Regulation of Spontaneous Transient Outward Potassium Currents in Human Coronary Arteries

Abstract

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 μmol/L Cd 2+ and 10 μmol/L verapamil, which block voltage-dependent Ca 2+ channels. STOCs were inhibited by iberiotoxin (100 nmol/L), a selective blocker of Ca 2+ -activated potassium channels (BK Ca ), and disappeared in a Ca 2+ -free bath. Iberiotoxin depolarized the VSMCs within 20 minutes from −44±7 to −18±5 mV (n=17). The Ca 2+ ionophore A23187 increased intracellular Ca 2+ and stimulated whole-cell BK Ca current. Depletion of Ca 2+ from the sarcoplasmic reticulum with caffeine (4 mmol/L) abolished STOCs for several minutes. Ryanodine (50 μmol/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 μmol/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 Ca 2+ to zero or chelating Ca 2+ intracellularly with BAPTA-AM (30 μmol/L). Conclusions STOCs resulted from BK Ca activity and were dependent on extracellular Ca 2+ but not significantly on voltage-dependent Ca 2+ channels. STOCs were dependent on intracellular Na + and intracellular calcium store refilling state. We suggest that Ca 2+ entry into the cell through reverse-mode Na + /Ca 2+ exchange determines calcium store refilling, which in turn regulates STOC generation in human coronary VSMCs.