Anion channels in rat liver canalicular plasma membranes reconstituted into planar lipid bilayers

Abstract

Previous studies from this laboratory have demonstrated a Cl(-)-HCO3- exchanger and have provided evidence for a Cl- conductance in rat liver canalicular plasma membrane vesicles. To further investigate the apical Cl- conductance, we performed single-channel analysis after incorporation of canalicular liver plasma membrane vesicles into planar lipid bilayers. This was necessary, because the canalicular membrane is not accessible for the patch-clamp technique. Two types of anion channels could be identified (30- and 90-pS conductance) corresponding to the class of small and intermediate channels, respectively. The kinetics of the small channel were found to be voltage dependent with a maximum for the open probability at -20 mV. In contrast, intermediate channel kinetics were voltage independent. The anion channels described above could allow electrogenic Cl- efflux, to compensate Cl- influx via the electroneutral Cl(-)-HCO3- exchanger. Further studies will be required to prove their functional importance in bile formation.

Effect of glucagon on intracellular pH regulation in isolated rat hepatocyte couplets.

“…In rat hepatocytes, Rp-cAMPS inhibited glucagon-induced glycogenolysis, a cAMP-mediated effect (43 involved in glucagon stimulation of the exchanger. Different Cl -channels could, however, be involved also in light of a recent description, in rat liver canalicular plasma membranes, of two types (30-and 90-pS conductance) of Cl-channels with NPPB-insensitive conductance and kinetics (45). In different cell types, several ion transport systems regulated by protein kinase A show inverse sensitivity to protein kinase C (46,47).…”

Leukotriene D4 activates a chloride conductance in hepatocytes from lipopolysaccharide-treated rats.

“…It increased conductance of LPS cells slightly, but this effect did not reach statistical significance. 4 -activated conductance was decreased from 1,889Ϯ221 pS/pF (n ϭ 4) to 654Ϯ129 pS/pF (n ϭ 5, P Ͻ 0.001). This result suggests that direct LTD 4 -mediated conductance activation is partially Ca 2ϩ dependent, unlike the conductance in day 1 LPS cells.…”

“…Fig. 7 shows that when control hepatocytes were patched without LTD 4 in the pipet, a constant intracellular Ca 2ϩ was maintained (138Ϯ17 nM at 3 min). When LTD 4 was added to the pipet, there was no change in intracellular Ca 2ϩ in control hepatocytes.…”

“…Effects of LTD 4 on membrane conductance. The ability of exogenous leukotrienes to directly activate the conductance was tested in both control hepatocytes and hepatocytes derived from LPS-treated rats.…”

“…The cysteinyl leukotrienes (LT), 1 LTC 4 , LTD 4 , and LTE 4 , are potent mediators of inflammation but have also been shown to regulate ion channels in a number of systems. These include K ϩ currents in cardiac myocytes (9, 10), somatostatin-activated Cl Ϫ currents in hippocampal neurons (11), and Na ϩ channel activity in polarized renal epithelial cells (12).…”