Properties of two calcium transport systems of isolated rat ileal epithelial cells: effects of Ca2+ channel modulators and membrane potential examined with fluorescent dye, fura-2.
Calcium transport systems of isolated ileal epithelial cells were investigated. The concentration of cytosolic free calcium ions, [Ca2+]i, was monitored with a fluorescent Ca2+ dye, fura-2. The fluorescence intensity ratio (I340/I380) was used as an index of [Ca2+]i. [Ca2+]i of the cells suspended in the nominally Ca(2+)-free solution was estimated at 52 +/- 3 nM. Ca2+ uptake was followed for as long as 5 min in the presence of 100-1000 microM added CaCl2. Most of the experiments were performed at 200 microM CaCl2. The Ca2+ uptake was abolished by 0.8 mM Ni2+ and 50 microM Mn2+ and partitally antagonized by 50 microM verapamil and 50 microM diltiazem but not affected by 20 microM nifedipine. The Ca2+ entry was reduced by increasing concentrations of extracellular K+ in the presence of valinomycin, suggesting a voltage-dependent nature of the uptake. On the other hand, the Ca2+ transport doubled in the presence of Bay K8644 (8 microM), a Ca2+ channel agonist. The Bay-K-8644-induced uptake was inhibited by either 10 microM nifedipine, 10 microM verapamil or 10 microM diltiazem and was relatively independent of extracellular K+ concentration. These results suggest that there are at least two distinct Ca2+ transport systems in the rat ileal epithelial cells, one resistant to organic Ca2+ channel blockers but relatively sensitive to membrane potential (basal uptake) and another inducible by Bay K 8644 and sensitive to the channel blockers but relatively independent of membrane potential.