BPS2026 – Sex differences in sinoatrial node function and drug response

The sinoatrial node (SAN) is the primary pacemaker of the heart, and its automaticity is regulated by a coupled-clock system integrating membrane ion currents and rhythmic Ca 2+ releases from the sarcoplasmic reticulum. These processes are coordinated by Ca 2+ signaling, which not only tunes pacemaker activity but also drives metabolic regulation. Cytosolic Ca 2+ uptake through the mitochondrial calcium uniporter (MCU) complex and its regulators (MICU1/2) stimulates oxidative phosphorylation and ATP production, providing energetic support required for SAN function, particularly during stress-induced responses. Previous studies reported no sex differences in basal SAN activity, yet males exhibit greater stress-induced heart rate increases and higher heart rate variability (HRV). The molecular and cellular mechanisms underlying these sex-specific differences remain poorly understood. SAN tissues from adult male and female C57BL/6JOlaHsd mice were isolated; Ca 2+ dynamics were assessed using Fluo-4 AM imaging, PKA activity was monitored with adenoviral AKAR4 FRET, and proteomic profiles were obtained by LC-MS/MS analyzed with DIA-NN and Perseus. At baseline, the beat interval (BI) did not show a significant difference between male (445.4 ± 88.5 ms) and female (376.9 ± 80.7 ms) tissues. Activation of adenylate cyclase with forskolin (20 μM) shortened the BI by 26.0 ± 6.9% in males but only 10.9 ± 6.6% in females, a trend also observed with the phosphodiesterase inhibitor IBMX (100 μM). PKA activity measurements revealed greater forskolin-induced activation in males (13.8 ± 9.9%) compared to females (7.9 ± 4.5%). Inhibition of PKA with H89 (10 μM) prolonged the BI and reduced PKA activity similarly in both sexes. Proteomics analysis revealed sex-dependent differences in proteins associated with mitochondrial Ca 2+ uptake and bioenergetics, including MCU complex regulators, and blocking. These findings demonstrate sex-dependent differences in SAN responses to cAMP/PKA-modulating drugs. The results suggest that mitochondrial Ca 2+ handling and energy balance contribute to the observed disparities in pacemaker regulation.