a different result, and would suggest coupling between the L-type channel and the BK channel. I think that Mark Nelson will discuss coupling between these channels independent of Ca2+ release. We see similar results with respect to the activation of spontaneous inward currents. 

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