Mild Osmotic Stress Alters mTOR Pathway Signaling

Abstract

Increased osmotic stress reduces cell volume, thus disrupting homeostatic processes. This could impact signaling that promotes muscle growth and maintenance at rest and in response to mechanical stimuli. To date, the detrimental effect of osmotic stress on muscle protein turnover has been established in in vitro and in situ models, but the effects on muscular anabolic signaling in humans are unknown. Purpose: To examine the effect of fluid restriction (FR)-induced mild osmotic stress (plasma osmolality ≥295.0 mmol×kg−1) on muscular anabolic signaling in response to resistance exercise (RE) in trained young men. Hypothesis: We hypothesized that FR would alter the Akt/mTOR signaling. Method: RE-trained men (n=11, 21±1yr, 175.9±6.2cm, 79.2±12.3kg, 18.4±6.7% fat) completed 2 identical RE bouts either with (FR) or without (CON) fluid restriction from 24 hrs before to 3 hrs after RE. RE consisted of bilateral leg press and knee extensions (5 sets x 10 repetitions at 80% of 1 repetition maximum). The two conditions were 2-3 wks apart. Plasma osmolality (PO) was measured before (PRE), and 5 min, 15 min, 30 min, 1 hr, and 3 hrs after RE. Muscle samples were collected at PRE and 1 and 3 hrs after RE. Western blots were used to analyze contents of proteins in the mTOR signaling pathway: Akt, p-AktS473, mTOR, p-mTORS2448, p70S6K, p-p70S6KT389, rpS6, p-rpS6S240/244, eEF2, and p-eEF2T56 and normalized to Ponceau S staining (total protein). Results: Increased PO was successfully induced in FR vs. CON (condition effect, p<0.0001; 299±2 vs 286±1 mmol/kg). P-AktS473 significantly (p≤0.05) increased from PRE to 1 and 3 hrs in CON, and from PRE to 1h with a return to PRE by 3h in FR (condition x time). P-AktS473 was higher at 1h and lower at 3h in FR versus CON. P-mTORS2448 increased from PRE to 1 and 3 hrs regardless of condition (main time). P-p70S6KT389 was greater in FR than CON regardless of time point (main condition). P-rpS6S240/244 increased from PRE to 1 and 3 hrs for CON and FR, and was greater for FR than CON at PRE (condition x time). Conclusion: Baseline anabolic signaling appeared elevated when individuals were subjected to mild osmotic stress. After RE, signaling through mTOR was elevated as expected when plasma was iso-osmotic, although anticipated time effects for p-p70S6K and p-eEF2 were not detected. Upstream of mTOR, activation of Akt was stronger yet shorter in duration with mild osmotic stress. Downstream of mTOR, p70S6K and rpS6 activation were greater with mild osmotic stress. While these results suggest increased anabolic signaling with mild osmotic stress, concomitant catabolic signaling should be assessed. Together, the present results and future measures will inform how conditions that confer osmotic stress, such as dehydration, may alter intramuscular protein homeostasis at rest and in response to an anabolic stimulus. This study was funded by National Strength and Conditioning Association Foundations. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.