Apelin is a peptide secreted from various tissues and has been classically characterized as an adipokine, though it has recently been described as a myokine (Besse-Patin et al., 2014; Kleinz and Davenport, 2005). Apelin is purported to assist in regulating glucose homeostasis (Dray et al., 2010; Dray et al., 2008) and plays an integral role in insulin sensitivity in mice (Yue et al., 2010). Additionally, apelin was noted to improve peripheral glucose uptake in normal and insulin-resistant mice (Dray et al., 2008) and apelin injections during a hyperinsulinemic-euglycemic clamp enhanced glucose disposal (Dray et al., 2010). Apelin-stimulated glucose uptake was confirmed in isolated adipocytes from healthy (Attane et al., 2011; Than et al., 2014) and type 2 diabetic (T2D) subjects (Dray et al., 2010). These results support apelin's role as an exogenous insulin sensitizer under hyperinsulinemic conditions. However, apelin's effect under normal insulinemia and its role as an endogenous glucose regulator in healthy individuals during and following exercise has not been determined and requires further clarity (Alexiadou et al., 2012).
Apelin is chiefly influenced by insulin, hypoxia and adiposity, among other factors (Kleinz and Davenport, 2005). Insulin is considered the prime regulator of apelin stimulating its synthesis and release (Boucher et al., 2005). Conversely, apelin exerts a depressive effect on insulin, acting via G proteins in islet cells (Ringstrom et al., 2010). Acute aerobic exercise, as a result of repeated muscular contractions, stimulates insulin-independent glucose uptake via GLUT translocation in skeletal muscle, and subsequently confers both peripheral and hepatic sensitization to insulin that typically lasts hours to days (Henriksen, 2002; Holloszy, 2005). This improved sensitization to insulin after acute aerobic exercise may be partially attributable to apelin. Previous studies concerning exercise and apelin have focused on apelin's response to training in obese and T2Ds (Besse-Patin et al., 2014; Krist et al., 2013), populations known to exhibit considerable variability in basal apelin concentration due to a variety of contributing factors (Alexiadou et al., 2012, Castan-Laurell et al., 2012, Castan-Laurell et al., 2008, Cavallo et al., 2012, Dray et al., 2010, Krist et al., 2013).
Enhanced contraction-induced glucose uptake lasts up to two hours post-exercise, however, improved insulin sensitivity may last longer (Borghouts and Keizer, 2000; Holloszy, 2005). The benefits of insulin sensitivity improvements are transient and return to normal 12-48 hours post-exercise (Borghouts and Keizer, 2000; Christiansen et al., 2010; Fontana et al., 2010; Holloszy, 2005; Magkos et al., 2010). Endurance training studies have reported insulin sensitivity reversal after cessation of acute exercise (Heath et al., 1983; LeBlanc et al., 1981). Thus, insulin sensitivity improvements appear to be facilitated primarily by single exercise bouts (Goodyear and Kahn, 1998). Given this, acute aerobic exercise represents an excellent model by which the effect of apelin upon insulin sensitivity can be assessed. High-intensity aerobic exercise (e.g. V[O.sub.2]max) demonstrably elevates blood glucose and insulin, while moderate-intensity aerobic exercise maintains blood glucose and insulin (Marliss and Vranic, 2002); thus, these represent two exercise conditions with readily reproducible, disparate outcomes. Therefore, the primary aim of this study was to determine if plasma apelin concentration is altered by either bout of acute aerobic exercise or whether this is related to insulin-mediated sensitization in a healthy adult population.
The IRB of UNC Greensboro approved this study and all participants signed informed consent on their first visit. Twelve (n = 7 male; n = 5 female) apparently healthy subjects (22.8 [+ or -] 2.9 years), non-tobacco users and not taking medications or supplements for at least 6 months that could alter metabolism, oxidation status, blood glucose and/or insulin, were recruited. Subjects were instructed to maintain their normal diet throughout the study and dietary logs were reviewed to ensure compliance.
Volunteers (18-35 yrs) were screened for medical, metabolic, cardiovascular and activity factors, per American Heart Association (AHA) and American College of Sports Medicine (ACSM) guidelines (Pescatello, 2014). Obese, hypertensive or pregnant participants were excluded. Resting heart rate (HR), blood pressures (BP), % body fat (%BF) (7-site determination via Siri equation, Jackson and Pollock, 1978), weight and height were determined. Once all criteria were satisfied participants became enrolled subjects. Subjects were provided food logs to record diet intake three days prior to each visit, and for the 24-hour period post-treatment prior to the fourth blood draw time point.
Maximal and submaximal exercise and negative control condition procedures
Subjects reported to the laboratory (6-9 A.M.) on three different days to complete two different exercise conditions (graded to maximal [V[O.sub.2]max] and steady state [70-75% V[O.sub.2]max] aerobic exercise on treadmill) and a negative control glucose challenge (GC) of 50g, in a post-absorptive state (10-12 hrs) and after having not exercised for >24 hrs. Each condition was separated by at least three days, but not more than 14 days and subjects returned 24 hrs after each condition. The first visit was randomized, either V[O.sub.2]max or GC, with the 30-minute bout (70-75% V[O.sub.2]max) necessarily performed after V[O.sub.2]max (2nnd or 3rd visit).
Maximal treadmill graded exercise
Subjects completed a warm-up (5 min) on the treadmill until HR rose to 130 bpm. The test starting speed ranged between 5.5 and 8 mph amongst all subjects. Thereafter, both grade (by 2.5%) and speed (by 0.5 mph) increased each minute until reaching V[O.sub.2]max or volitional fatigue. Expired gases were collected and analyzed by a Parvo Medics True One 2400 analyzer system (Parvo Medics, Sandy, UT, USA) calibrated to known gases. Ratings of perceived exertion (RPE) were recorded each minute (Noble et al., 1983). All subjects satisfied at least four of five required criteria for a true V[O.sub.2]max (Midgley et al, 2007; Pescatello, 2014). Subjects were given water before and after the graded to maximal exercise trial and encouraged to drink water ad libitum (up to 1 L) in the 1-hr recovery period.
Submaximal treadmill exercise
Subjects completed a warm-up (3-5 min) on the treadmill. HR was continually monitored during the 70-75% V[O.sub.2]max exercise (Polar Electro Inc., Bethpage, NY, USA) and recorded every 5 minutes...