FIGURE 4. Rhesus monkey on the right papilla. Relaxed myofibrils cut lengthwise show the distinct pattern of sarcoma units. Each sarcomere is delimited by opaque Z bands and also contains a dark central A band and two « half » I bands (whole I band denoted I band I). The central set of bands in the sarcomere is known in the heart as the « pseudo-H » zone (psH). The side-by-side register is not always accurate between adjacent myofibrils (see also Fig. 1). Sarcoma modeling has its basis in organized networks of protein-containing filaments: actin is found alone in the I bands, overlaps myosin in all areas except the pseudo-H zone of the A band, and blends into the dark substance of the Z band. Some cellular structures of the myocardium appear preferably junctional next to the Z-band, including the corbulistic SR vesicles (C-SR), a form of « extended » sarcoplasmic reticulum. Scale bar: 1 μm. As mentioned earlier, the accumulations of contractile proteins of myocardial cells known as myofibrils (or sometimes « myofilamentary masses ») make up the bulk of the muscle cell. The archetype of the « tonic » skeletal muscle, such as the frog sartorius, exhibits an extreme degree of regularity of its myofibrils, each of which is cylindrical, has a uniform diameter and is closely related to its neighbors in terms of sarcoma register.
The presence of uniform myofibrils of small diameter is said to represent a structure of fibrils, while larger and amorphous masses of myofilaments are called field structure. In the fast-beating hearts of small mammals such as shrews and mice, the myofibrils of working cardiomyocytes tend to have smaller diameters (i.e., a more fragile structure; see e.B. Fig. 3) than those of the cells of the slower-beating hearts (e.B. Dog, monkey), whose myofibrils are thicker and more variable in the cross-section profile (field structure) (Fig. 4 and 5). An obvious consequence (or perhaps an advantage) of a contractile system of conformation of the structure of the fibrils is the transfer of a larger myofibrillar area per unit of cell volume; On these myofibrillary surfaces, a vast system of anastomosing tubules and sacrules of the sarcoplasmic reticulum is applied, responsible for the movements of calcium inside and outside the underlying myofibrils – the basis of the contractile cycle of the cell. The first events of the estradiol effect in the myometrium and endometrium (and, by the way, all steroid hormones in their target cells) seem to be identical: the hormone bound to the receptor protein enters the cell nucleus and « induces » the synthesis of a protein whose function is still unknown. But the ultimate response of the two tissues to estradiol is profoundly different: massive protein synthesis in the myometrium and cell proliferation in the endometrium.
Obviously, it is better to look at the two tissues separately. The contractile functional unit of myofibrils is called sarcomeres, which is about 1.6 to 2.0 μm long. Due to its binding to contractile proteins, ADP remains invisible by SRM 31P. Many attempts have been made to calculate the concentration of free ADP, which has a major influence on energy processes. This concentration is usually calculated from the mass-effect ratio of the creatine kinase reaction. Chance and colleagues proposed that the Pi/PCr ratio be considered as a measure of unrelated PDA concentration (Chance et al., 1981). This hypothesis applies only under pH and ATP homeostasis conditions. The equality between the Cr and Pi changes during the exercise is also taken into account, a valid assumption as long as the ATP concentration does not change.
However, since the rest level Pi is only half of the rest level Cr, the values of Cr and Pi are different (Chance et al., 1990). As highlighted by Chance et al. (1990), the use of Pi/PCr instead of Cr/PCr leads to an overestimation of the concentration of free ADP. The study of muscle bioenergetics under limited conditions of pH homeostasis is also questionable, especially for pathophysiological purposes. If the concentration of free ADP is estimated from the equilibrium of creatine kinase under conditions of pH change, the calculation may be inaccurate. Another condition that must be met for this calculation is the constancy of the concentration of Mg2+ on the free cells from rest to movement. If changes in pH are detected during exercise, mg2+ binding sites are reduced and therefore the concentration of free Mg2+ is increased (Sahlin 1991). This increase is clearly reflected in the chemical transition of Pβ from ATP to the 31P-NMR spectra recorded on trained rat muscles (Dawson et al., 1977). Given the crucial importance of free ADP concentration in metabolic regulation, an assessment of this concentration in resting and contracting muscles is of great importance, and the indirect determinations proposed so far are not entirely satisfactory. .