Figure 6.

Vascular K_IR channels and their currents. Panel A shows the topology of K_IR channels with two membrane spanning domains. Functional channels are composed of a tetramer these subunits (see Figure 2). Panel B shows a schematic of the current-voltage-relationship for VSM K_IR channels for a cell with 5 mM K⁺ in the extracellular solution (140 mM K⁺ intracellular) and is based on data from (Filosa et al., 2006). At membrane potentials more negative than the K⁺ equilibrium potential (E_K, ~−90 mV in 5 mM K⁺), the channel conducts K⁺ into the cells, as shown by the negative current density values. At potentials more positive than E_K up to ~−30 mV, K_IR channels conduct K⁺ ions out of the cell, and contribute to the resting membrane potential as denoted by the small positive currents at the assumed resting membrane potential of −35 mV. Note that anything that hyperpolarizes the membrane will recruit outward, positive current through K_IR channels, effectively amplifying the initial hyperpolarization. Panel C demonstrates the effects of increasing extracellular K⁺ from 5 mM (red dashed curve) to 15 mM K⁺ (green solid curve): increased extracellular K⁺ shifts the E_K from −90 mV to ~−60 mV. Note that there is now an elevated outward K⁺ current at the original resting membrane potential. This enhanced outward K⁺ current will hyperpolarize the VSM cell membrane from its resting value toward E_K, leading to vasodilation.