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During direct stimulation of m. Soleus by trains of 5, 10 and 50 stimuli with a frequency of 20 Hz in control experiments (n = 16) a biphasic change was observed in the amplitude of the last contractile responses (LCRN) depending on N, where N is the number of individual contractile responses within the te- tanus. Thus, an initial decrease of LCRN amplitude (up to 54 ± 8 % for LCR₅) was replaced by their subsequent growth (up 218 ± 14 % for LCR5o) associated with a significant shortening of their half-relaxation time relative to the initial response (to 44 ± 8 % for LCR₅₀). Caffeine at concentrations of 5 mM (n = 6) and 10 mM (n = 4), at the background of developing characteristic stationary contracture respon- ses, increased LCR5 depression during the initial inhibitory phase (31 ± 8 % and 15 ± 4 %, respectively). The subsequent growth of LCRN amplitude was significantly lower than in the control (114 ± 18 % and 46 ± 9 % for LCR₅₀ at 5 and 10 mM caffeine, respectively). LCR₅₀ half-relaxation time during the action of both caffeine concentrations remained still considerably shorter than the individual responses recorded both in the presence of caffeine and in control. In contrast to the control and caffeine effects, LCR5 and ₁₀ (to 143 ± 14 %) than was observed in the control muscle. Additionally, dantrolene enhanced muscle relaxation at rest. Caffeine (10 mM), at the background of dantrolene, restored the dynamics of changes of amplitude time characteristics of the last contractile responses to values close to the control. The time-amplitude characteristics of the extracellular AP recorded in individual muscle fibers in m. Soleus did not change significantly during tetanic stimulation under protocol similar to that used for mechanografical experiments. These data can be interpreted to support the previously suggested theory about the participation of <> as an additional mechanism of excitation-contraction coupling in skeletal muscle under conditions of tetanic stimulation [1, 2].

We studied inotropic reaction of isolated myocardium of patients with heart failure induced by ischemic and rheumatic cardiac involvement after periods of rest at the background of amiodarone. The studies were carried out on muscular trabeculi separated from right atrial auricle during surgery. It was shown that inotropic reaction of insufficient myocardium could be of 2 types. In type I inotropic reaction first contraction after period of rest either exceeded basal contractions or remained at their level. Type 2 was characterized by significant lowering of amplitude of contractions relative to regular ones. Exposure of the myocardium to amiodarone (1 micro/L) led to significant potentiation of trabeculi contraction after periods of rest of muscles with type I reaction and did not affect response of muscles with type II reaction. These results allow to conclude that the state of calcium transporting systems of sarcoplasmic reticulum (SR) is different in ischemic and rheumatic heart failure and that realization of therapeutic effect of amiodarone depends on functional state of SR.

It was shown in experiments on Wistar rats that the processes of postinfarction remodeling of the heart in animals that survived myocardial infarction were characterized by lowering of excitability of cardiac muscle, impairment of ability of sarcoplasmic reticulum to accumulate calcium ions. High level of content of lipid peroxidation products both in blood serum and myocardial tissue, and lowering of activity of antioxidant enzymes were also noted. A conclusion was made that high level of lipid peroxidation, lowering of defense properties of endogenous antioxidants and impairment of myocardial contractile function induced by myocardial infarction were sustained minimally for subsequent 45 days. This could serve as a factor, capable to significantly affect the process of postinfarction remodeling and development of heart failure.

Membrane lipid phase has been studied in the sarcoplasmic reticulum (SR) of skeletal muscles, using spin probes. Hypercholesterolemia was found to increase rotation and decrease hydrophobicity of water-soluble probe medium located inside SR vesicles. This is probably indicative of the reduction in SR vesicle membranes. At the same time reduced rotation and increased hydrophobicity and regularity of fat-acid probe micro-environment are observed in hypercholesterolemia, with the differences disappearing nearer to the centre of the membrane. It is suggested that POL activation and cholesterol accumulation in SR membranes in hypercholesterolemia lead to greater density of phospholipid molecules in the membrane.