Biomechanical characteristics of unconditioned and conditioned latissimus dorsi muscles used for cardiocirculatory assistance.

An understanding of the biomechanical characteristics of striated skeletal muscles involved in cardiocirculatory assistance is a prerequisite to assess their efficacy and to evaluate their haemodynamic benefits. Six goats had their latissimus dorsi muscles evaluated by isometric strain gauge testing. Total tension, and both active and passive force development at different preloads were measured. The relationship between muscle impedance and starting length was also studied. Four additional muscles were submitted to isometric and isotonic strain gauge testing after 3 months of chronic electrical stimulation (Broussais Hospital protocol) with the contralateral muscle serving as a control. In isometric testing, both conditioned and unconditioned goat latissimus dorsi displayed a Frank-Starling length-tension curve, and a linear relationship between muscle impedance and starting length was found. Chronic stimulation preserved muscle mass and isometric force. Transformed muscles showed a mean 59% reduction of maximal shortening velocity; means (s.d.) residual shortening velocity at maximal work and power output was 0.17(0.07) m/s. The work and power output were both reduced 65% after stimulation, and the residual maximal power at optimal preload varied from approximately 7.7 and 9.6 W/kg. It is concluded that, following the Broussais protocol, the goat latissimus dorsi muscle retained mass and most of its isometric force-generating capacity, but lost significant work and power potential. The residual power output did not, however, preclude the possibility of a significant cardiocirculatory contribution, providing that the conditions for optimal energy transduction are adequately delineated.

Progressive latissimus dorsi muscle denervation for free-flap dynamic cardiomyoplasty.

BACKGROUND: The creation of free muscle grafts for surgical myoplasty is limited by the dependence of muscle on its original nerve supply. The aim of this study was to develop a model of gradual denervation of a large skeletal muscle (latissimus dorsi) and evaluate the possibility that atrophic degeneration and loss of function would be reduced using progressive nerve compression instead of surgical division of the nerve. The effects of chronic stimulation prior to, and after, denervation were also evaluated. METHODS: Electrodes connected to a myostimulator were implanted on 24 latissimus dorsi muscles of 12 goats. Denervation of these muscles was achieved either by sectioning of the nerve by progressive compression using ameroid rings placed around the nerve. Electrostimulation of the muscle started either 5 weeks before (prestimulation), or immediately after the denervation. RESULTS: The model of gradual nerve compression was successfully created and did have less atrophy and loss of function at mid-term when compared with nerve division. Chronic electrostimulation of the muscle after nerve division had a beneficial effect on function and on the atrophic process. Chronic electrostimulation in our model of gradual nerve compression did not mirror these beneficial results. Detrimental results were observed in groups in which chronic electrostimulation was applied prior to nerve division or constriction.

Type II to type I transformation of chronically stimulated goat latissimus dorsi muscle: a histoenzymological, biochemical, bioenergetic, and functional study.

Five goat latissimus dorsi muscles (LDM) were submitted to a progressive chronic electrostimulation program to reach an integrated understanding of the fast-to-slow transformation process in large mammals. LDM were regularly sampled and followed during a period of 8 months. Each sample was simultaneously assessed for histoenzymological study, myosin and LDH isoforms and bioenergetic capacities [NADH dehydrogenase cytochrome c oxidoreductase (NADH Cyt c OR), succinate dehydrogenase cytochrome c oxidoreductase (Succ Cyt c OR), cytochrome c oxidase (Cyt c Ox) and LDH]. Such muscles were also tested with and without completion of II to I transformation for their mechanical properties in isometric and isotonic strain gauge testing. The conversion of fast-to-slow myosin monitored by heavy chain (HC I) and light chain slow component (LC2s) began a few days after stimulation and was almost 100% after 100 days. The H-LDH isoforms evolved similarly but did not reach 100% conversion after 200 days. The activity of respiratory chain oxidases increased within 36 h but to a variable extent and peaked after 32 days, corresponding to a 75% transformation of myosin compared to initial levels. NADH Cyt c OR, Succ Cyt c OR, and Cyt c Ox, respectively increased 10-, 5- and 5-fold. These activities then significantly decreased before the completion of the myofibrillar transformation and reached a plateau with stable activities that remained 2- to 3-fold higher than the unstimulated LDM. LDH activity sharply decreased until day 62 (5-fold) and then plateaued. Functionally, muscle showed a reduced speed of contraction and moderate reduction in power output but had become fatigue-resistant. This study documents the transformation process in large mammals and suggests the dynamic relation between workload, aerobic-anaerobic metabolism and the contractile myofibrillar system.