Sex Differences in the Mechanical Properties of Rat Myocardium After Exposure to Simulated Microgravity

Abstract

Listen

Translate article

Female astronauts are more likely than male astronauts to develop presyncope and orthostatic intolerance when they return to normal gravity after space-flights. The underlying mechanisms for this are unclear but could include myocardial adaptations to micro-gravity conditions. This study was designed to test whether there are sex-specific changes in the mechanical properties of myocardial tissue in rats subjected to a period of simulated microgravity. Multicellular myocardial preparations were isolated from control male and female Sprague-Dawley rats and corresponding experimental animals that were hind-limb unloaded (HLU) for 14 days. Preparations were subjected to standard mechanical tests imposed under muscle length control in solutions with pCa values ranging from 9.0 to 4.5. The rates of tension recovery (ktr) measured in pCa 4.5 solution were significantly higher (p<0.002, 2-way ANOVA) in the preparations isolated from the HLU animals than in preparations isolated from the control groups (ktr mean values ± SEM measured in pCa 4.5 were: 7.45±0.37 s−1 for female control; 9.54±0.50 s−1 female HLU; 7.70±0.46 s−1 male control and 8.42±0.37 s−1 male HLU). The relative content of slower beta-Myosin Heavy Chain decreased in both HLU groups compared to the corresponding control groups (p<0.01, 2-way ANOVA). There was no interaction for this parameter between sex and unloading condition (p=0.93, 2-way ANOVA). The heart-to-body weight ratios increased significantly in the HLU groups (p<0.0001, 2-way ANOVA) but again there was no interaction with sex (p=0.84, 2-way ANOVA). If similar mechanisms operate in humans, female astronauts may be more likely to develop presyncope after space-flights than male astronauts because their hearts contain a greater relative content of the alpha-Myosin Heavy Chain when they return to Earth.