52 SECTION ICellular & Molecular Basis of Medical Physiology
distinct pathways for primary messengers to alter transcrip-
tion of cells. First, as is the case with steroid or thyroid hor-
mones, the primary messenger is able to cross the cell
membrane and bind to a nuclear receptor, which then can di-
rectly interact with DNA to alter gene expression. A second
pathway to gene transcription is the activation of cytoplasmic
protein kinases that can move to the nucleus to phosphorylate
a latent transcription factor for activation. This pathway is a
common endpoint of signals that go through the mitogen ac-
tivated protein (MAP) kinase cascade. MAP kinases can be
activated following a variety of receptor ligand interactions
through second messenger signaling. They comprise a series
of three kinases that coordinate a stepwise phosphorylation to
activate each protein in series in the cytosol. Phosphorylation
of the last MAP kinase in series allows it to migrate to the nu-
cleus where it phosphorylates a latent transcription factor. A
third common pathway is the activation of a latent transcrip-
tion factor in the cytosol, which then migrates to the nucleus
and alters transcription. This pathway is shared by a diverse
set of transcription factors that include nuclear factor kappa
B (NFκB; activated following tumor necrosis family receptor
binding and others), and signal transducers of activated
transcription (STATs; activated following cytokine receptor
binding). In all cases the binding of the activated transcription
factor to DNA increases (or in some cases, decreases) the
transcription of mRNAs encoded by the gene to which it
binds. The mRNAs are translated in the ribosomes, with the
production of increased quantities of proteins that alter cell
function.
INTRACELLULAR Ca2+
AS A SECOND MESSENGER
Ca2+ regulates a very large number of physiological processes
that are as diverse as proliferation, neural signaling, learning,
contraction, secretion, and fertilization, so regulation of intra-
cellular Ca2+ is of great importance. The free Ca2+ concentra-
tion in the cytoplasm at rest is maintained at about 100 nmol/
L. The Ca2+ concentration in the interstitial fluid is about
12,000 times the cytoplasmic concentration (ie, 1,200,000
nmol/L), so there is a marked inwardly directed concentration
gradient as well as an inwardly directed electrical gradient.
Much of the intracellular Ca2+ is stored at relatively high con-
centrations in the endoplasmic reticulum and other organelles
(Figure 2–21), and these organelles provide a store from which
Ca2+ can be mobilized via ligand-gated channels to increase
the concentration of free Ca2+ in the cytoplasm. Increased cy-
toplasmic Ca2+ binds to and activates calcium-binding pro-
teins. These proteins can have direct effects in cellular
physiology, or can activate other proteins, commonly protein
kinases, to further cell signaling pathways.
Ca2+ can enter the cell from the extracellular fluid, down its
electrochemical gradient, through many different Ca2+ chan-
nels. Some of these are ligand-gated and others are voltage-
gated. Stretch-activated channels exist in some cells as well.
Many second messengers act by increasing the cytoplasmic
Ca2+ concentration. The increase is produced by releasing Ca2+
from intracellular stores—primarily the endoplasmic reticu-
lum—or by increasing the entry of Ca2+ into cells, or by both
mechanisms. IP 3 is the major second messenger that causes
Ca2+ release from the endoplasmic reticulum through the direct
activation of a ligand-gated channel, the IP 3 receptor. In effect,
the generation of one second messenger (IP 3 ) can lead to the
release of another second messenger (Ca2+). In many tissues,
transient release of Ca2+ from internal stores into the cytoplasm
triggers opening of a population of Ca2+ channels in the cell
membrane (store-operated Ca2+ channels; SOCCs). The
resulting Ca2+ influx replenishes the total intracellular Ca2+
supply and refills the endoplasmic reticulum. The exact identity
of the SOCCs is still unknown, and there is debate about the
signal from the endoplasmic reticulum that opens them.
As with other second messenger molecules, the increase in
Ca2+ within the cytosol is rapid, and is followed by a rapid
decrease. Because the movement of Ca2+ outside of the cytosol
(ie, across the plasma membrane or the membrane of the inter-
nal store) requires that it move up its electrochemical gradient,
it requires energy. Ca2+ movement out of the cell is facilitated
by the plasma membrane Ca2+ ATPase. Alternatively, it can be
transported by an antiport that exchanges three Na+ for each
Ca2+ driven by the energy stored in the Na+ electrochemical
gradient. Ca2+ movement into the internal stores is through the
action of the sarcoplasmic or endoplasmic reticulum Ca2+
AT Pa s e, also known as the SERCA pump.CALCIUM-BINDING PROTEINS
Many different Ca2+-binding proteins have been described, in-
cluding troponin, calmodulin, and calbindin. Troponin is theFIGURE 2–21 Ca2+ handling in mammalian cells. Ca2+ is
stored in the endoplasmic reticulum and, to a lesser extent, mitochondria
and can be released from them to replenish cytoplasmic Ca2+. Calcium-
binding proteins (CaBP) bind cytoplasmic Ca2+ and, when activated in
this fashion, bring about a variety of physiologic effects. Ca2+ enters the
cells via voltage-gated (volt) and ligand-gated (lig) Ca2+ channels and
store-operated calcium channels ( SOCCs). It is transported out of the cell
by Ca, Mg ATPases (not shown), Ca, H ATPase and an Na, Ca antiport. It is
also transported into the ER by Ca ATPases.CaBP EffectsCa^2 +Ca^2 +Ca^2 +2H+3Na+ATPMitochondrion Endoplasmic reticulumCa^2 +
(volt)Ca^2 +
(lig)
Ca^2 +
(SOCC)