Bile Acid Inhibition of N-type Calcium Channel Currents from Sympathetic Ganglion Neurons

Under some pathological conditions as bile flow obstruction or liver diseases with the enterohepatic circulation being disrupted, regurgitation of bile acids into the systemic circulation occurs and the plasma level of bile acids increases. Bile acids in circulation may affect the nervous system. We examined this possibility by studying the effects of bile acids on gating of neuronal (N)-type Ca(2+) channel that is essential for neurotransmitter release at synapses of the peripheral and central nervous system. N-type Ca(2+) channel currents were recorded from bullfrog sympathetic neuron under a cell-attached mode using 100 mM Ba(2+) as a charge carrier. Cholic acid (CA, 10(-6) M) that is relatively hydrophilic thus less cytotoxic was included in the pipette solution. CA suppressed the open probability of N-type Ca(2+) channel, which appeared to be due to an increase in null (no activity) sweeps. For example, the proportion of null sweep in the presence of CA was ~40% at +40 mV as compared with ~8% in the control recorded without CA. Other single channel properties including slope conductance, single channel current amplitude, open and shut times were not significantly affected by CA being present. The results suggest that CA could modulate N-type Ca(2+) channel gating at a concentration as low as 10(-6) M. Bile acids have been shown to activate nonselective cation conductance and depolarize the cell membrane. Under pathological conditions with increased circulating bile acids, CA suppression of N-type Ca(2+) channel function may be beneficial against overexcitation of the synapses.