Abstract
1. Substance P and bradykinin, endothelium-dependent vasodilators of pig coronary artery, trigger in endothelial cells a rise in cytosolic Ca2+ concentration ([Ca2+]i) and membrane hyperpolarization. The aim of the present study was to determine the type of Ca2+-dependent K+ (KCa) currents underlying the endothelial cell hyperpolarization. 2. The substance P-induced increase in [Ca2+]i was 30 % smaller than that induced by bradykinin, although the two peptides triggered a membrane hyperpolarization of the same amplitude. The two agonists evoked a large outward K+ current of the same conductance at maximal stimulation. Agonists applied together produced the same maximal current amplitude as either one applied alone. 3. Iberiotoxin (50 nM) reduced by about 40 % the K+ current activated by bradykinin without modifying the substance P response. Conversely, apamin (1 microM) inhibited the substance P-induced K+ current by about 65 %, without affecting the bradykinin response. Similar results were obtained on peptide-induced membrane hyperpolarization. 4. Bradykinin-induced, but not substance P-induced, endothelium-dependent relaxation resistant to NG-nitro-L-arginine and indomethacin was partly inhibited by 3 microM 17-octadecynoic acid (17-ODYA), an inhibitor of cytochrome P450 epoxygenase. Similarly, the bradykinin-induced K+ current was reduced by 17-ODYA. 5. Our results show that responses to substance P and bradykinin result in a hyperpolarization due to activation of different KCa currents. A current consistent with the activation of large conductance (BKCa) channels was activated only by bradykinin, whereas a current consistent with the activation of small conductance (SKCa) channels was stimulated only by substance P. The observation that a similar electrical response is produced by different pools of channels implies distinct intracellular pathways leading to KCa current activation.
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