Hypoxia inhibits transients KCa currents to limit cerebral artery dilation

Abstract

In brain, a reduction in dissolved oxygen partial pressure (PO2) leads to vasodilation, but the mechanism is unclear. Here, we investigated the function of cerebral artery smooth muscle cell large-conductance Ca2+-activated K+ (KCa) channels in hypoxic vasodilation. In arterial smooth muscle cells, several KCa channels are activated by localized intracellular Ca2+ transients termed “Ca2+ sparks”, resulting in a transient KCa current. Transient KCa current activation leads to membrane hyperpolarization, a reduction in voltage-dependent Ca2+ channel activity, and dilation. In voltage-clamped (−40 mV) cells, a decrease in PO2 from 150 to 25 mmHg (hypoxia) reduced transient KCa current frequency from 0.76±0.18 to 0.39±0.11 Hz and amplitude from 20±2 to 16±2 pA. In cells in which Ca2+ sparks were blocked with thapsigargin (100 nM), a SR Ca2+-ATPase blocker, hypoxia also reduced KCa channel activity (NPo) from 0.14±0.04 to 0.08±0.025. Hypoxia did not alter the amplitude of caffeine (10 mM)-induced intracellular Ca2+ transients, indicating that Ca2+ load of the sarcoplasmic reticulum, the Ca2+ spark source, did not change. In pressurized cerebral arteries, hypoxia caused similar amplitude dilations in control (45±9 μm) and in the presence of thapsigargin (100 nM, 40±12 μm). Although thapsigargin constricted cerebral arteries by ~16 μm in normoxia, thapsigargin did not alter diameter when applied in hypoxia. Data suggest hypoxic inhibition of smooth muscle cell transient KCa currents is a negative-feedback mechanism that limits hypoxic dilation.