Novel benzamil-sensitive sodium-independent choline transport in the cestode Hymenolepis diminuta: Evidence for sodium channels uptake at low pH

Abstract

The uptake of choline by the tegument of Hymenolepis diminuta was investigated. The Q₁₀ at pH 7.0 was 1.7, with an Ea of 90 kJ·mol⁻¹. Choline transport was pH sensitive: At pH 5.0, a Na⁺-independent mechanism predominated, which was inhibited by 100 nM benzamil, 130 mM Na⁺, and 300 μM verapamil. At pH 7.0, the Na⁺-independent mechanism was inhibited by 130 mM Na⁺, amiloride, and EIPA with IC₅₀'s of 130 μM and 30 μM, respectively, and by benzamil with IC₅₀'s of 100 pM (high-potency Benzamil Sensitive Component; HBSC) and 70 μM (low-potency Benzamil Sensitive Component; LBSC). Calcium-free saline enhanced choline uptake non-specifically. Lanthanum³⁺, Gd³⁺, gramicidin, nigericin, and high-K⁺ did not affect choline uptake at pH 5.0 or pH 7.0, and 10 μM verapamil was without effect at pH 5.0, suggesting no significant role for the electrical potential difference across the brush-border membrane, a $\text{Na}{}^{\mathrm{+}}\text{H}{}^{\mathrm{+}}$ antiporter, a $\text{Na}{}^{\mathrm{+}}\text{Ca}{}^{\mathrm{2+}}$ antiporter, or Ca²⁺ channels in choline uptake. Under physiological conditions, the HBSC accounts for ∼25% of the total choline taken up at pH 5.0, while the LBSC accounts for ∼55% of the choline taken up at pH 7.0. The data suggest novel choline transporting mechanisms; an HBSC which displays properties in common with apical Na⁺ channels, and a unique LBSC of choline transport.