Hyperpolarization of isolated capillaries from guinea-pig heart induced by K+ channel openers and glucose deprivation

The present study was designed to test if microvascular coronary endothelial cells express ATP-sensitive K+ channels (K-ATP 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, [DiBAC(4)(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 (E-K) could be induced by application of the K+ channel openers HOE 234 (100 nM to 1 mu M), diazoxide (10 pm to 100 nM) or pinacidil (100 nM). Subsequent addition of glibenclamide (200 nM to 2 mu M) reversed this hyperpolarization. 4. A glibenclamide-sensitive hyperpolarization of coronary capillaries to values near E-K was also observed upon omission of D-glucose (10 mar) from the superfusing solution or by substituting L-glucose for D-glucose. Maximum hyperpolarization was reached in less than IO min. 5. Our results suggest that microvascular coronary endothelial cells express K-ATP channels which may be activated during hypoglycaemia.