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⁺ (K_Ca) currents underlying the endothelial cell hyperpolarization.

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.

Iberiotoxin (50 nM) reduced by about 40 % the K⁺ current activated by bradykinin without modifying the substance P response. Conversely, apamin (1 μm) inhibited the substance P-induced K⁺ current by about 65 %, without affecting the bradykinin response. Similar results were obtained on peptide-induced membrane hyperpolarization.

Bradykinin-induced, but not substance P-induced, endothelium-dependent relaxation resistant to N^G-nitro-L-arginine and indomethacin was partly inhibited by 3 μm 17-octadecynoic acid (17-ODYA), an inhibitor of cytochrome P450 epoxygenase. Similarly, the bradykinin-induced K⁺ current was reduced by 17-ODYA.

Our results show that responses to substance P and bradykinin result in a hyperpolarization due to activation of different K_Ca currents. A current consistent with the activation of large conductance (BK_Ca) channels was activated only by bradykinin, whereas a current consistent with the activation of small conductance (SK_Ca) 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 K_Ca current activation.