Effect of aging on changes in plasma potassium during exercise

Invasive Assessment of Hemodynamic, Metabolic and Ionic Consequences During Blood Flow Restriction Training

Purpose: Medically recommended training often faces the dilemma that necessary mechanical intensities for muscle adaptations exceed patients' physical capacity. In this regard, blood flow restriction (BFR) training is becoming increasingly popular because it enables gains in muscle mass and strength despite using low-mechanical loads combined with external venous occlusion. Since the underlying mechanisms are still unknown, we applied invasive measurements during exercise with and without BFR to promote physiological understanding and safety of this popular training technique.

Methods: In a randomized cross-over design, ten healthy men (28.1 ± 6.5 years) underwent two trials of unilateral biceps curls either with (BFR) and without BFR (CON). For analysis of changes in intravascular pressures, blood gases, oximetry and electrolytes, an arterial and a venous catheter were placed at the exercising arm before exercise. Arterial and venous blood gases and intravascular pressures were analyzed before, during and 5 min after exercise.

Results: Intravascular pressures in the arterial and venous system were more increased during exercise with BFR compared to CON (p < 0.001). Furthermore, arterial and venous blood gas analyses revealed a BFR-induced metabolic acidosis (p < 0.05) with increased lactate production (p < 0.05) and associated elevations in [K⁺], [Ca²⁺] and [Na⁺] (p < 0.001).

Conclusion: The present study describes for the first time the local physiological changes during BFR training. While BFR causes greater hypertension in the arterial and venous system of the exercising extremity, observed electrolyte shifts corroborate a local metabolic acidosis with concurrent rises in [K⁺] and [Na⁺]. Although BFR could be a promising new training concept for medical application, its execution is associated with comprehensive physiological challenges.

Intense interval training in healthy older adults increases skeletal muscle [3H]ouabain-binding site content and elevates Na+,K+-ATPase α2 isoform abundance in Type II fibers

Young adults typically adapt to intense exercise training with an increased skeletal muscle Na⁺,K⁺-ATPase (NKA) content, concomitant with reduced extracellular potassium concentration [K⁺] during exercise and enhanced exercise performance. Whether these changes with longitudinal training occur in older adults is unknown and was investigated here. Fifteen older adults (69.4 ± 3.5 years, mean ± SD) were randomized to either 12 weeks of intense interval training (4 × 4 min at 90-95% peak heart rate), 3 days/week (IIT, n = 8); or no exercise controls (n = 7). Before and after training, participants completed an incremental cycle ergometer exercise test until a rating of perceived exertion of 17 (very hard) on a 20-point scale was attained, with measures of antecubital venous [K⁺]_v Participants underwent a resting muscle biopsy prior to and at 48-72 h following the final training session. After IIT, the peak exercise work rate (25%), oxygen uptake (16%) and heart rate (6%) were increased (P < 0.05). After IIT, the peak exercise plasma [K⁺]_v tended to rise (P = 0.07), while the rise in plasma [K⁺]_v relative to work performed (nmol.L^-1J^-1) was unchanged. Muscle NKA content increased by 11% after IIT (P < 0.05). Single fiber measurements, increased in NKA α₂ isoform in Type II fibers after IIT (30%, P < 0.05), with no changes to the other isoforms in single fibers or homogenate. Thus, intense exercise training in older adults induced an upregulation of muscle NKA, with a fiber-specific increase in NKA α₂ abundance in Type II fibers, coincident with increased muscle NKA content and enhanced exercise performance.

Cell specific differences in the protein abundances of GAPDH and Na(+),K(+)-ATPase in skeletal muscle from aged individuals

Na(+), K(+)-ATPase (NKA) isoforms (α1,α2,α3,β1,β2,β3) are involved in the maintenance of membrane potential and hence are important regulators of cellular homeostasis. Given the age-related decline in skeletal muscle function, we investigated whether the natural physiological process of aging is associated with altered abundance of NKA isoforms (α1,α2,α3,β1,β2,β3) or of the commonly used control protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Importantly, measurements were made in both whole muscle or specific fiber types obtained from skeletal muscle biopsies. Seventeen healthy older (AGED, 69.4 ± 3.5 years, mean ± SD) and 14 younger (YOUNG, 25.5 ± 2.8 years) adults underwent a muscle biopsy for biochemical analyses. Comparing homogenates from AGED and YOUNG individuals revealed higher β3 isoform (p<0.05) and lower GAPDH (p<0.05). Analysis of individual fibers in muscle from YOUNG individuals, showed greater α3 and β2 isoforms, and more GAPDH in Type II compared with Type I fibers (p<0.05). In the AGED, GAPDH was higher in Type II compared with Type I fibers (p<0.05), there were no fiber type differences in the NKA isoforms (p>0.05). Compared with the same fiber type in YOUNG, α1 was greater (Type I) and α3 lower (Type II), while in both fiber types, β2 was lower, β3 greater and GAPDH lower, in muscle from AGED individuals (all p<0.05). Overall, we demonstrate that (i) GAPDH is an inappropriate choice of protein for normalization in all skeletal muscle research and (ii) full understanding of the role of NKA isoforms in human skeletal muscle requires consideration of age and muscle fiber type.

Unchanged [3H]ouabain binding site content but reduced Na+-K+ pump α2-protein abundance in skeletal muscle in older adults

Aging is associated with reduced muscle mass, weakness, and increased fatigability. In skeletal muscle, the Na+-K+ pump (NKA) is important in regulating Na+-K+ gradients, membrane excitability, and thus contractility, but the effects of aging on muscle NKA are unclear. We investigated whether aging is linked with reduced muscle NKA by contrasting muscle NKA isoform gene expression and protein abundance, and NKA total content in 17 Elderly (66.8 ± 6.4 yr, mean ± SD) and 16 Young adults (23.9 ± 2.2 yr). Participants underwent peak oxygen consumption assessment and a vastus lateralis muscle biopsy, which was analyzed for NKA α1-, α2-, α3-, β1-, β2-, and β3-isoform gene expression (real-time RT-PCR), protein abundance (immunoblotting), and NKA total content ([3H]ouabain binding sites). The Elderly had lower peak oxygen consumption (−36.7%, P = 0.000), strength (−36.3%, P = 0.001), NKA α2- (−24.4%, 11.9 ± 4.4 vs. 9.0 ± 2.7 arbitrary units, P = 0.049), and NKA β3-protein abundance (−23.0%, P = 0.041) than Young. The β3-mRNA was higher in Elderly compared with Young ( P = 0.011). No differences were observed between groups for other NKA isoform mRNA or protein abundance, or for [3H]ouabain binding site content. Thus skeletal muscle in elderly individuals was characterized by decreased NKA α2- and β3-protein abundance, but unchanged α1 abundance and [3H]ouabain binding. The latter was likely caused by reduced α2 abundance with aging, preventing an otherwise higher [3H]ouabain binding that might occur with a greater membrane density in smaller muscle fibers. Further study is required to verify reduced muscle NKA α2 with aging and possible contributions to impaired exercise capability and daily living activities.

Aging augments interstitial K+concentrations in active muscle of rats

Skeletal muscle performance declines with advancing age, and the underlying mechanism is not completely understood. A large body of convincing evidence has demonstrated a crucial role for interstitial K+concentration ([K+]o) in modulating contractile function of skeletal muscle. The present study tested the hypothesis that during muscle contraction there is a greater accumulation of [K+]oin aged compared with adult skeletal muscle. Twitch muscle contraction was induced by electrical stimulation of the sciatic nerves of 8- and 32-mo-old Fischer 344 × Brown Norway rats. Levels of [K+]owere measured continuously by a microdialysis technique with the probes inserted into the gastrocnemius muscle. Stimulation at 1, 3, and 5 Hz elevated muscle [K+]oby 52, 64, and 88% in adult rats, and by 78, 98, and 104% in aged rats, respectively, and the increase was significantly higher in aged than in adult rats. Recovery for [K+]o, as measured by the time for [K+]oto recover by 20 and 50% from peak response after stimulation, was slower in aged rats. Ouabain (5 mM), a specific inhibitor of the Na+-K+pump, was added in the perfusate to inhibit the reuptake of K+into the cells to assess the role of the pump in the overall K+balance. Ouabain elevated muscle [K+]oat rest, and the effect was significantly attenuated in aged animals. The present data demonstrated an augmented [K+]oin aged skeletal muscle compared with adult skeletal muscle, and the data suggested that an alteration in the function of the Na+-K+pump may contribute, in part, to the deficiency in K+balance in skeletal muscle of aged rats.

Biology of renal aging in humans

Advancing age is usually accompanied by a decline in glomerular filtration rate and an increased incidence of certain renal and electrolyte disorders. These include an increased susceptibility to acute renal failure, hypo- and hypernatremia, hyperkalemia, and hypertension. This report discusses anatomic and physiological observations related to the aged human kidney and explores the various theories and postulated mechanisms underlying these changes.

Age-associated differential expression of Na(+)-K(+)-ATPase subunit isoforms in skeletal muscles of F-344/BN rats

Skeletal muscle expresses multiple isoforms of the Na(+)-K(+)-ATPase. Their expression has been shown to be differentially regulated under pathophysiological conditions. In addition, previous studies suggest possible age-dependent alterations in Na(+)-K(+) pump function. The present study tests the hypothesis that advancing age is associated with altered Na(+)-K(+)-ATPase enzyme activity and isoform-specific changes in expression of the enzyme subunits. Red and white gastrocnemius (Gast) as well as soleus muscles of male Fischer 344/Brown Norway (F-344/BN) rats at 6, 18, and 30 mo of age were examined. Na(+)-K(+)-ATPase activity, measured by K(+)-stimulated 3-O-methylfluorescein phosphatase activity, increased by approximately 50% in a mixed Gast homogenate from 30-mo-old compared with 6- and 18-mo-old rats. Advancing age was associated with markedly increased alpha(1)- and beta(1)-subunit, and decreased alpha(2)- and beta(2)-subunit in red and white Gast. In soleus, there were similar changes in expression of alpha(1)- and alpha(2)-subunits, but levels of beta(1)-subunit were unchanged. Functional Na(+)-K(+)-ATPase units, measured by [(3)H]ouabain binding, undergo muscle-type specific changes. In red Gast, high-affinity ouabain-binding sites, which are a measure of alpha(2)-isozyme, increased in 30-mo-old rats despite decreased levels of alpha(2)-subunit. In white Gast, by contrast, decreased levels of alpha(2)-subunit were accompanied by decreased high-affinity ouabain-binding sites. Finally, patterns of expression of the four myosin heavy chain (MHC) isoforms (type I, IIA, IIX, and IIB) in these muscles were similar in the three age groups examined. We conclude that, in the skeletal muscles of F-344/BN rats, advancing age is associated with muscle type-specific alterations in Na(+)-K(+)-ATPase activity and patterns of expression of alpha- and beta-subunit isoforms. These changes apparently occurred without obvious shift in muscle fiber types, since expression of MHC isoforms remained unchanged. Some of the alterations occurred between middle-age (18 mo) and senescence (30 mo), and, therefore, may be attributed to aging of skeletal muscle.

Age-related blunting of growth hormone secretion during exercise may not be soley due to increased somatostatin tone

Age-related declines in growth hormone (GH) secretion may result from augmented somatostatin (SRIH) tone and/or diminished GH-releasing hormone (GHRH) secretion. We assessed GH release during exercise without and with pyridostigmine (PYR), which indirectly suppresses SRIH. GH levels were measured throughout exercise and recovery in 12 young men (mean +/- SEM, 20.8 +/- 0.4 years) and seven old men (66.1 +/- 1.9). The area under the GH curve (GH-AUC) was greater in young versus old men during a short-term maximal exercise test (12.9 +/- 2.8 v 1.5 +/- 0.2 ng x min(-1) x mL(-1), P = .002) and a 1-hour 60% maximal (submaximal, 10.0 +/- 1.5 v 3.0 +/- 1.0 ng x min(-1) x mL(-1), P = .001) cycle exercise bout. PYR increased the GH-AUC in young and old men during maximal (20.9 +/- 5.2 v 4.9 +/- 1.8) and submaximal (12.3 +/- 1.6 v 4.7 +/- 1.5) exercise (P < .05). The greater GH response to maximal versus submaximal exercise suggests a role for adrenergic modulation of GHRH during exercise. However, the failure of PYR to restore the responses of the old to those of the young suggests that increased SRIH tone does not completely explain the age difference in GH secretion during exercise.