Reductionist approaches to studying exercise metabolism have generated a wealth of knowledge regarding individual organs but, by their nature, have yielded fewer insights into how health benefits are communicated at the global level of an organism. The increased recognition of exerkines (104) — signaling molecules that are released by tissues into the circulation in response to exercise stimuli and impart health effects through local (autocrine or paracrine) or distant (endocrine) means — serves as a paradigm to help better understand the interorgan communication that takes place in response to exercise. While this concept has long been recognized, much of the focus has centered on muscle-secreted factors (myokines), beginning with the identification of interleukin-6 (IL-6) as a metabolic signaling molecule (105, 106). Over the past two decades, emerging high-throughput molecular profiling technologies have further enabled the identification of small-molecule metabolites, lipids, peptides, and nucleic acids that come from adipose tissue (adipokines), liver (hepatokines), bone (osteokines), and the nervous system (neurokines) in addition to myokines (Figure 2). Despite this, the exact tissue sources of many of the exercise-induced plasma biochemicals remain uncertain (107, 108), although efforts are being made to help localize exerkines (109). The topic of exerkines has been reviewed recently (110); however, we further expand upon questions regarding their temporal effects and modes of transport into circulation, before focusing on cardiovascular-specific exerkines.

Exerkines according to primary tissue source. Exercise-induced secreted and bioactive factors according to primary tissue origin. Note that several exerkines have multiple tissue sources.

Temporal effects: acute versus chronic exercise stimuli. An important consideration in the field is the temporal relationship between exercise bouts, training, and exerkine effects. While the acute and chronic effects of exercise-induced transcription at the skeletal muscle level have been well established (54), less is known about circulating proteins and additional signaling molecules. Acute bouts of exercise may lead to substantial changes in circulating proteins, metabolites, or microRNAs; however, these changes may be transient, may or may not lead to changes in the resting levels of individual plasma biochemicals after repetitive bouts of exercise, and, further, may or may not lead to directionally concordant changes (111–113). For example, IL-6 levels increase markedly during acute bouts of exercise, enhancing insulin-stimulated glucose uptake and fat oxidation and promoting an antiinflammatory milieu (106, 114, 115); however, elevated basal (or resting) levels of IL-6 have been widely associated with an increased risk of incident CVD and T2D (116–118), and limited human data suggest that basal levels remain largely unchanged after regular exercise (119, 120). Taken together, these data and others suggest a context-dependent effect of IL-6 in exercise (121), with its pulsatile increases during acute exercise — as opposed to gradual changes in its basal levels — conferring its beneficial effects. Other exerkines, such as cathepsin B (CTSB) and GLPD1, may require increased basal levels after cumulative bouts to subsequently impart physiologic effects (122–124). Ultimately, the kinetic, physiologic effects of individual exerkines are likely to vary among the heterogeneous collection of biochemicals.

Modes of transport. As described above, exerkines represent a diverse group of compounds that enter the circulation through different means. It has long been established that proteins with a secreted signal peptide sequence (“classically” secreted proteins) can be stimulated by exercise and released into the circulation and impart health effects (125). Similarly, small-molecule metabolites — including lactate — are readily transported into circulation by solute carriers (126). In contrast to these more well-established biologic processes, it is increasingly recognized that many proteins are secreted by nonclassical means (127). In particular, extracellular vesicles (EVs) have emerged as an important source of nonclassical protein secretion — as well as other bioactive cargo — in response to exercise. EVs are a heterogeneous (by size) group of lipid membrane–enclosed spheres that house biologic material, including proteins, DNAs, and RNA species, and transport it between cells (128). Recent studies have demonstrated substantial EV responsiveness to acute aerobic and resistance exercise bouts (129–131), highlighted a large number of EV proteins that are not annotated as classically secreted proteins (reviewed in ref. 125), and shown a broad range of EV tissue sources (132). While this topic has been reviewed elsewhere (133, 134), it represents an important reminder that much work remains to be done regarding our understanding of the plasma secretome.

Exerkines and the cardiovascular system. Exercise and increased physical activity provide protection against the development of CVD that extends well beyond that expected from their effects on traditional risk factors (135). Indeed, their effects on nontraditional risk factors such as improvements in autonomic nervous system and skeletal muscle health and endothelial function have been proposed as potentially filling these gaps (136, 137).

The diverse effects of exerkines on metabolic and vascular health coupled with increasing data supporting interorgan crosstalk with the heart (138) support the potential for exerkines to play an important role in mediating cardiovascular wellness. In addition to the well-established roles of nitric oxide and vascular endothelial growth factor in endothelial function and angiogenesis in response to exercise (139–142), we highlight emerging data regarding relevant exerkines and the cardiovascular system (Figure 3). The following cardiovascular-specific exerkines represent a wide range of circulating chemicals, from small-molecule metabolites to immunologic proteins to natriuretic peptide–like muscle-derived hormones.

Cardiovascular-specific exerkines. Exercise-induced secreted and bioactive factors relevant in cardiovascular physiology. Fractalkine, angiopoietin-1, VEGF, and nitric oxide have effects on vascular biology, including the coronary arteries, whereas 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME), IL-6, musclin, and myonectin have direct effects on the myocardium.

12,13-diHOME. 12,13-Dihydroxy-9Z-octadecenoic acid (12,13-diHOME) is an oxidized linoleic acid metabolite secreted by brown adipose tissue (BAT) that was recently shown to increase significantly in male and female human subjects after cold (14°C) exposure as well as in response to an acute bout of moderate-intensity exercise (143, 144). 12,13-diHOME is inversely associated with insulin sensitivity and BMI, and has been shown to enhance fatty acid uptake in both BAT and skeletal muscle. More recently, Pinckard and colleagues (145) extended their previous findings by implicating 12,13-diHOME’s favorable impact on the heart. Through a series of experiments including BAT transplantation in age- and sex-matched C57BL/6 mice, 8 weeks of interval-based treadmill training in mice, acute treatment with and sustained overexpression of 12,13-diHOME using tissue nanotransfection, and both echocardiography and invasive hemodynamics, the investigators showed that 12,13-diHOME led to favorable cardiac remodeling and improved ino- and lusitropy (i.e., cardiac contractility and relaxation) in mice. They further demonstrated that in addition to increasing fatty acid uptake in the heart (as previously shown in skeletal muscle), 12,13-diHOME enhanced cardiomyocyte respiration in a nitric oxide synthase type 1–dependent (NOS1-dependent) manner. And finally, plasma levels of 12,13-diHOME were lower in men and women with heart failure compared with healthy subjects. Taken together, these data support 12,13-diHOME as a lipokine that imparts direct, beneficial effects on the myocardium in an endocrine-like manner.

Interleukin-6. Perhaps the most well-recognized myokine, IL-6 is an important metabolic regulator in skeletal muscle whose exercise effects were briefly described above and have been extensively reviewed elsewhere (106, 121). More recently, a secondary analysis from a randomized controlled trial of a 12-week aerobic exercise training intervention with or without IL-6 antagonism (via monthly injections of tocilizumab) in 52 mostly female (79%) abdominally obese adults showed that IL-6 blockade led to significant attenuation in pericardial fat loss and physiologic hypertrophy as well as increases in epicardial fat mass (146). These data suggest an IL-6–dependent role for favorable cardiac adaptations to aerobic exercise training.

Fractalkine. Fractalkine (CX3CL1) is an additional immunologic protein and unique chemokine that is both membrane-bound and secreted into the circulation. Human studies have shown that acute bouts of endurance exercise lead to significant increases in both skeletal muscle mRNA and protein levels — mainly in endothelial cells — as well as circulating protein levels (147, 148). Fractalkine mediates survival in several cell lines, including monocytes, and has been shown to promote atherosclerotic lesion progression in Apoe–/– mouse models (149, 150). However, its role in atherogenesis may be context-dependent in regard to lesion development, and furthermore, fractalkine mediates smooth muscle cell recruitment and demonstrates antifibrotic effects in a liver injury model as well as tissue healing, all supporting a likely diverse role for fractalkine in vascular biology (151). Thus, additional study of the relationship between exercise-secreted fractalkine and its cardiovascular effects is warranted.

Myonectin. Murine studies of myonectin, a member of the C1q/TNF-related protein (CTRP) family, have shown that its skeletal muscle expression and circulating levels increase in response to regular aerobic exercise (152, 153). Using a combination of a mouse model with global myonectin knockout, a gain-of-function model in skeletal muscle, and in vitro cardiomyocyte studies, Otaka et al. demonstrated a crucial role for myonectin in attenuating ischemia/reperfusion injury through the sphingosine-1-phosphate (S1P)/cAMP/Akt (protein kinase B) signaling pathway in exercise-trained mice (153). More investigation is needed to understand myonectin’s mechanistic role in the myocardium, and whether it mediates cardioprotection in additional pathologic states.

Musclin. Musclin is a secreted peptide with high homology to the natriuretic peptides. In a series of in vivo murine studies and primary myoblast lines, Subbotina et al. (154) showed that skeletal muscle expression and circulating levels of musclin were greatly increased after 5 days of treadmill running (45 min/d) through a Ca+2/Akt/FOXO1 signaling pathway. Using a murine musclin (Ostn) knockout model, the authors then showed that Ostn-knockout mice had reduced exercise tolerance despite exhibiting an otherwise normal resting metabolic phenotype, and that musclin administration “rescued” their exercise capacity. Ostn-knockout mice demonstrated reduced VO2max after 5 days of training, an effect mediated by musclin’s potentiation of the effects of atrial natriuretic peptide (ANP) on cyclic guanosine monophosphate (cGMP)/PGC-1α regulation and subsequent mitochondrial biogenesis (154). More recently, investigators showed that Ostn-knockout mice demonstrate exaggerated cardiac dysfunction in a murine pressure overload (transverse aortic constriction) model, and that AAV6-mediated skeletal muscle musclin overexpression attenuates cardiac dysfunction and myocardial fibrosis in the same model (155). Mechanistically, musclin enhances cardiomyocyte contractility and prevents fibroblast activation through a C-type natriuretic peptide (CNP) signaling pathway. The authors then showed in a small human sample that serum musclin levels were significantly lower in patients with heart failure than in healthy controls. Taken together, these data support the potential of musclin as a relevant exerkine in cardiovascular health.

Angiopoietin-1. Angiopoietin-1 (ANG1) is a secreted member of the angiopoietin/TIE growth factor receptor pathway that mediates vascular protection through inhibition of vascular inflammation and plasma leakage and attenuates fibrosis (156). Its therapeutic potential is highlighted by promising preclinical studies of ANG1 ligands (157). Conflicting and limited data exist regarding the effect of both acute and chronic exercise on ANG1. ANG1 skeletal muscle expression increased significantly in a group of sedentary male human subjects who demonstrated substantial improvements in CRF after 6 weeks of endurance exercise training (158); however, expression levels were mixed in heterogeneous skeletal muscle samples (fast-twitch white and red fibers and slow-twitch fibers) from Sprague-Dawley rats after regular aerobic exercise (159). Similarly, skeletal muscle expression of ANG1 was unchanged in a small (N = 7) cohort of young, sedentary male human participants after acute resistance exercise (160).