A stepwise approach to resolving small ionic currents in vascular tissue

Arterial membrane potential (V_m) is set by an active interplay among ion channels whose principal function is to set contractility through the gating of voltage-operated Ca²⁺ channels. To garner an understanding of this electrical parameter, the activity of each channel must be established under near-physiological conditions, a significant challenge given their small magnitude. The inward rectifying K⁺ (K_IR) channel is illustrative of the problem, as its outward "physiological" component is almost undetectable. This study describes a stepwise approach to dissect small ionic currents at physiological V_m using endothelial and smooth muscle cells freshly isolated from rat cerebral arteries. We highlight three critical steps, beginning with the voltage clamping of vascular cells bathed in physiological solutions while maintaining a giga-ohm seal. K_IR channels are then inhibited (micromolar Ba²⁺) so that a difference current can be created, once Ba²⁺ traces are corrected for the changing seal resistance and subtle instrument drift, pulling the reversal potential rightward. The latter is a new procedure and entails the alignment of whole cell current traces at a voltage where K_IR is silent and other channels exhibit limited activity. We subsequently introduced corrected and uncorrected currents into computer models of the arterial wall to show how these subtle adjustments markedly impact the importance of K_IR in V_m and arterial tone regulation. We argue that this refined approach can be used on an array of vascular ion channels to build a complete picture of how they dynamically interact to set arterial tone in key organs like the brain.NEW & NOTEWORTHY This work describes a stepwise approach to resolve small ionic currents involved in controlling V_m in resistance arteries. Using this new methodology, we particularly resolved the outward component of the K_IR current in native vascular cells, voltage clamped in near-physiological conditions. This novel approach can be applied to any other vascular currents and used to better interpret how vascular ion channels cooperate to control arterial tone.