The effect of raised pH on pacemaker activity and ionic currents in cardiac purkinje fibers

Abstract

• 1. Cardiac Purkinje fibers exposed to alkaline solutions (pH 8 to 9.5) show an increase in rate of diastolic depolarization, eventually resulting in induction of spontaneous activity or an increase of the spontaneous firing rate.

• 2. The voltage-clamp analysis of the transmembrane currents in pH 9–9.5 shows: i) a shift in the depolarizing direction of the activation (s∞) and time constants (τ _s ) curve of the$i_{{\text{K}}_{\text{2}} }$ current, ii) a small increase in the maximal value of the fully activated$i_{{\text{K}}_{\text{2}} }$ current, iii) no significant change of background current in the pacemaker region of membrane potentials.

• 3. The effect of NH₄Cl was studied as a means to vary intracellular pH. In the presence of Tris buffer the addition of 5 mM NH₄Cl resulted in i) a shift in the depolarizing direction and a decrease in slope of the activation curve of the$i_{{\text{K}}_{\text{2}} }$ current, ii) a shift in the depolarizing direction of the time-constants curve together with an increase in the maximum value of τ _s , iii) an increase in the maximum value of the fully activated$i_{{\text{K}}_{\text{2}} }$ current and a depolarizing shift of the reversal potential, similar to the effect of addition of K_c. In the presence of CO₂−HCO₃ buffer the addition of NH₄Cl had no significant effect on the kinetics of the$i_{{\text{K}}_{\text{2}} }$ current. Since intracellular pH is only affected by NH₄Cl in the presence of Tris buffer, the results suggest that intracellular alkalinization is responsible for the change in$i_{{\text{K}}_{\text{2}} }$ kinetics.

• 4. Based on the findings with NH₄Cl it is suggested that perfusion with Tris buffered alkaline solutions not only affects net negative surface charges on the outside but also and to a larger extent increases negative surface charges on the inside of the cell membrane.