The Ca 2+ currents and homeostasis during the aging process of skeletal muscle

Aims: The mechanisms involved in sarcopenia, the decline in muscle mass with aging coupled with loss of force and function, has been actively investigated in animal and human models over the last years [reviewed in Di Iorio et al., Sarcopenia: age-related skeletal muscle changes from determinants to physical disability, Int. J. Immunopathol. Pharmacol. 19 (2006) 703-719]. An important age-associated deficit may be the alteration of the mechanisms controlling Ca2+ handling. Moreover, it has already been proposed that defective fibres in old humans could result from a reduced efficiency of aged satellite cells (a distinct muscle cell subtype, responsible for post-natal growth and repair of damaged fibres) in properly differentiating into myotubes with a mature E-C coupling mechanism [see: Lorenzon et al., Aging affects the differentiation potential of human myoblasts, Exp. Gerontol. 39 (2004) 1545-1554]. Proceeding from these results, the main goal of the present Ph.D. thesis was to investigate whether the inefficiency of aged satellite cells to generate functional skeletal muscle fibres could be partly due to defective voltage-dependent Ca2+ currents. Methods: The whole-cell patch clamp and the videoimaging techniques were employed to measure respectively T- and L-type Ca2+ currents and [Ca2+]i transients in myoblasts and/or myotubes derived from murine and human satellite cells, obtained respectively from young murine skeletal muscle and then aged in vitro under culture conditions, and from human skeletal muscle tissue of healthy donors aged 2, 12, 76 and 86 years. Results: First of all, I confirmed that both murine and human senescent satellite cells fuse more slowly and less efficiently, leading to smaller and thinner myotubes, as known from previous work. Moreover, I showed for the first time that both in myotubes derived from in vitro aged murine satellite cells and in human myotubes derived from satellite cells of old donors the functional expression and the biophysical properties of T- and L-type voltage-dependent Ca2+ channels are impaired. In fact, extensively, less Ca2+ can be available via T-type and L-type channels in old myotubes than in the young ones, and this can be put in relation to the age-related decrease in the quality of myoblast fusion. I also confirmed a specific responsibility of the decrease of the L-type channel number and/or activity for the age-related lowering of intracellular Ca2+ release (the so-called E-C uncoupling; see: Delbono et al., Excitation-calcium release uncoupling in aged single human skeletal muscle fibers, J. Membr. Biol. 148 (1995) 211-222]. Conclusions: From these results one can infer a clear parallelism between the results obtained with the in vitro aging of murine satellite cells model and that concerning the physiological process of human skeletal muscle aging in vivo. In the final analysis, aging effects on voltage-dependent L- and T-type currents could be one of the causes of the inability of old satellite cells to efficiently counteract age-related impairment in muscle force. So, a further strong evidence has been given that in humans, as in other mammals, the satellite cells and the regulation of Ca2+ homeostasis have a decisive role in the physiological process of skeletal muscle aging.