A new method to detect rapid oxygen changes around cells: How quickly do calcium channels sense oxygen in cardiomyocytes?

摘要

Acute hypoxia is thought to trigger protective responses that, in tissues like heart and carotid body, include rapid (5–10 s) suppression of Ca²⁺ and K⁺ channels. To gain insight into the mechanism for the suppression of the cardiac l-type Ca²⁺ channel, we measured O2-dependent fluorescence in the immediate vicinity of voltage-clamped cardiac cells subjected to rapid exchange of solutions with different O2 tensions. This was accomplished with an experimental chamber with a glass bottom that was used as a light guide for excitation of a thin ruthenium-based O2-sensitive ORMOSIL coating. Fluorescence imaging showed that steady-state Po2 was well controlled within the entire stream from an electromagnetically controlled solution “puffer” but that changes were slower at the periphery of the stream (τ1/2 ∼ 500 ms) than immediately around the voltage-clamped myocyte (τ1/2 ∼ 225 ms) where, in turn, firmly attached cells produced an additional local delay of 50–100 ms. Performing simultaneous voltage clamp and O2 measurements, we found that acute hypoxia gradually and reversibly suppressed the Ca²⁺ channel (CaV1.2). Using Ba²⁺ as charge carrier, the suppression was significant after 1.5 s, reached ∼10% after 2.5 s, and was nearly completely reversible in 5 s. The described fluorescence measurements provide the means to check and fine tune solution puffers and suggest that changes in Po2 can be accomplished within ∼200 ms. The rapid and reversible suppression of barium current under hypoxia is consistent with the notion that the cardiac Ca²⁺ channel is directly modulated by O2.