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Institute of Physiology, University of Marburg, Germany. langhein@mailer.uni-marburg.de

1. The present study was designed to test if microvascular coronary endothelial cells express ATP-sensitive K+ channels (KATP channels). We performed microfluorometric measurements of the membrane potential of freshly isolated guinea-pig coronary capillaries equilibrated with the voltage-sensitive dye bis-oxonol (bis-[1,3-dibutylbarbituric acid] trimethineoxonol, [DiBAC4(3)]). 2. The resting membrane potential of capillaries in physiological salt solution was -46 +/- 4.2 mV (n = 8) at room temperature (22 degrees C) as determined after calibration of the fluorescence using the Na(+)-K+ ionophore gramicidin in the presence of different K+ concentrations. Spontaneous membrane potential fluctuations of 10-20 mV amplitude were often observed. 3. A reversible, sustained hyperpolarization to a new membrane potential close to the K+ equilibrium potential (EK) could be induced by application of the K+ channel openers HOE 234 (100 nM to 1 microM), diazoxide (10 PM to 100 nM) or pinacidil (100 nM). Subsequent addition of glibenclamide (200 nM to 2 microM) reversed this hyperpolarization. 4. A glibenclamide-sensitive hyperpolarization of coronary capillaries to values near EK was also observed upon omission of D-glucose (10 mM) from the superfusing solution or by substituting L-glucose for D-glucose. Maximum hyperpolarization was reached in less than 10 min. 5. Our results suggest that microvascular coronary endothelial cells express KATP channels which may be activated during hypoglycaemia.