Mice Null for Calsequestrin 1 Exhibit Deficits in Functional Performance and Sarcoplasmic Reticulum Calcium Handling

In skeletal muscle, the release of calcium (Ca2+) by ryanodine sensitive sarcoplasmic reticulum (SR) Ca2+ release channels (i.e., ryanodine receptors; RyR1s) is the primary determinant of contractile filament activation. Much attention has been focused on calsequestrin (CASQ1) and its role in SR Ca2+ buffering as well as its potential for modulating RyR1, the L-type Ca2+ channel (dihydropyridine receptor, DHPR) and other sarcolemmal channels through sensing luminal [Ca2+]. The genetic ablation of CASQ1 expression results in significant alterations in SR Ca2+ content and SR Ca2+ release especially during prolonged activation. While these findings predict a significant loss-of-function phenotype in vivo, little information on functional status of CASQ1 null mice is available. We examined fast muscle in vivo and in vitro and identified significant deficits in functional performance that indicate an inability to sustain contractile activation. In single CASQ1 null skeletal myofibers we demonstrate a decrease in voltage dependent RyR Ca2+ release with single action potentials and a collapse of the Ca2+ release with repetitive trains. Under voltage clamp, SR Ca2+ release flux and total SR Ca2+ release are significantly reduced in CASQ1 null myofibers. The decrease in peak Ca2+ release flux appears to be solely due to elimination of the slowly decaying component of SR Ca2+ release, whereas the rapidly decaying component of SR Ca2+ release is not altered in either amplitude or time course in CASQ1 null fibers. Finally, intra-SR [Ca2+] during ligand and voltage activation of RyR1 revealed a significant decrease in the SR[Ca2+]free in intact CASQ1 null fibers and a increase in the release and uptake kinetics consistent with a depletion of intra-SR Ca2+ buffering capacity. Taken together we have revealed that the genetic ablation of CASQ1 expression results in significant functional deficits consistent with a decrease in the slowly decaying component of SR Ca2+ release

INTRASTRAIN VARIATION IN EXERCISE TRAINABILITY IS LINKED TO CHANGES IN BODY WEIGHT IN HET3 MICE

Gabriel Reichert Blume1, Adam B. Salmon2, Hanyu Liang2, Elizabeth Fernandez2, Nicolas Musi2, Peter J. Hornsby2, Michael P. Massett3, and Heather L. Vellers1 1University of Oklahoma, Norman, OK; 2University of Texas Health Science Center at San Antonio, San Antonio, TX; 3Texas Tech University, Lubbock TX PURPOSE: The purpose of this study was to characterize endurance trainability in the agenetically heterogenous HET3 aging mouse model by assessing change in exercise capacity after completing a standardized treadmill training protocol. Secondarily, we also aimed to assess the association between exercise capacity changes with body composition measures in response to the training. METHODS: 40-week-old male mice were randomly assigned to endurance training (ET, n = 23) or sedentary control (SED, n = 20) groups. Both groups underwent a graded treadmill exercise capacity test before and after four weeks of exercise training. ET mice trained five days per week for four weeks at 65% of the maximal work output calculated in joules (J = kg*gravity*vertical speed*time). Additionally, we analyzed training-induced changes in body composition (total body weight, lean mass, and fat mass). An independent sample t-test was employed to compare changes in total work and linear regression analysis to assess the association between post-training exercise work and body composition measures (body weight, fat mass, and lean mass) and exercise work (alpha level = 0.05). RESULTS:While the mean change in exercise capacity was significantly greater in ET versus SED mice (ET: 483.3 ± 48.8 joules, SED: 194.7 ± 52.0 joules; p = 0.0003), there was considerable variance in change within each group (Range: ET = 74.4 – 925.4 joules; SED = 85.1 – 432.0 joules). In the ET mice, change in exercise capacity was positively correlated with post-training body weight (37.3 ± 1.3 gms, p = 0.004) and the change in body weight (-9.5 ± 1.8 gms, p = 0.01) but no other pre- or post-training body composition factors. There were no associations between body composition measures and exercise capacity in the untrained animals. CONCLUSION: While the standardized treadmill training protocol increased exercise capacity in HET3 ET mice, these changes were variable. Interestingly, greater declines in body weight with training were associated with lower exercise capacity measures that were not linked to pre-training body weight, possibly suggesting food consumption and/or spontaneous cage activity may have influenced results. Taken together, the variable responses in endurance trainability and factors contributing to change in body weight may be, at least partially, explained by their heterogeneous nature. ACKNOWLEDGMENTS: San Antonio Nathan Shock Center New Investigator Award Fundin

Physiological significance of P2X receptor-mediated vasoconstriction in five different types of arteries in rats

P2X1 receptors, the major subtype of P2X receptors in the vascular smooth muscle, are essential for α,β-methylene adenosine 5′-triphosphate (α,β-MeATP)-induced vasoconstriction. However, relative physiological significance of P2X1 receptor-regulated vasoconstriction in the different types of arteries in the rat is not clear as compared with α1-adrenoceptor-regulated vasoconstriction. In the present study, we found that vasoconstrictive responses to noncumulative administration of α,β-MeATP in the rat isolated mesenteric arteries were significantly smaller than those to single concentration administration of α,β-MeATP. Therefore, we firstly reported the characteristic of α,β-MeATP-regulated vasoconstrictions in rat tail, internal carotid, pulmonary, mesenteric arteries, and aorta using single concentration administration of α,β-MeATP. The rank order of maximal vasoconstrictions for α,β-MeATP (Emax·α,β-MeATP) was the same as that of maximal vasoconstrictions for noradrenaline (Emax·NA) in the internal carotid, pulmonary, mesenteric arteries, and aorta. Moreover, the value of (Emax·α,β-MeATP/Emax·KCl)/(Emax·NA/Emax·KCl) was 0.4 in each of the four arteries, but it was 0.8 in the tail artery. In conclusion, P2X1 receptor-mediated vasoconstrictions are equally important in rat internal carotid, pulmonary, mesenteric arteries, and aorta, but much greater in the tail artery, suggesting its special role in physiological function