Abstract
Background
Trimeric intracellular cation (TRIC) channels are expressed on the surface of the sarcoplasmic reticulum and compensate for calcium release from ryanodine receptors. Tric-a knock-out (KO) mice showed diminished calcium release from ryanodine receptors in vascular smooth muscle cells. The cardiac pacemaker is controlled by the surface membrane and intracellular calcium clocks. In spontaneously firing sinus node action potentials, the membrane and calcium clocks work together via numerous interactions modulated by membrane voltage, intracellular calcium release, and protein phosphorylation. Intracellular calcium changes modulate cardiac pacemaking in the sinus node, but the physiological importance of TRIC channels in cardiac rhythm formation is still obscure.
Purpose
In this study, we aimed to clarify the importance of TRIC channels on cardiac pacemaking using Tric-a KO mice.
Methods
The expression level of mRNA and proteins in the sinus node was examined by RT-PCR and immunoblotting. Systolic blood pressure was measured with tail-cuff method. Heart rate was measured by ECG, and heart rate variability was examined. The atrial contractile force from isolated hearts was measured with a force transducer. Cardiac action potential and spontaneous sinus rate from isolated hearts were measured with a microelectrode. Isoproterenol was used for sympathetic nerve manipulation.
Results
Tric-a KO heart showed increased adrenergic β1-receptor expression in immunoblotting. Although there was no significant difference in basal systolic blood pressure between Tric-a KO and wild type (WT) mice, basal heart rate in Tric-a KO mice was significantly lower than that in WT mice (660±10 and 698±10 bpm, n=15 and 19, Tric-a KO mice and WT mice, respectively, p=0.017). Tric-a KO mice showed limited heart rate changes to isoproterenol (24±6 and 99±15 bpm, n=9 and 10, Tric-a KO mice and WT mice, respectively, p<0.001). In the action potential recordings, Tric-a KO atria showed only limited sinus rate changes to isoproterenol (35±9 and 71±10 bpm, n=8 and 6, Tric-a KO mice and WT mice, respectively, p=0.038). WT mice and Tric-a KO mice atrial contractile force showed dose-dependent changes in response to isoproterenol (10–100 nM), but Tric-a KO mice atria showed limited contractile force changes to isoproterenol (116 and 169%, n=7 and 6, Tric-a KO mice and WT mice, respectively, p<0.01). In heart rate variability, Tric-a KO mice showed unstable RR intervals and longer standard deviation of RR intervals than WT mice.
Conclusion
Tric-a KO mice showed decreased cardiac pacemaking in the sinus node and attenuated responses to beta-adrenergic stimulus, which indicates the involvement of TRIC channels in cardiac rhythm formation and sympathetic nerve regulation.
