Ca2+-activated K+ channels of big (hBK), intermediate (hIK) or small (rSK3) conductance were co-expressed with aquaporin 1 (AQP1) in Xenopus laevis oocytes. hBK channels were activated by depolarization, whereas hIK and rSK3 channels were activated by direct injection of Ca2+ or Cd2+ into the oocyte cytoplasm, before the oocytes were subjected to hyperosmolar or hypoosmolar (+/-50 mOsm mannitol) challenges. In all cases, the oocytes responded rapidly to the osmotic changes with shrinkage or swelling and the effects on the K+ currents were measured. hIK and rSK3 currents were highly sensitive to volume changes and increased immediately to 178% (hIK) or 165% (rSK3) of control in response to swelling and decreased to 64% (hIK) or 61% (rSK3) of control after shrinkage. These responses were dependent on the channels being pre-activated and were almost totally abolished after injection of cytochalasin D into the oocyte cytoplasm (final concentration: 1 microM). In contrast, hBK channels showed only a minor sensitivity to volume changes; the hBK channel activity decreased approximately 20% after swelling and increased approximately 20% after shrinkage. The opposite effects of volume changes on hIK/rSK3 and hBK channels suggest that the significant stimulation of hIK and rSK3 channels during swelling is not mediated by changes in intracellular Ca2+, but rather through interactions with the cytoskeleton, provided that a sufficient basal concentration of intracellular Ca2+ or Cd2+ is present.