Human Toxicology and Effects of Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 360

signalling by oxidation of thiol groups, for example in the Ca 2+ ATPase pumps.
These pumps are responsible for sequestering calcium into stores such as the endo-
plasmic reticulum and for pumping calcium out of the cell via the plasma membrane
(Barnes et al. , 2000; Kourie, 1998). In addition, there is evidence that ROS can
impact upon membrane calcium channels and calcium binding proteins (Kourie,
1998). Dysfunction of these calcium controlling mechanisms due to ROS results in
an infl ux of calcium into the cytoplasm and potentially uncontrolled calcium signal-
ling. In a monocytic (immature macrophage) cell line known as Monomac 6, and
in primary rat alveolar macrophages, 14 nm carbon black has been found to induce
a signifi cant calcium infl ux (Stone et al. , 2000a,b). This effect was not observed with
260 nm carbon black particles. Similar results were also demonstrated for polysty-
rene particles of different sizes (Brown et al. , 2001). The calcium signalling induced
by carbon black nanoparticles was shown to be important in activating the tran-
scription factor NF κ B and in stimulating the production of the pro-infl ammatory

cytokine TNF . Addition of antioxidants, to block the ROS effects, prevented the

14 nm carbon black calcium signal NF

α

κ B activation (as outlined above) and TNF α

production, suggesting a strong link between the ability of these nanoparticles to
generate ROS and infl ammation.

9.2.4 Uptake of Nanoparticles into Cells

Each cell is surrounded by a plasma membrane, which segregates the internal and
external environments of a cell and is responsible for regulating the entry and exit
of substances into and out of the cell (Conner and Schmid, 2003). There are a
number of clearly defi ned mechanisms for crossing the plasma membrane that
include:

• diffusion
• facilitated diffusion
• active transport
• endocytosis.

The plasma membrane consists of a phospholipids bilayer and proteins and is
therefore a lipophyllic environment that allows selective permeability to low
molecular weight lipophillic substances by passive diffusion (e.g. alcohol). In con-
trast, hydrophilic or high molecular weight substances must gain access via chan-
nels, transporter proteins or endocytosis, all of which are active processes.
The extracellular surface of the plasma membrane consists of the negatively
charged phosphate heads of the phospholipids molecules, resulting in a negatively
charged surface that could infl uence the way in which charged particles interact
with the cell membrane. In addition, there is an overall intracellular negative
charge, caused by unequal distribution of charged ions across the plasma mem-
brane. This unequal distribution of ions is driven by the activity of ion channels and
pumps such as the sodium/potassium (Na/K ATPase) pump, which transports two
potassium ions into the cell for every three sodium ions pumped out. The cell actu-
ally uses this negative electrical (and chemical) membrane potential to drive the
transport of substances, often against a concentration gradient, into or out of the