Tohoku J. Exp. Med., 2007, 211(2)

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

MARIKO NISHIKITANI,¹ YUKIKO YASUOKA,¹ HIDEAKI KAWADA¹ and KATSUMASA KAWAHARA¹

¹Department of Physiology, Kitasato University School of Medicine, Sagamihara, Japan

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 (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.

keywords —— verapamil; cadmium; voltage-gated Ca²⁺ channel; colon; cholinergic neurons

© 2007 Tohoku University Medical Press

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Tohoku J. Exp. Med., 2007, 211, 151-160

Correspondence: Prof. Katsumasa Kawahara, M.D., Ph.D., Department of Physiology, Kitasato University School of Medicine, Kitasato 1-15-1, Sagamihara 228-8555, Japan.

e-mail: kawahara@nc.kitasato-u.ac.jp