TRPC4 currents have properties similar to the pacemaker current in interstitial cells of Cajal

Abstract

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, TRPC4α and TRPC4β, from GI muscles. TRPC4β 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 TRPC4β channels recorded at the whole cell level had characteristics similar to the nonselective cation current in ICC. The single-channel conductance of TRPC4β was determined to be 17.5 pS. Application of calmidazolium to cells expressing TRPC4β 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 TRPC4β led us to conclude that a TRPC4-like current encodes the plasmalemmal pacemaker current in murine small intestine.