The cellular basis of regulatory volume decrease (RVD) by mudpuppy (Necturus maculosus) red blood cells (RBCs) was examined. Volume regulation was inhibited by replacing extracellular Na+ with K+. In contrast, addition of gramicidin (5 microM) to the extracellular medium enhanced RVD. The K(+)-channel blocker quinine (1 mM) also inhibited RVD, and this inhibition was reversed by gramicidin (5 microM). In addition, a 0 Ca(2+)-EGTA Ringer blocked RVD, whereas the Ca2+ ionophore A23187 ( microM) enhanced recovery of cell volume. The stretch-activated ion channel antagonist gadolinium (10 microM) inhibited RVD, and this effect was reversed by A23187 (2 microM). Furthermore, the calmodulin inhibitors pimozide (10 microM) and N-(6-aminohexyl)-5-chloro-1-napthalene-sulfonamide (0.1 mM) blocked RVD, and this inhibition was reversed with gramicidin (5 microM). Consistent with these findings, a K(+)-selective membrane conductance was activated by exposing RBCs to a 0.5x Ringer solution (observed with the whole cell patch clamp technique). This conductance was inhibited by quinine (1 mM), gadolinium (10 microM), and pimozide (10 microM). These results indicate that cell swelling activates a K+ conductance by a Ca(2+)-calmodulin-dependent mechanism and that this channel mediates K+ loss during RVD.