Intracellular Ca2+-Mediated Mechanisms for the Pacemaker Depolarization of the Mouse and Guinea Pig Sinus Node Tissue

Abstract

Intracellular Ca²⁺-mediated mechanisms for pacemaker depolarization were studied in sinus node tissue preparations from mice and guinea pigs. Microelectrode recordings revealed that the sinus node of the mouse, which had a higher beating rate, had a steeper slope of the pacemaker depolarization than that of the guinea pig. BAPTA and ryanodine, agents that interfere with intracellular Ca²⁺, significantly decreased the slope of the pacemaker depolarization in both species. In contrast, SEA0400, a specific inhibitor of the Na⁺-Ca²⁺ exchanger (NCX), as well as change to low Na⁺ extracellular solution, significantly decreased the slope in the mouse, but not in the guinea pig. Niflumic acid, a blocker of the Ca²⁺ activated Cl^- channel, decreased the slope in both species. Confocal microscopy revealed the presence of spontaneous Ca²⁺ oscillations during the interval between Ca²⁺ transients; such phenomenon was more pronounced in the mouse than in the guinea pig. Thus, although intracellular Ca²⁺-mediated mechanisms were involved in the pacemaker depolarization of the sinus node in both species, the NCX current was involved in the mouse but not in the guinea pig.

Keywords: Na+-Ca2+ exchanger; cardiac pacemaking; pacemaker depolarization; sinus node.

Figure 1

Effects of BAPTA and ryanodine on the sinus node action potential of the mouse and guinea pig. Typical traces before (black lines) and after (red lines) application of 300 μM BAPTA (upper) and 0.1 μM ryanodine (lower).

Figure 2

Effect of SEA0400 on the sinus node action potential of the mouse and guinea pig. Typical traces before (black lines) and after (red lines) application of 1 μM (upper) and 10 μM (lower).

Figure 3

Effect of low Na⁺ solution on the sinus node action potential of the mouse and guinea pig. Typical traces before (black lines) and after (red lines) rapid change of the extracellular solution to low Na⁺ solution.

Figure 4

Effect of niflumic acid on the sinus node action potential of the mouse and guinea pig. Typical traces before (black lines) and after (red lines) application of 30 μM niflumic acid.

Figure 5

Effect of SEA0400 on spontaneously occurring Ca²⁺ transients in the mouse sinus node tissue. (A) Typical x-y images of the myocardium loaded with Cal-590 before (a–c) and 10 min after the addition of 1μM SEA0400 (d–f). Panels c and f are the images at the peak of the Ca²⁺ transient. Note that Ca²⁺ oscillations were observed during the interval between the Ca²⁺ transients (arrows). (B) Time course of the changes in fluorescence before addition (a–c) and 10 min after the addition of 1 μM SEA0400 (d–f). Time course of the fluorescence intensity quantified in the whole field of view (a,d), cell 1 (b,e), and cell 2 (c,f) as shown in panel (A)/(c).

Figure 6

Effect of SEA0400 on spontaneously occurring Ca²⁺ transients in the guinea pig sinus node tissue. (A) Typical x-y images of the myocardium loaded with Cal-590 before (a–c) and 10 min after the addition of 1μM SEA0400 (d–f). Panels c and f are the images at the peak of the Ca²⁺ transient. Note that no Ca²⁺ oscillation was observed during the interval between the Ca²⁺ transients (a,b,d,e). (B) Time course of the changes in fluorescence before addition (a–c) and 10 min after the addition of 1 μM SEA0400 (d–f). Time course of the fluorescence intensity quantified in the whole field of view (a,d), cell 1 (b,e), and cell 2 (c,f) as shown in panel (A)/(c).