Regulation of spontaneous transient outward potassium currents in human coronary arteries

Background Spontaneous transient outward potassium currents (STOCs) induce myogenic relaxation in small cerebral vessels. We found STOCs in human coronary artery vascular smooth muscle cells (VSMCs) and studied their regulation. Methods and results K+ currents were recorded in human coronary VSMCs by current- and voltage-clamp techniques. STOCs were recorded in the presence of 200 mu mol/L Cd2+ and 10 mu mol/L verapamil, which block voltage-dependent Ca2+ channels. STOCs were inhibited by iberiotoxin (100 nmol/L), a selective blocker of Ca2+-activated potassium channels (BKCa), and disappeared in a Ca2+-free bath. Iberiotoxin depolarized the VSMCs within 20 minutes from -44+/-7 to -18+/-5 mV (n=17). The Ca2+ ionophore A23187 increased intracellular Ca2+ and stimulated whole-cell BKCa current. Depletion of Ca2+ from the sarcoplasmic reticulum with caffeine (4 mmol/L) abolished STOCs for several minutes. Ryanodine (50 mu mol/L) transiently stimulated STOCs but then completely inhibited STOCs within 10 minutes. The firing frequency of STOCs was directly correlated with intracellular Na+ concentrations from 0 to 24 mmol/L. Lowering intracellular Na+ to zero abolished STOCs. We next gave monensin (30 mu mol/L) to increase intracellular Na+. This maneuver resulted in an increase in whole-cell current fluctuations and STOCs. Monensin-induced STOCs were abolished by either lowering extracellular Ca2+ to zero or chelating Ca2+ intracellularly with BAPTA-AM (30 mu mol/L). Conclusions STOCs resulted from BKCa activity and were dependent on extracellular Ca2+ bur not significantly on voltage-dependent Ca2+ channels. STOCs were dependent on intracellular Na+ and intracellular calcium store refilling state. We suggest that Ca2+ entry into the cell through reverse-mode Na+/Ca2+ exchange determines calcium store refilling, which in turn regulates STOC generation in human coronary VSMCs.