Increased myofilament calcium sensitivity is associated with decreased cardiac troponin I phosphorylation in the diabetic rat heart.

NEW FINDINGS: What is the central question of this study? In Zucker Diabetic Fatty rats, does cardiomyocyte myofilament function change through the time course of diabetes and what are the mechanisms behind alterations in calcium sensitivity? What is the main finding and its importance? Zucker Diabetic Fatty rats had increased myofilament calcium sensitivity and reduced phosphorylation at cardiac troponin I without differential O-GlcNAcylation. ABSTRACT: The diabetic heart has impaired systolic and diastolic function independent of other comorbidities. The availability of calcium is altered, but does not fully explain the cardiac dysfunction seen in the diabetic heart. Thus, we explored if myofilament calcium regulation of contraction is altered while also categorizing the levels of phosphorylation and O-GlcNAcylation in the myofilaments. Calcium sensitivity (pCa50 ) was measured in Zucker Diabetic Fatty (ZDF) rat hearts at the initial stage of diabetes (12 weeks old) and after 8 weeks of uncontrolled hyperglycaemia (20 weeks old) and in non-diabetic (nDM) littermates. Skinned cardiomyocytes were connected to a capacitance-gauge transducer and a torque motor to measure force as a function of pCa (-log[Ca2+ ]). Fluorescent gel stain (ProQ Diamond) was used to measure total protein phosphorylation. Specific phospho-sites on cardiac troponin I (cTnI) and total cTnI O-GlcNAcylation were quantified using immunoblot. pCa50 was greater in both 12- and 20-week-old diabetic (DM) rats compared to nDM littermates (P = 0.0001). Total cTnI and cTnI serine 23/24 phosphorylation were lower in DM rats (P = 0.003 and P = 0.01, respectively), but cTnI O-GlcNAc protein expression was not different. pCa50 is greater in DM rats and corresponds with an overall reduction in cTnI phosphorylation. These findings indicate that myofilament calcium sensitivity is increased and cTnI phosphorylation is reduced in ZDF DM rats and suggests an important role for cTnI phosphorylation in the DM heart.

Adaptiveness for Online Learning: Conceptualising 'Online Learning Dexterity' from Higher Education Students' Experiences.

Online learning dexterity, or the ability to effortlessly adapt to online learning situations, has become critical since the COVID-19 pandemic, but its processes are not well-understood. Using grounded theory, this study develops a paradigm model of online learning dexterity from semi-structured interviews with 32 undergraduate and postgraduate students from a university in New Zealand. Through students' online learning experiences during the pandemic from 2020 to 2021, online learning dexterity is found to be how students make online learning 'just as good' as face-to-face learning by creating and adjusting five learning manoeuvres according to developing online learning circumstances. Undergraduates and postgraduates re-use familiar study strategies as deep learning manoeuvres, but undergraduates restrict support-seeking manoeuvres to lecturers. Technical problems with online systems and poor course organisation by lecturers affected learning productivity, resulting in the need for more time optimisation manoeuvres. Social support helped students activate persistence manoeuvres to sustain online class attendance. However, undergraduates had more problems sustaining interest and engagement during class as they were not as proficient with using learning presence manoeuvres as postgraduates enrolled in distance learning programmes. The theoretical and practical significance of online learning dexterity for post-pandemic higher education is discussed.

Treadmill running increases the calcium sensitivity of myofilaments in diabetic rats.

The cardiovascular benefits of regular exercise are unequivocal, yet patients with type 2 diabetes respond poorly to exercise due to a reduced cardiac reserve. The contractile response of diabetic cardiomyocytes to ß-adrenergic stimulation is attenuated, which may result in altered myofilament calcium sensitivity and posttranslational modifications of cardiac troponin I (cTnI). Treadmill running increases myofilament calcium sensitivity in nondiabetic rats, and thus we hypothesized that endurance training would increase calcium sensitivity of diabetic cardiomyocytes and alter site-specific phosphorylation of cTnI. Calcium sensitivity, or pCa50, was measured in Zucker diabetic fatty (ZDF), nondiabetic (nDM), and diabetic (DM) rat hearts after 8 wk of either a sedentary (SED) or progressive treadmill running (TR) intervention. Skinned cardiomyocytes were connected to a capacitance-gauge transducer and a torque motor to measure force as a function of pCa (-log[Ca2+]). Specific phospho-sites on cTnI and O-GlcNAcylation were quantified by immunoblot and total protein phosphorylation by fluorescent gel staining (ProQ Diamond). The novel finding in this study was that training increased pCa50 in both DM and nDM cardiomyocytes (P = 0.009). Phosphorylation of cTnI amino acid residues Ser23/24, a crucial protein kinase A site, and Threonine (Thr)144 was lower in DM hearts, but there was no effect of training on site-specific phosphorylation. In addition, total phosphorylation and O-GlcNAcylation levels were not different between SED and TR groups. These findings suggest that regular exercise may benefit the diabetic heart by specifically targeting myofilament contractile function.NEW & NOTEWORTHY We examined the effects of training on the myofilament calcium in diabetic rat hearts. After 8 wk of treadmill running, both nondiabetic and diabetic cardiomyocytes had increased myofilament calcium sensitivity compared with their sedentary counterparts, but there was no effect of training on the phosphorylation or O-GlcNAcylation status of myofilament proteins measured in this study. These data highlight one potential mechanism capable of reversing, in part, reduced cardiac reserve in the diabetic heart.

Sex differences in skeletal muscle alterations in a model of colorectal cancer.

Cancer cachexia is the loss of lean muscle mass with or without loss of fat mass that is often highlighted by a progressive loss of skeletal muscle mass and function. The mechanisms behind the cachexia-related loss of skeletal muscle are poorly understood, including cachexia-related muscle functional impairments. Existing models have revealed some potential mechanisms, but appear limited to how the cancer develops and the type of tumors that form. We studied the C57BL6/J (B6) ApcMin/+ Tg::Fabp1-Cre TG::PIK3ca* (CANCER) mouse. In this model, mice develop highly aggressive intestinal cancers. We tested whether CANCER mice develop cancer cachexia, if muscle function is altered and if sex differences are present. Both female and male mice, B6 (CONTROL) and CANCER mice, were analyzed to determine body weight, hindlimb muscle mass, protein concentration, specific force, and fatigability. Female CANCER mice had reduced body weight and hindlimb muscle mass compared with female CONTROL mice, but lacked changes in protein concentration and specific force. Male CANCER mice had reduced protein concentration and reduced specific force, but lacked altered body weight and muscle mass. There were no changes in fatigability in either group. Our study demonstrates that CANCER mice present an early stage of cachexia, have reduced specific force in male CANCER mice and develop a sex-dependent cachexia phenotype. However, CANCER mice lack certain aspects of the syndrome seen in the human scenario and, therefore, using the CANCER mice as a preclinical model does not seem sufficient in order to maximize the translation of preclinical findings to humans.