Regulation of Cell Membrane Ion Channels 
Michael J. Welsh, M.D. — Investigator 
Dr. Welsh is also Professor of Internal Medicine and Physiology and Biophysics at the University of Iowa 
College of Medicine. He earned his M.D. degree from the University of Iowa. He completed his residency 
at the University of Iowa College of Medicine; held clinical and research fellowships in pulmonary diseases 
and cardiovascular research at the University of California, San Francisco; and did postgraduate research 
in physiology and cell biology at the University of Texas, Houston. He then returned to the University of 
Iowa as a faculty member. 
CELL membrane ion channels control the in- 
tracellular ionic environment and play an 
important role in signal transduction. In epithe- 
lial cells they are also responsible for secretion 
and absorption of electrolytes and water. 
In the lungs, ion transport, including chloride 
secretion by the airway epithelium, controls the 
quantity and composition of the respiratory tract 
fluid. Thus it helps effect mucociliary clearance, 
a normal pulmonary defense mechanism. 
Net vectorial ion transport requires the asym- 
metric distribution of ion channels and other 
transporters in polar epithelial cells. For active 
chloride secretion, chloride enters the cell at the 
basolateral membrane (interstitial side of the 
cell) by a sodium-coupled cotransport process 
that accumulates intracellular chloride above 
electrochemical equilibrium. Chloride then 
leaves the cell passively, exiting across the apical 
membrane (luminal side of the cell) via a chlo- 
ride channel. 
Intracellular sodium is maintained at a low 
concentration by the basolateral sodium-potas- 
sium ATPase, and potassium that enters on the 
sodium pump is recycled by basolateral potas- 
sium channels. Effective secretion requires coor- 
dinated regulation of chloride channels at the 
apical membrane and potassium channels at the 
basolateral membrane. The regulation of these 
channels controls the rate of transepithelial chlo- 
ride secretion. An understanding of the function 
and regulation of epithelial ion channels is a ma- 
jor goal of this work. 
Understanding the regulation of apical chlo- 
ride channels is also important in understanding 
cystic fibrosis (CF) , a common lethal genetic dis- 
ease of Caucasians. In CF the regulation of apical 
chloride channels is defective. Apical membrane 
chloride channels are activated and chloride se- 
cretion is stimulated by a variety of hormones and 
neurotransmitters that increase intracellular lev- 
els of cAMP. In CF the apical membrane is chlo- 
ride impermeable, an abnormality that may ex- 
plain several of the pathologic manifestations. In 
CF airway epithelial cells, hormonal secreta- 
gogues stimulate cAMP accumulation appropri- 
ately but fail to activate (or open) chloride 
channels. 
In many cells the biologic effects of cAMP re- 
sult from activation of cAMP-dependent protein 
kinase (PKA) , resulting in phosphorylation of tar- 
get proteins. To determine if the enzyme PKA reg- 
ulates chloride channels, we used excised, in- 
side-out patches of membrane and added the 
purified catalytic subunit of PKA plus ATP to the 
internal surface of the channel. We found that 
chloride channels were activated by PKA plus 
ATP in cell-free patches of membrane from nor- 
mal cells but were not activated in CF cells. 
Protein kinase C (PKC) , another important en- 
zyme, regulates cell function by phosphoryla- 
tion. In airway epithelia, several secretagogues 
increase the cellular mass of diacylglycerol, an 
activator of PKC, suggesting that the latter also 
regulates the chloride channel. In experiments 
using excised cell-free patches, we found that 
PKC could have a dual effect on chloride chan- 
nels. At a high calcium concentration, it inacti- 
vated chloride channels, and at a low calcium 
concentration, it activated them. In CF cells, 
PKC-dependent channel inactivation was normal, 
but activation was defective. Thus it appears that 
PKC also regulates the chloride channel by phos- 
phorylation and suggests that two different sites 
on the channel, or an associated membrane pro- 
tein, may be involved in regulation, with one of 
these being defective in CF. 
Although phosphorylation-dependent activa- 
tion of chloride channels is defective in CF, an 
increase in the cell calcium concentration stimu- 
lates chloride secretion in airway epithelia. Cal- 
cium-stimulated chloride secretion has the po- 
tential to bypass the CF secretory defect. We 
found that calcium and cAMP activate different 
chloride channels in the apical membrane. This 
conclusion is based on four observations: 1) cal- 
cium-activated chloride channels are present in 
the apical membrane of airway, but not intestinal, 
epithelia; 2) cAMP-activated but not calcium- 
activated chloride channels are defective in CF 
airway epithelia; 3) calcium- and cAMP-activated 
channels have different halide permeabilities; 
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