Epoxyeicosatrienoic acids, potassium channel blockers and endothelium-dependent hyperpolarization in the guinea-pig carotid artery

1. Using intracellular microelectrodes, we investigated the effects of 17-octadecynoic acid (17-ODYA) on the endothelium-dependent hyperpolarization induced by acetylcholine in the guinea-pig isolated internal carotid artery with endothelium. 2. In the presence of N(ω)-nitro-L-arginine (L-NOARG, 100 μM) and indomethacin (5 μM) to inhibit nitric oxide synthase and cyclo-oxygenase, acetylcholine (1 μM) evoked an endothelium-dependent hyperpolarization which averaged -16.4 mV starting from a resting membrane potential of -56.8 mV. There was a negative correlation between the amplitude of the hyperpolarization and the absolute values of the resting membrane potential. 3. The acetylcholine-induced endothelium-dependent hyperpolarization was not altered by charybdotoxin (0.1 μM) or iberiotoxin (30 nM). It was partially but significantly reduced by apamin (0.5 μM) to -12.8 ± 1.2 mV (n = 10) or the combination of apamin plus iberiotoxin (-14.3 ± 3.4 mV, n = 4). However, the combination of charybdotoxin and apamin abolished the hyperpolarization and under these conditions, acetylcholine evoked a depolarization (+7.1 ± 3.7 mV, n = 8). 4. 17-ODYA (10 μM) produced a significant hyperpolarization of the resting membrane potential which averaged -59.6 mV and a partial but significant inhibition of the acetylcholine-induced endothelium-dependent hyperpolarization (-10.9 mV). 5. Apamin did not modify the effects of 17-ODYA but in the presence of charybdotoxin or iberiotoxin, 17-ODYA no longer influenced the resting membrane potential or the acetylcholine-induced hyperpolarization. 6. When compared to solvent (ethanol, 1% v/v), epoxyeicosatrienoic acids (EpETrEs) (5,6-, 8,9-, 11,12- and 14,15-EpETrE, 3 μM) did not affect the cell membrane potential and did not relax the guinea-pig isolated internal carotid artery. 7. These results indicate that, in the guinea-pig internal carotid artery, the involvement of metabolites of arachidonic acid through the cytochrome P450 pathway in endothelium-dependent hyperpolarization is unlikely. Furthermore, the hyperpolarization mediated by the endothelium-derived hyperpolarizing factor (EDHF) is probably not due to the opening of BK(Ca) channels.link_to_subscribed_fulltex

K + channels in cultured bovine retinal pericytes: Effects of β-adrenergic stimulation

Retinal pericytes are key cells involved in the regulation of retinal blood flow. The purpose of this work was to identify the K + channel population expressed in cultured bovine retinal pericytes and to determine whether β-adrenergic stimulation alters the activity of these channels. Isolated pericytes were obtained by homogenization and filtration of bovine retina and K + channels were studied with the whole-cell configuration of the patch-clamp technique on 3-5 passaged pericytes. Pericytes expressed an inward current dependent on extracellular K + concentration which was sensitive to micromolar concentrations of barium, a characteristic of an inward-rectifying K + current. Furthermore, two voltage-dependent outward currents were also observed. Their activation and inactivation properties, as well as their respective sensitivity to 4-aminopyridine and iberiotoxin, were indicative of voltage-sensitive and large-conductance calcium-activated K + channels (BK Ca). Isoproterenol and dibutyryl cyclic adenosine monophosphate enhanced the activity of BK Ca without affecting the other potassium currents. In conclusion, bovine retinal pericytes express mainly two outward potassium currents, K v and BK Ca, as well as an inward rectifying K + current, K ir. Physiologic stimuli such as an increase in extracellular potassium concentration or β-adrenergic receptor stimulation enhance the activity of K ir and BK Ca, respectively, suggesting a potential role for these channels in the control of retinal blood flow.link_to_subscribed_fulltex