We compared the effects of isocapnic hypoxia (IHO) and hyperoxic hypercapnia (HC) on sympathetic nerve activity (SNA) recorded from a peroneal nerve in 13 normal subjects. HC caused greater increases in blood pressure (BP), minute ventilation (VE), and SNA [53 +/- 14% (SE) during HC vs. 21 +/- 7% during IHO; P less than 0.05]. Even at equivalent levels of VE, HC still elicited greater SNA than IHO. However, apnea during HC caused a lesser (P less than 0.05) increase in SNA (91 +/- 26% compared with apnea on room air) than apnea during IHO (173 +/- 50%). Hypercapnic hypoxia resulted in a greater absolute increase in VE (23.6 +/- 2.8 l/min) than the additive increases due to HC alone plus IHO alone (18.0 +/- 1.8 l/min, P less than 0.05). SNA also increased synergistically by 108 +/- 23% with the combined stimulus compared with the additive effect of HC alone plus IHO alone (68 +/- 19%; P less than 0.05). We conclude that 1) HC causes greater increases in VE and SNA than does hypoxia; 2) for the same increase in VE, hypercapnia still causes a greater increase in SNA than hypoxia; however, during apnea, hypoxia causes a much greater increase in SNA than hypercapnia; 3) the inhibitory influence of ventilation on SNA is greater during hypoxia (i.e., predominantly peripheral chemoreceptor stimulation) than hypercapnia (i.e., predominantly central chemoreceptor stimulation); and 4) combined hypoxia and hypercapnia have a synergistic effect on SNA as well as on VE.

Activation of stretch-sensitive baroreceptor neurons exerts acute control over heart rate and blood pressure. Although this homeostatic baroreflex has been described for over 80 years, the molecular identity of baroreceptor mechanosensitivity remains unknown. We discovered that mechanically activated ion channels PIEZO1 and PIEZO2 are together required for baroreception. Genetic ablation of both Piezo1 and Piezo2 in the nodose and petrosal sensory ganglia abolished drug-induced baroreflex and aortic depressor nerve activity. Awake, behaving animals that lack Piezos had labile hypertension and increased blood pressure variability, consistent with phenotypes in baroreceptor-denervated animals and humans with baroreflex failure. Optogenetic activation of Piezo2+ sensory afferents was sufficient to initiate baroreflex in mice. These findings suggest that PIEZO1 and PIEZO2 are the long-sought baroreceptor mechanosensors critical for acute blood-pressure control.

The sympathetic response to hypoxia depends on the interaction between chemoreceptor stimulation (CRS) and the associated hyperventilation. We studied this interaction by measuring sympathetic nerve activity (SNA) to muscle in 13 normal subjects, while breathing room air, 14% O2, 10% O2, and 10% O2 with added CO2 to maintain isocapnia. Minute ventilation (VE) and blood pressure (BP) increased significantly more during isocapnic hypoxia (IHO) than hypocapnic hypoxia (HHO). In contrast, SNA increased more during HHO [40 +/- 10% (SE)] than during IHO (25 +/- 19%, P less than 0.05). To determine the reason for the lesser increase in SNA with IHO, 11 subjects underwent voluntary apnea during HHO and IHO. Apnea potentiated the SNA responses to IHO more than to HHO. SNA responses to IHO were 17 +/- 7% during breathing and 173 +/- 47% during apnea whereas SNA responses to HHO were 35 +/- 8% during breathing and 126 +/- 28% during apnea. During ventilation, the sympathoexcitation of IHO (compared with HHO) is suppressed, possibly for two reasons: 1) because of the inhibitory influence of activation of pulmonary afferents as a result of a greater increase in VE, and 2) because of the inhibitory influence of baroreceptor activation due to a greater rise in BP. Thus in humans, the ventilatory response to chemoreceptor stimulation predominates and restrains the sympathetic response. The SNA response to chemoreceptor stimulation represents the net effect of the excitatory influence of the chemoreflex and the inhibitory influence of pulmonary afferents and baroreceptor afferents.

Randomized clinical trials initially used heart failure (HF) patients with low left ventricular ejection fraction (LVEF) to select study populations with high risk to enhance statistical power. However, this use of LVEF in clinical trials has led to oversimplification of the scientific view of a complex syndrome. Descriptive terms such as ‘HFrEF’ (HF with reduced LVEF), ‘HFpEF’ (HF with preserved LVEF), and more recently ‘HFmrEF’ (HF with mid-range LVEF), assigned on arbitrary LVEF cut-off points, have gradually arisen as separate diseases, implying distinct pathophysiologies. In this article, based on pathophysiological reasoning, we challenge the paradigm of classifying HF according to LVEF. Instead, we propose that HF is a heterogeneous syndrome in which disease progression is associated with a dynamic evolution of functional and structural changes leading to unique disease trajectories creating a spectrum of phenotypes with overlapping and distinct characteristics. Moreover, we argue that by recognizing the spectral nature of the disease a novel stratification will arise from new technologies and scientific insights that will shape the design of future trials based on deeper understanding beyond the LVEF construct alone.

We determined whether angiotensin II (ANG II) modulates the arterial baroreflex control of lumbar sympathetic nerve activity (LSNA) in chloralose-anesthetized rabbits. Intravenous infusion (iv) of ANG II caused significantly less reflex bradycardia and less inhibition of LSNA than iv phenylephrine (PE) for equivalent increments in arterial pressure. During a background iv infusion of ANG II, which caused a small sustained increase in arterial pressure, the reflex inhibition of heart rate (HR) and LSNA in response to further increases in pressure with graded doses of PE was attenuated, but the reflex increase in HR and LSNA in response to hypotension with graded doses of nitroprusside was unchanged. This modulation of the baroreflex by ANG II is specific since a similar background infusion of PE did not alter baroreflex responses to further increases or to decreases in arterial pressure. The frequency of aortic baroreceptors was comparable for equivalent increases in pressure caused by iv ANG II or PE. When ANG II was confined to the isolated carotid sinuses, the reflex inhibition of HR and LSNA during distension of carotid sinuses was unchanged. An excitatory effect of ANG II on the efferent limb of the baroreflex that would oppose the reflex bradycardia or inhibition of LSNA is unlikely because when the pressor effect of ANG II was prevented by nitroprusside, there were no changes in HR and LSNA. We conclude that through an effect on the central nervous system iv ANG II has a selective effect on the arterial baroreflex; it impairs reflex decreases in HR and LSNA during hypertension but not reflex increases in HR and LSNA during hypotension.