Differences in Ca²⁺-mediation of hypotonic and Na⁺-nutrient regulatory volume decrease in suspensions of jejunal enterocytes

Summary

We determined differences in the Ca²⁺ signalling of K* and Cl⁻ conductances required for Regulatory Volume Decrease (RVD) in jejunal villus enterocytes passively swollen (0.5 or 0.95, isotonic) compared with swelling because of the absorption of d-glucose (d-Glc) or l-Alanine (l-Ala). Cell volume was measured using electronic cell sizing. In nominally Ca²⁺-free medium containing EGTA (100 μ m) RVD after 0.5 or 0.95, isotonic challenge was prevented. l-Ala swelling and subsequent RVD was influenced in Ca²⁺-free medium. Villus cells were incubated with 10 μ m of the acetomethoxy derivative of 1,2.bis (2-aminophenoxy) ethane N,N,N¹,N¹ tetracetic acid (BAPTA-AM) and RVD after 0.5 · isotonic swelling or l-Ala swelling was prevented. Niguldipine (0.1 μ m), nifedipine (5 μ m), diltiazem (100 μ m), Ni²⁺, and Co²⁺ (1 mm) all prevented hypotonic RVD but had no effect on RVD after l-Ala addition. Charybdotoxin (25 nm) a potent inhibitor of Ca²⁺-activated K⁺ channels, had no effect on hypotonic RVD but prevented RVD of villus cells swollen by d-Glc. We used the calmodulin antagonists, napthalene sulfonamide derivatives W-7 and W-13, to assess calmodulin activation of K⁺ and Cl⁻ conductance in these two models. l-Ala swelling and subsequent RVD was not influenced by 25 μ m W-7; hypotonic RVD was prevented by 25 μ m W-7 or 100 μ mW-13. The W-13 inhibition of RVD was by-passed with 0.5 μm gramicidin. Our data show that hypotonic RVD requires extracellular Ca²⁺ and that the K⁺ conductance activated is not charybdotoxin sensitive but requires calmodulin. Na⁺-nutrient RVD requires intracellular calcium mobilization to activate a charybdotoxin-sensitive K⁺ conductance. The signalling for RVD after cell swelling because of transport of osmotically active Na⁺ nutrient is different than the signalling for RVD after passive hypotonic cell swelling.