The large-conductance Ca(2+)-activated K(+) (BK) channels play an important role in the regulation of cellular excitability in response to changes in intracellular metabolic state and Ca(2+) homeostasis. In vascular smooth muscle, BK channels are key determinants of vasoreactivity and vital-organ perfusion. Vascular BK channel functions are impaired in diabetes mellitus, but the mechanisms underlying such changes have not been examined in detail. We examined and compared the activities and kinetics of BK channels in coronary arterial smooth muscle cells from Lean control and Zucker Diabetic Fatty (ZDF) rats, using single-channel recording techniques. We found that BK channels in ZDF rats have impaired Ca(2+) sensitivity, including an increased free Ca(2+) concentration at half-maximal effect on channel activation, a reduced steepness of Ca(2+) dose-dependent curve, altered Ca(2+)-dependent gating properties with decreased maximal open probability, and a shortened mean open-time and prolonged mean closed-time durations. In addition, the BK channel beta-subunit-mediated activation by dehydrosoyasaponin-1 (DHS-1) was lost in cells from ZDF rats. Immunoblotting analysis confirmed a 2.1-fold decrease in BK channel beta(1)-subunit expression in ZDF rats, compared with that of Lean rats. These abnormalities in BK channel gating lead to an increase in the energy barrier for channel activation, and may contribute to the development of vascular dysfunction and complications in type 2 diabetes mellitus.