Biphasic rises in cytosolic free Ca2+ in association with activation of K+ and Cl? conductance during the regulatory volume decrease in cultured human epithelial cells

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Abstract
During exposure to a hypotonic solution (55% osmolarity), cultured human epithelial (Intestine 407) cells exhibit a regulatory volume decrease after osmotic swelling. This process is known to involve parallel activation of volume-regulatory K+ and Cl− conductances. Biphasic increase in the cytosolic free Ca2+ concentration ([Ca2+]i) were observed by microspectrofluorometry, in fura-2-loaded cells upon hypotonic stress. Electrophysiological studies with Ca2+-selective and conventional microelectrodes indicated that a biphasic [Ca2+]i increase was associated with a biphasic hyperpolarization, whereas an interposing [Ca2+]i decrease coincided with a transient depolarization. A Ca2+ ionophore, ionomycin, produced a sustained Ca2+ increase and a prolonged hyperpolarization which was sensitive to the K+ channel blocker, quinine. A subsequent hypotonic challenge gave rise to a depolarization, which was sensitive to a stilbene-derivative Cl− channel blocker, without inducing further changes in [Ca2+]i. Normal cell volume regulation in a hypo-osmotic medium could take place even in the presence of ionomycin. It is concluded that a biphasic [Ca2+]i increase is closely associated with activation of the volume-regulatory K+ conductance, and that the interposing [Ca2+]i decrease is neither a causative factor for activation of the volume-regulatory Cl− conductance nor a prerequisite for regulatory volume decrease in epithelial cells exposed to a hypotonic solution.