Fig. 8. SR Ca²⁺ release is involved in the activation of I_K1 during hypoxia.

A. Strong intracellular Ca²⁺ buffering abolishes activation of I_K1 by CN. Current densities at −60 mV were measured before and after application of 1 mM CN (∼3 min) using different concentrations of Ca²⁺ and types of Ca²⁺ buffer in the pipette solution (n=4, 11 and 6 from left to right). pCa_i values were calculated using the following concentrations of Ca²⁺-binding components and Ca²⁺. pCa²⁺_i ∼5 − 5 mM ATP, 2 mM EGTA 190 μM Ca²⁺; pCa²⁺_i ∼8 − 5 mM ATP, 170 μM Ca²⁺; pCa²⁺_i >12 − 5 mM ATP, 20 mM BAPTA, 0.1 μM Ca²⁺; pH 7.3, T=25 °C. Calculations were performed using WinMaxc programs, http://www.stanford.edu/∼cpatton/maxc.html. B. Removal of extracellular Ca²⁺_o does not affect the activation of I_K1 by CN (n=6). In contrast, depletion of Ca²⁺ stores by extracellular application of 10 mM caffeine (Caff) plus 10 μM ryanodine (Rya) or the inhibition of Ca²⁺ release by 10 μM ruthenium red (RR; included into pipette solution) abolishes I_K1 activation (n=7 and 5, respectively).