One of the mechanisms by which the renal cortical collecting duct (CCD) maintains cellular composition and volume, in the face of changes in transepithelial transport, is by the activation of volume regulatory pathways. A previous study has demonstrated that rabbit isolated CCDs regulate their volume on exposure to a hypotonic shock (Strange, 1988), with ducts demonstrating regulatory volume decrease (RVD) in response to hypotonic shock induced cell swelling. The aim of the following study was to examine the effect of K⁺ channel blockers on hypotonic shock induced volume regulation in mouse CCDs. C57/B6 mice were humanely killed by cervical dislocation and CCDs isolated by enzyme digestion (Schafer et al. 1997). Tubule diameter was measured using an optical system. CCDs were superfused with NaCl Ringer solution which contained (mM): 112 NaCl, 3 KCl, 2 CaCl₂, 1 MgCl₂, 10 HEPES and 60 mannitol, and then exposed to a hypotonic solution (removal of 40 mM mannitol). This was carried out in the presence of either 5 mM Ba²⁺ (general K⁺ channel inhibitor), 100 nM apamin (small conductance Ca²⁺-activated K⁺ channel blocker), 100 nM iberiotoxin (large conductance Ca²⁺-activated K⁺ channel blocker), 20 nM tertiapin (inhibitor of Kir channels) or in the absence of extracellular Ca²⁺. All values are expressed as means ±SEM. Statistical significance was tested using ANOVAS and assumed at the 5% level. The control diameter of CCDs was 25.6 ± 0.55 μm (n=35). Hypotonic shock increased diameter by 0.94 ± 0.07 μm and this was followed by RVD. At steady-state after RVD diameter was 0.27 ± 0.12 μm above the pre-shock diameter. Iberiotoxin had no effect on volume regulation, steady-state diameter was 0.23 ± 0.25 μm above the pre-shock level (n=9). In contrast, Ba²⁺, apamin, tertiapin and zero Ca²⁺ were all associated with inhibition of RVD. Steady-state diameters were 1.23 ± 0.34 μm (n=7), 1.21 ± 0.29 μm (n=8), 0.84 ± 0.20 μm (n=10) and 1.72 ± 0.18 μm (n=7) above the pre-shock level with Ba²⁺, apamin, tertiapin and zero Ca²⁺, respectively. In conclusion, these data indicate that K⁺ channels play an important role in RVD in mouse CCDs. The inhibitory actions of apamin and zero Ca²⁺ suggest that small conductance Ca²⁺-activated K⁺ channels are involved in this process.