L-type Ca²⁺ channels in the enteric nervous system mediate oscillatory Cl^- secretion in guinea pig colon

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 neuro-epithelial junction. We investigated the inhibitory effects of organic and inorganic Ca²⁺, channel antagonists on the short circuit current (I_sc) 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 I_sc 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 I_sc 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 μM 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 I_sc,, was unchanged over the range of concentrations of the Ca₂₊ channel antagonists used. Our results show that the oscillatory I_sc 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.