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Department of Physiology and Cell Biology, University of Nevada, School of Medicine, Reno, Nevada 89557
Interstitial cells of Cajal (ICC) are the
pacemaker cells responsible for the generation and propagation of
electrical slow waves in phasic muscles of the gastrointestinal (GI)
tract. The pacemaker current that initiates each slow wave derives from
a calcium-inhibited, voltage-independent, nonselective cation channel. This channel in ICC displays properties similar to that reported for
the transient receptor potential (TRP) family of nonselective cation
channels, particularly those seen for TRPC3 and TRPC4. We have
identified transcripts for TRPC4 in individually isolated ICC and have
cloned the two alternatively spliced forms of TRPC4, TRPC4
and
TRPC4
, from GI muscles. TRPC4
is missing an 84-amino acid segment
from the carboxy terminus. Expression of either form using the whole
cell patch-clamp technique led to calcium-inhibited, nonselective
cation channels as determined by
N-methyl-D-glucamine replacement experiments and
BAPTA dialysis. Expression of TRPC4
channels recorded at the whole
cell level had characteristics similar to the nonselective cation
current in ICC. The single-channel conductance of TRPC4
was
determined to be 17.5 pS. Application of calmidazolium to cells
expressing TRPC4
led to a significant increase in the inward current
of these cells at both the whole cell and single-channel level, and
currents were sensitive to block by 10 µM lanthanum, niflumic acid,
and DIDS. Comparison of the properties reported for the nonselective
cation current in ICC and those identified here for TRPC4
led us to
conclude that a TRPC4-like current encodes the plasmalemmal pacemaker
current in murine small intestine.
cation channel; gastrointestinal; smooth muscle; calmodulin
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