The aim of this study was to investigate the influence of hypotonic challenge on the effects of potassium channel openers (PCO), rilmakalim and pinacidil, on activation of the ATP-sensitive K(+) current. The whole cell configuration of the patch-clamp technique was applied to guinea pig ventricular myocytes exposed to isotonic and hypotonic solutions. Difference in osmolarity was about 100 mOsm due to different mannitol concentrations. Rilmakalim, a second generation PCO [(3S,4R)-3-hydroxy-2,2-dimethyl-4-(2-oxo-l-pyrrolidinyl)-6-phenylsulfonylchromanhemihydrate], activated time-independent K(+) current in the isotonic solution with pD2 (-log EC(50)) = 6.42 +/- 0.12 and E(max) = 19.74 +/- 2,16 pA/pF, n = 7, at 0 mV. The effects of the cyanoguanidine compound pinacidil were similar to those of rilmakalim, but the action appeared slower and with about 600-fold less potency than the former. Efficacy of rilmakalim, but not pinacidil, was enhanced in hypotonic solution, with E(max) = 30.87 +/- 5.40 pA/pF (P<0.05, n = 7), and the current was completely inhibited by glibenclamide. Additionally, rilmakalim concentration-effects correlation coefficient (R) decreased from 0.96 to 0.86 and Hill's coefficient increased from 1.21 to 1.45. Pretreatment with phalloidin (20 microM), a cytoskeleton stabilizer, prevented an intensification of the effects of rilmakalim in hypotonic solution and returned R and Hill's coefficients to the control values. We conclude that osmotic stress increases efficacy of rilmakalim to activate K(ATP) channels in guinea pig ventricular myocytes due to the specific interaction with actin filaments.