Hypotonicity activates a lanthanide-sensitive pathway for K+ release in A6 epithelia

The nature of the pathway forK⁺ release activated duringregulatory volume decrease (RVD) in A6 epithelia was investigated bymeasuring cell thickness (T_c) asan index of cell volume and by probingK⁺ efflux with⁸⁶Rb as tracer forK⁺(R_Rb). Cell swelling was inducedby sudden reduction of basolateral osmolality (from 260 to 140 mosmol/kgH₂O). Experiments wereperformed in the absence of Na⁺transport. Apical R_Rb wasnegligible in iso- and hyposmotic conditions. On the other hand,osmotic shock increased basolateralR_Rb(R^bl_Rb) rapidly, reaching a maximum 7 minafter the peak in T_c. Quinine (0.5 mM) completely inhibited RVD and R^bl_Rb.Also verapamil (0.2 mM) impeded volume recovery considerably; lidocaine(0.2 mM) did not exert a noticeable effect. TheK⁺ channel blockerBa²⁺ (30 mM) delayed RVD but couldnot prevent complete volume recovery. Cs⁺ inhibited RVD noticeably atconcentrations <40 mM. With large Cs⁺ concentrations (>40 mM), theinitial osmometric swelling was followed by a gradual increase ofT_c, suggesting activation of Cs⁺ influx. Chronic exposure ofthe basolateral surface to 0.5 mM La³⁺ orGd³⁺ completely abolished RVD andR^bl_Rb. Acute administration oflanthanides at the time of osmolality decrease did not affect theinitial phase of RVD and reduced R^bl_Rbonly slightly. Apical Gd³⁺ exertedan inhibitory effect on RVD and R^bl_Rb.The effect of Gd³⁺ shouldtherefore be localized at an intracellular site. The role ofCa²⁺ entry could be excluded byfailure of extracellular Ca²⁺removal to inhibit volume recovery. In contrast to lanthanides, chronically and acutely administeredMg²⁺ (0.5 mM) inhibited RVD andR^bl_Rb by ~50%. These data suggest thatK⁺ excretion during RVD occursthrough a rather poorly selective pathway that does not seem to bedirectly activated by membrane stretch.