Spontaneous quantal transmitter secretion from myocytes and fibroblasts: comparison with neuronal secretion

Abstract

When exogenous ACh is loaded into the cytoplasm of cultured amphibian myocytes and fibroblasts, the cells undergo spontaneous quantal ACh secretion, as detected by the appearance of pulsatile membrane currents in Xenopus myocytes which are manipulated into contact with the cells. These currents resemble in many ways the miniature endplate currents (MEPCs) observed at developing neuromuscular synapses formed on these Xenopus myocytes. Analyses of the frequency, amplitude, and time course of these currents suggests similarity in the cellular mechanisms involved in the packaging and secretion of ACh quanta in fibroblasts, myocytes, and developing neurons. The size of the ACh packets released by the non-neuronal cells were found to be very similar to the size of the neuronal ACh quanta, which are thought to result from the exocytotic release of synaptic vesicles. Moreover, the kinetics with which the ACh packets are discharged from all three cell types are comparable, although the speed of secretion in non-neuronal cells is somewhat slower and more irregular. The spontaneous quantal ACh secretion from neurons and myocytes was decreased by reducing cytosolic Ca2+ level and enhanced by activation of protein kinase C with phorbol ester, but secretion from fibroblasts was unaffected by both treatments. The spontaneous secretion from fibroblasts did show some sensitivity to a rise in cytosolic Ca2+ after treatment with a Ca2+ ionophore. These observations support the hypothesis that the basic machinery for transmitter secretion operating in neurons derive from a more ubiquitous mechanism used for constitutive secretion and membrane trafficking in non-neuronal cells, and neuronal differentiation involves expression of additional unique components for the regulation of the spontaneous quantal secretion.

The Vesicular Acetylcholine Transporter Is Required for Neuromuscular Development and Function

“…Previous results indicated that both the exocytosis-endocytosis of synaptic vesicles and the quantal release of the neurotransmitter occur in developing axons (24,34). Our experiments indicate that VAChT regulates a key step for physiologically relevant neurotransmission during the development of the NMJ.…”

“…However, alternative mechanisms of secretion have been proposed (20,56,61). Quantal ACh release, comparable to that seen in developing nerve terminals, has been detected in myocytes and fibroblasts in culture, which presumably do not express VAChT (14,24). More recently, it was found that the correct targeting of Drosophila photoreceptor axons is disrupted in flies with null mutations in ChAT (64).…”

Long-Term Potentiation of Exocytosis and Cell Membrane Repair in Fibroblasts

“…2ϩ -regulated exocytosis is apparently universally present in cells (Dan and Poo, 1992;Steinhardt et al, 1994;Bi et al, 1995;Girod et al, 1995;Coorssen et al, 1996;Ninomiya et al, 1996;Togo et al, 1999;Andrews, 2000) and plays an essential role in maintaining the integrity of the cell membrane (McNeil and Steinhardt, 1997). If a cell experiences a membrane disruption in the micrometer diameter range, Ca 2ϩ influx at the wound site triggers exocytosis that is essential for successful cell membrane repair (Steinhardt et al, 1994;Bi et al, 1995Bi et al, , 1997Miyake and McNeil, 1995;Togo et al, 1999;Reddy et al, 2001).…”

cAMP Regulates Ca2+-dependent Exocytosis of Lysosomes and Lysosome-mediated Cell Invasion by Trypanosomes

“…Recently, however, evidence has been accumulating indicating that it is a ubiquitous process observed in a variety of animal cells (1)(2)(3)(4)(5). A previous study from our laboratory showed that between 10 and 20% of conventional lysosomes in fibroblasts and epithelial cells fuse with the plasma membrane and release their contents upon elevation of the intracellular free Ca 2ϩ concentration ([Ca 2ϩ ] i ) (6).…”