Concomitant sympathetic nerve stimulation increased the asymptotic low-frequency gain of the vagal heart rate transfer function during high-intensity vagal stimulation (7.39 vs 4.77 beats/min/Hz; P=0.011).
Does concomitant sympathetic nerve stimulation augment the dynamic gain of the vagal heart rate transfer function during different levels of vagal nerve stimulation in anesthetized rats?
Background sympathetic tone enables finer heart rate control by increasing the dynamic gain of the vagal heart rate transfer function regardless of vagal nerve stimulation intensity.
Absolute Event Rate: 7.39% vs 4.77%
p-value: p=0.011
Accentuated antagonism refers to a phenomenon in which the vagal effect on heart rate (HR) is augmented by background sympathetic tone. The dynamic aspect of accentuated antagonism remains to be elucidated during different levels of vagal nerve stimulation (VNS) intensity. We performed VNS on anesthetized rats ( n = 8) according to a binary white noise signal with a switching interval of 500 ms at three different stimulation rates (low-intensity: 0–10 Hz, moderate-intensity: 0–20 Hz, and high-intensity: 0–40 Hz). The transfer function from VNS to HR was estimated with and without concomitant tonic sympathetic nerve stimulation (SNS) at 5 Hz. The asymptotic low-frequency (LF) gain (in beats/min/Hz) of the transfer function increased with SNS regardless of the VNS rate low-intensity: 3.93 ± 0.70 vs. 5.82 ± 0.65 ( P = 0.021), moderate-intensity: 3.87 ± 0.62 vs. 5.36 ± 0.53 ( P = 0.018), high-intensity: 4.77 ± 0.85 vs. 7.39 ± 1.36 ( P = 0.011). Moreover, SNS slightly increased the ratio of high-frequency (HF) gain to the LF gain. These effects of SNS were canceled by the pretreatment of ivabradine, an inhibitor of hyperpolarization-activated cyclic nucleotide-gated channels, in another group of rats ( n = 6). Although background sympathetic tone antagonizes the vagal effect on mean HR, it enables finer HR control by increasing the dynamic gain of the vagal HR transfer function regardless of VNS intensity. When interpreting the HF component of HR variability, the augmenting effect from background sympathetic tone needs to be considered.
Kawada et al. (2022) studied this question. Concomitant tonic sympathetic nerve stimulation (SNS) vs. Without concomitant tonic sympathetic nerve stimulation was evaluated on Asymptotic low-frequency (LF) gain of the transfer function from VNS to HR (p=0.011). Concomitant sympathetic nerve stimulation increased the asymptotic low-frequency gain of the vagal heart rate transfer function during high-intensity vagal stimulation (7.39 vs 4.77 beats/min/Hz; P=0.011).