L-Type Ca2+ Channels in the Enteric Nervous System Mediate Oscillatory Cl- Secretion in Guinea Pig Colon

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The enteric nervous system regulates epithelial ion and fluid secretion. Our previous study has shown that the low (0.2-1 mM) concentrations of Ba2+, a K+ channel inhibitor, evoke Ca2+-dependent oscillatory Cl− secretion via activation of submucosal cholinergic neurons in guinea pig distal colon. However, it is still unclear which types of Ca2+ channels are involved in the oscillation at the neuroepithelial junction. We investigated the inhibitory effects of organic and inorganic Ca2+ channel antagonists on the short circuit current (Isc) of colonic epithelia (mucosa-submucosa sheets) mounted in Ussing chambers. The amplitude (412 ± 37 μA cm−2) and frequency (2.6 ± 0.1 cycles min−1) of the Ba2+-induced Isc in normal (1.8 mM) Ca2+ solution (n = 26) significantly decreased by 37.6% and 38.5%, respectively, in the low (0.1 mM) Ca2+ solution (n = 14). The Isc amplitude was reversibly inhibited by either verapamil (an L-type Ca2+ channel antagonist) or divalent cations (Cd2+, Mn2+, Ni2+) in a concentration-dependent manner. The concentration of verapamil for half-maximum inhibition (IC50) was 4 and 2 μ</i>M in normal and low Ca2+ solution, respectively. The relative blocking potencies of metal ions were Cd2+ > Mn2+, Ni2+ in normal Ca2+ solution. In contrast, the frequency of Isc was unchanged over the range of concentrations of the Ca2+ channel antagonists used. Our results show that the oscillatory Isc evoked by Ba2+ involves L-type voltage-gated Ca2+ channels. We conclude that L-type Ca2+ channels play a key role in the oscillation at the neuroepithelial junctions of guinea pig colon.

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