title: Mechanical and energetic properties of papillary muscle from ACTC E99K transgenic mouse models of hypertrophic cardiomyopathy.
abstract: We compared the contractile performance of papillary muscle from a mouse model of hypertrophic cardiomyopathy [α-cardiac actin (ACTC) E99K mutation] with nontransgenic (non-TG) littermates. In isometric twitches, ACTC E99K papillary muscle produced three to four times greater force than non-TG muscle under the same conditions independent of stimulation frequency and temperature, whereas maximum isometric force in myofibrils from these muscles was not significantly different. ACTC E99K muscle relaxed slower than non-TG muscle in both papillary muscle (1.4×) and myofibrils (1.7×), whereas the rate of force development after stimulation was the same as non-TG muscle for both electrical stimulation in intact muscle and after a Ca²⁺ jump in myofibrils. The EC₅₀ for Ca²⁺ activation of force in myofibrils was 0.39 ± 0.33 μmol/l in ACTC E99K myofibrils and 0.80 ± 0.11 μmol/l in non-TG myofibrils. There were no significant differences in the amplitude and time course of the Ca²⁺ transient in myocytes from ACTC E99K and non-TG mice. We conclude that hypercontractility is caused by higher myofibrillar Ca²⁺ sensitivity in ACTC E99K muscles. Measurement of the energy (work + heat) released in actively cycling heart muscle showed that for both genotypes, the amount of energy turnover increased with work done but with decreasing efficiency as energy turnover increased. Thus, ACTC E99K mouse heart muscle produced on average 3.3-fold more work than non-TG muscle, and the cost in terms of energy turnover was disproportionately higher than in non-TG muscles. Efficiency for ACTC E99K muscle was in the range of 11-16% and for non-TG muscle was 15-18%.
authors.affiliation: National Heart and Lung Institute, Imperial College London, London, United Kingdom.
authors.forename: Weihua
authors.initials: W
authors.lastname: Song
authors.forename: Petr G
authors.initials: PG
authors.lastname: Vikhorev
authors.forename: Mavin N
authors.initials: MN
authors.lastname: Kashyap
authors.forename: Christina
authors.initials: C
authors.lastname: Rowlands
authors.forename: Michael A
authors.initials: MA
authors.lastname: Ferenczi
authors.forename: Roger C
authors.initials: RC
authors.lastname: Woledge
authors.forename: Kenneth
authors.initials: K
authors.lastname: MacLeod
authors.forename: Steven
authors.initials: S
authors.lastname: Marston
authors.forename: Nancy A
authors.initials: NA
authors.lastname: Curtin
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D020013:Calcium Signaling / Q000502:physiology; D006332:Cardiomegaly / Q000235:genetics* / Q000503:physiopathology*; D004558:Electric Stimulation; D004734:Energy Metabolism / Q000235:genetics* / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D066298:In Vitro Techniques; D007537:Isometric Contraction; D051379:Mice; D008822:Mice, Transgenic; D009200:Myocardial Contraction / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000378:metabolism / Q000502:physiology; D009210:Myofibrils / Q000502:physiology; D010210:Papillary Muscles / Q000033:anatomy & histology / Q000502:physiology*; D022722:Thermogenesis; D014159:Transducers
keywords: animal models of human disease; calcium cycling/excitation-contraction coupling; efficiency; energy metabolism; hypertrophic cardiomyopathy