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Medical AI Inquiry: You was designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.  Labels: BACKGROUND, METHODS, RESULTS. Meshes: Actin Cytoskeleton, Animals, Atrophy, Calcium, Disease Models, Animal, Heart Transplantation, Heart Ventricles, Heart-Assist Devices, Male, Myocardial Contraction, Myocytes, Cardiac, Rats, Rats, Inbred Lew, Sarcoplasmic Reticulum, Ventricular Dysfunction, Left, Ventricular Remodeling.
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Does prolonged mechanical unloading reduce myofilament sensitivity to calcium and sarcoplasmic reticulum calcium uptake leading to contractile dysfunction? Context: Prolonged unloading using left ventricular (LV) assist devices (LVADs) leads to unloading-induced atrophy with altered cardiomyocyte contractility. The causes for this time-dependent deterioration of myocardial function are unclear. Our aim was to determine the effects of prolonged mechanical unloading on cardiomyocyte function and, more specifically, on Ca(2+) cycling and myofilament sensitivity to Ca(2+). LV unloading was induced by heterotopic abdominal transplantation (UN) in rats for 5 weeks. Recipient hearts were used as controls (REC). LV myocytes were isolated and cardiomyocyte area measured by planimetry, sarcomere length measured by Fourier analysis of digitized cardiomyocyte images, and cytoplasmic [Ca(2+)] monitored using Indo-1. Myofilament sensitivity to Ca(2+) was assessed as the slope of the linear relationship between Indo-1 ratio and sarcomere shortening during relaxation. UN cardiomyocyte area was smaller compared with REC (mean +/- SEM: UN 2,503 +/- 78 microm(2) [n = 132], REC 3,856 +/- 89 microm(2) [n = 116]; p < 0.001). UN cardiomyocytes had a smaller sarcomere shortening amplitude (UN 0.08 +/- 0.01 microm [n = 37], REC 0.11 +/- 0.01 microm [n = 38]; p < 0.01), despite normal Ca(2+) transient amplitude (UN 0.13 +/- 0.01 Indo-1 ratio units [n = 37], REC 0.11 +/- 0.01 Indo-1 ratio units [n = 38]; p = non-significant). Myofilament sensitivity to Ca(2+) was reduced in UN (UN 2.0 +/- 1.2 microm/ratio unit [n = 20], REC 3.7 +/- 0.4 microm/ratio unit [n = 22]; p < 0.01). Sarcoplasmic reticulum (SR) Ca(2+) uptake (assessed by 20 mmol/liter caffeine) was also reduced in UN (UN 84.3 +/- 0.79% relative contribution [n = 22], REC 89.8 +/- 0.67% relative contribution [n = 24]; p < 0.001).
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Prolonged myocardial unloading causes depressed contractility due to reduced SR Ca(2+) uptake and myofilament sensitivity to Ca(2+). These effects may be relevant with regard to myocardial performance after prolonged LVAD support. Final Decision: yes