Question:

Human BASIC (bile acid-sensitive ion channel) is a member of the ENaC/degenerin family of ion channels. It was previously termed INaC (intestine Na⁺ channel). The new name BASIC pays tribute to the fact that it is activated by bile acids (Wiemuth D. et al., FASEB J. 2012). This channel may play an important role in ion transport in cholangiocytes and intestinal epithelial cells. However, until now, little is known about its function and its electrophysiological properties. The aim of this study was to investigate the single-channel properties of human BASIC (hBASIC) and its activation by bile acids at the single-channel level.

Methods:

Human BASIC was heterologously expressed in Xenopus laevis oocytes. Since BASIC is inhibited by the diarylamidine diminazene, its activity was monitored by measuring the diminazene-sensitive current in whole oocytes using the two-electrode voltage-clamp technique and in excised outside-out patches using the patch-clamp technique. The bile acids hyodeoxycholic (HDCA) and deoxycholic (DCA) acids were used to activate the channel.

Results:

HDCA and DCA both activated diminazene-sensitive currents in human BASIC expressing oocytes. However, the effect of DCA was faster and more readily reversible than that of HDCA. Therefore, DCA was chosen for further experiments. We determined the single-channel conductance of BASIC using recordings in outside-out patches excised from BASIC expressing oocytes. The single-channel conductance of BASIC averaged 12.9 ± 0.4 pS (n=9). Under these experimental conditions, application of DCA led to an increase of BASIC single-channel activity without altering the single-channel current amplitude. DCA activated BASIC by a) reducing the single-channel closed time from 401 ± 89 to 104 ± 31 ms (n=15, p<0.001), and b) increasing the frequency of single-channel events from 9 ± 3 to 21 ± 5 s^-1 (n=15, p<0.01). There was also a non-significant trend of an increased single-channel open time from 3.1 ± 0.5 to 5.6 ± 1.9 ms (n=15, p=0.19).

Summary:

The single-channel conductance of human BASIC heterologously expressed in Xenopus laevis oocytes is about 13 pS. Channel activity is increased by DCA without changing the single-channel current amplitude. Stimulation of human BASIC by DCA is mainly attributed to a decrease of the single-channel closed time and an increased frequency of single-channel events.