Abstract
Anion transport proteins in mammalian cells participate in a wide
variety of cell and intracellular organelle functions, including
regulation of electrical activity, pH, volume, and the transport
of osmolites and metabolites, and may even play a role in the control
of immunological responses, cell migration, cell proliferation, and
differentiation. Although significant progress over the past decade
has been achieved in understanding electrogenic and electroneutral
anion transport proteins in sarcolemmal and intracellular membranes,
information on the molecular nature and physiological significance
of many of these proteins, especially in the heart, is incomplete.
Functional and molecular studies presently suggest that four primary
types of sarcolemmal anion channels are expressed in cardiac cells:
channels regulated by protein kinase A (PKA), protein kinase C,
and purinergic receptors (I(Cl.PKA)); channels regulated by changes
in cell volume (I(Cl.vol)); channels activated by intracellular Ca$^2+$
(I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In
most animal species, I(Cl.PKA) is due to expression of a cardiac
isoform of the epithelial cystic fibrosis transmembrane conductance
regulator Cl$^-$ channel. New molecular candidates responsible
for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2,
respectively) have recently been identified and are presently being
evaluated. Two isoforms of the band 3 anion exchange protein, originally
characterized in erythrocytes, are responsible for Cl$^-$/HCO$^3-$
exchange, and at least two members of a large vertebrate family of
electroneutral cotransporters (ENCC1 and ENCC3) are responsible
for Na$^+$-dependent Cl$^-$ cotransport in heart. A 223-amino
acid protein in the outer mitochondrial membrane of most eukaryotic
cells comprises a voltage-dependent anion channel. The molecular
entities responsible for other types of electroneutral anion exchange
or Cl$^-$ conductances in intracellular membranes of the sarcoplasmic
reticulum or nucleus are unknown. Evidence of cardiac expression
of up to five additional members of the ClC gene family suggest a
rich new variety of molecular candidates that may underlie existing
or novel Cl$^-$ channel subtypes in sarcolemmal and intracellular
membranes. The application of modern molecular biological and genetic
approaches to the study of anion transport proteins during the next
decade holds exciting promise for eventually revealing the actual
physiological, pathophysiological, and clinical significance of these
unique transport processes in cardiac and other mammalian cells.
Description
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