1 The effects of phentolamine (5-30 JAM) and glibenclamide (10 JM) on action potential characteristics were examined in guinea-pig papillary muscle exposed to either hypoxia or levcromakalim (20 gM).2 The hypoxia-induced abbreviation of action potential duration (APD) and effective refractory period (ERP) were attenuated but not abolished by glibenclamide (10 JAM). Hypoxia reduced APD by 24 ± 2 vs 65 ± 4% in glibenclamide-and vehicle-treated tissue, respectively.3 Phentolamine (10-30JM) was less effective than glibenclamide in attenuating the hypoxic shortening of APD since APD was reduced by 38 ± 10, 51 ± 6% vs 65 ± 4% in 10 and 30 JAM phentolamine and vehicle-treated muscle, respectively.4 Phentolamine, at concentrations of 10 and 30 JAM, also reduced the upstroke velocity of the action potential and at 5 JAM it increased the APD from 193 ± 9 to 221 ± 12 ms. 5 Glibenclamide completely abolished and phentolamine (30 JAM) significantly attenuated levcromakalim-induced changes in duration and ERP. Levcromakalim reduced APD by 71 ± 2 and 55 ± 2% in control and phentolamine pretreated muscle, respectively. 6 It is concluded that phentolamine may block KATP channels at concentrations that also block sodium channels.
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