Familial hypertrophic cardiomyopathy (FHC) is an inherited autosomal dominant disease caused by mutations in sarcomeric proteins. Among these, mutations that affect myosin binding protein-C (MyBP-C), an abundant component of the thick filaments, account for 20% to 30% of all mutations linked to FHC. However, the mechanisms by which MyBP-C mutations cause disease and the function of MyBP-C are not well understood. Therefore, to assess deficits due to elimination of MyBP-C, we used gene targeting to produce a knockout mouse that lacks MyBP-C in the heart. Knockout mice were produced by deletion of exons 3 to 10 from the endogenous cardiac (c) MyBP-C gene in murine embryonic stem (ES) cells and subsequent breeding of chimeric founder mice to obtain mice heterozygous (+/−) and homozygous (−/−) for the knockout allele. Wild-type (+/+), cMyBP-C ^+/− , and cMyBP-C ^−/− mice were born in accordance with Mendelian inheritance ratios, survived into adulthood, and were fertile. Western blot analyses confirmed that cMyBP-C was absent in hearts of homozygous knockout mice. Whereas cMyBP-C ^+/− mice were indistinguishable from wild-type littermates, cMyBP-C ^−/− mice exhibited significant cardiac hypertrophy. Cardiac function, assessed using 2-dimensionally guided M-mode echocardiography, showed significantly depressed indices of diastolic and systolic function only in cMyBP-C ^−/− mice. Ca ²⁺ sensitivity of tension, measured in single skinned myocytes, was reduced in cMyBP-C ^−/− but not cMyBP-C ^+/− mice. These results establish that cMyBP-C is not essential for cardiac development but that the absence of cMyBP-C results in profound cardiac hypertrophy and impaired contractile function.

Myocardial performance is likely affected by the relative expression of the two myosin heavy chain (MyHC) isoforms, namely α-MyHC and β-MyHC. The relative expression of each isoform is regulated developmentally and in pathophysiological states. Many pathophysiological states are associated with small shifts in the relative expression of each MyHC isoform, yet the functional consequence of these shifts remains unclear. The purpose of this study was to determine the functional effect of a small shift in the relative expression of α-MyHC. To this end, power output was measured in rat cardiac myocyte fragments that expressed ≈12% α-MyHC and in myocyte fragments that expressed ≈0% α-MyHC, as determined in the same cells by SDS-PAGE analysis after mechanical experiments. Myocyte fragments expressing ≈12% α-MyHC developed ≈52% greater peak normalized power output than myocyte fragments expressing ≈0% α-MyHC. These results indicate that small amounts of α-MyHC expression significantly augment myocyte power output.

Myosin binding protein-C (MyBP-C) is localized to the thick filaments of striated muscle where it appears to have both structural and regulatory functions. Importantly, mutations in the cardiac MyBP-C gene are associated with familial hypertrophic cardiomyopathy. The purpose of this study was to examine the role that MyBP-C plays in regulating force, power output, and force development rates in cardiac myocytes. Skinned cardiac myocytes from wild-type (WT) and MyBP-C knockout (MyBP-C ^−/− ) mice were attached between a force transducer and position motor. Force, loaded shortening velocities, and rates of force redevelopment were measured during both maximal and half-maximal Ca ²⁺ activations. Isometric force was not different between the two groups with force being 17.0±7.2 and 20.5±3.1 kN/m ² in wild-type and MyBP-C ^−/− myocytes, respectively. Peak normalized power output was significantly increased by 26% in MyBP-C ^−/− myocytes (0.15±0.01 versus 0.19±0.03 P/P _o · ML/sec) during maximal Ca ²⁺ activations. Interestingly, peak power output in MyBP-C ^−/− myocytes was increased to an even greater extent (46%, 0.09±0.03 versus 0.14±0.02 P/P _o · ML/sec) during half-maximal Ca ²⁺ activations. There was also an effect on the rate constant of force redevelopment (k _tr ) during half-maximal Ca ²⁺ activations, with k _tr being significantly greater in MyBP-C ^−/− myocytes (WT=5.8±0.9 s ⁻¹ versus MyBP-C ^−/− =7.7±1.7 s ⁻¹ ). These results suggest that cMyBP-C is an important regulator of myocardial work capacity whereby MyBP-C acts to limit power output.

The purpose of this study was to examine the role of myosin heavy chain (MHC) in determining loaded shortening velocities and power output in cardiac myocytes. Cardiac myocytes were obtained from euthyroid rats that expressed α-MHC or from thyroidectomized rats that expressed β-MHC. Skinned myocytes were attached to a force transducer and a position motor, and isotonic shortening velocities were measured at several loads during steady-state maximal Ca²⁺ activation (P_pCa4.5). MHC expression was determined after mechanical measurements using SDS-PAGE. Both α-MHC and β-MHC myocytes generated similar maximal Ca²⁺-activated force, but α-MHC myocytes shortened faster at all loads and generated ∼170% greater peak normalized power output. Additionally, the curvature of force-velocity relationships was less, and therefore the relative load optimal for power output (F_opt) was greater in α-MHC myocytes. F_opt was 0.31 ± 0.03 P_pCa4.5 and 0.20 ± 0.06 P_pCa4.5 for α-MHC and β-MHC myocytes, respectively. These results indicate that MHC expression is a primary determinant of the shape of force-velocity relationships, velocity of loaded shortening, and overall power output-generating capacity of individual cardiac myocytes.

The purpose of this study was to characterize the distribution of blood flow in the rat during hindlimb unweighting (HU) and post-HU standing and exercise and examine whether the previously reported (Witzmann et al., J. Appl. Physiol. 54: 1242–1248, 1983) elevation in anaerobic metabolism observed with contractile activity in the atrophied soleus muscle was caused by a reduced hindlimb blood flow. After either 15 days of HU or cage control, blood flow was measured with radioactive microspheres during unweighting, normal standing, and running on a treadmill (15 m/min). In another group of control and experimental animals, blood flow was measured during preexercise (PE) treadmill standing and treadmill running (15 m/min). Soleus muscle blood flow was not different between groups during unweighting, PE standing, and running at 15 m/min. Chronic unweighting resulted in the tendency for greater blood flow to muscles composed of predominantly fast-twitch glycolytic fibers. With exercise, blood flow to visceral organs was reduced compared with PE values in the control rats, whereas flow to visceral organs in 15-day HU animals was unaltered by exercise. These higher flows to the viscera and to muscles composed of predominantly fast-twitch glycolytic fibers suggest an apparent reduction in the ability of the sympathetic nervous system to distribute cardiac output after chronic HU. In conclusion, because 15 days of HU did not affect blood flow to the soleus during exercise, the increased dependence of the atrophied soleus on anerobic energy production during contractile activity cannot be explained by a reduced muscle blood flow.