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Several studies provided evidence for release of nucleotides by exocytosis.
Other mechanisms include the ATP binding cassette (i.e., ABC transporters, such
as cystic fibrosis transmembrane conductance regulator (CFTR)), or stretch- and
voltage-gated channels, such as plasmalemmal voltage-dependent anion channel
(pl-VDAC; a murine splicing variant of the mitochondrial ATP porin VDAC-
1), swelling-activated Gd 3+ -sensitive anion channel, the stretch-activated cation
channels, maxi-anion channels and P2X7 channels, and carriers (for review see
[19, 81]).
Recent findings have provided evidence that connexons (the constituent of gap
junctions; Fig. 10.1B) are also present in non-junctional parts of the plasma mem-
brane, where they form functional Cx hemichannels that can eventually open under
physiological and pathophysiological conditions (for review see [34, 45, 51, 90-
92, 38]). Furthermore, evidence has been presented that these hemichannels are
involved in ATP release, providing a pathway for PIC [108, 15, 29, 53].
Because gap junction channels and hemichannels are permeable to molecules
with molecular weight up to 1.2 kDa, it was thought that maintenance of gradi-
ents across the cell membrane requires that hemichannels should have very low
open probability under physiological conditions. However, a growing amount of
evidence indicates that opening of hemichannels does occur under both phys-
iological and pathological conditions in many cell types [24, 5, 108, 15, 91,
43, 74, 29, 3, 53, 89]. Electrophysiological studies indicate that hemichannels
have a low open probability under physiological conditions, but may have a
much higher open probability under certain pathological conditions. Cx hemichan-
nels tend to be closed by negative membrane potentials, high concentrations of
extracellular Ca 2+ and intracellular H + ions, gap junction blockers and protein
phosphorylation. They tend to be opened by positive membrane potentials and
low extracellular Ca 2+ , and possibly by as-yet-unidentified cytoplasmic signaling
molecules [92].
The permeability of hemichannels has been evaluated using a number of fluo-
rescent tracers. Cx43 hemichannels are shown to be permeable to lucifer yellow,
ethidium bromide, propidium iodide, carboxyfluorescein, 7-hydroxycoumarin-3-
carboxylic acid and fura-2 [61, 58, 78, 108]. Furthermore, Cx43 hemichannels are
permeable to small, physiologically significant molecules, including NAD + , gluta-
mate, ATP and IP 3 [18, 88, 108, 53] (for review see [90]), and may mediate paracrine
as well as autocrine signaling.
Low extracellular [Ca 2+ ] or application of quinine, two conditions known to
open hemichannels, elicit local and propagating Ca 2+ signals [108]. Mechanical
stimulation triggers ATP release through Cx43 hemichannels, and thus initiates
propagation of Ca 2+ waves in astrocytes and other electrically non-excitable cells
[5, 108]. When mechanically stimulated or exposed to extracellular medium with
low [Ca 2+ ], astrocytes showed membrane currents and uptake of small fluorescent
dyes that were inhibited by flufenamic acid or Gd 3+ , providing evidence for opening
of Cx hemichannels [5, 108].
Panx channels and the Panx-P2X7 complex have also been suggested to be
mechanisms for ATP release. Recent data support the concept that Panxs form
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