Low-Temperature Polymorphic Phase Transition in a Crystalline Tripeptide L-Ala-L-Pro-Gly·H2O Revealed by Adiabatic Calorimetry

We demonstrate application of precise adiabatic vacuum calorimetry to observation of phase transition in the tripeptide l-alanyl-l-prolyl-glycine monohydrate (APG) from 6 to 320 K and report the standard thermodynamic properties of the tripeptide in the entire range. Thus, the heat capacity of APG was measured by adiabatic vacuum calorimetry in the above temperature range. The tripeptide exhibits a reversible first-order solid-to-solid phase transition characterized by strong thermal hysteresis. We report the standard thermodynamic characteristics of this transition and show that differential scanning calorimetry can reliably characterize the observed phase transition with <5 mg of the sample. Additionally, the standard entropy of formation from the elemental substances and the standard entropy of hypothetical reaction of synthesis from the amino acids at 298.15 K were calculated for the studied tripeptide.National Institute of Biomedical Imaging and Bioengineering (U.S.) (EB-003151)National Institute of Biomedical Imaging and Bioengineering (U.S.) (EB-001960)National Institute of Biomedical Imaging and Bioengineering (U.S.) (EB-002026

Effects of Mechanical Interaction Between Two Rabbit Cardiac Muscles Connected in Parallel

Abstract. The hypothesis that myocardium mechanical inhomogeneity produces a substantial effect on mechanical function was tested. Muscle inhomogeneity was studied in isolated papillary muscles or trabeculae excised from rabbit right ventricle and connected in a parallel duplex. Each muscle was placed in a separate perfusion bath. One end of each muscle was fastened to an individual force transducer and the other to the common lever of a servomotor. This arrangement allowed both muscles, being excited independently, to pull jointly a load applied to the lever. Separate electrodes for each perfusion bath allowed to stimulate muscles with a time delay. Tension developed in the individual muscles and their interaction were studied. Developed tension was critically dependent on the timing and sequence of excitation. Using mathematical modeling, patterns of tension distribution experimentally observed in parallel duplexes were simulated. These results suggest that changes both in Ca 2+ transients and in the time course of Ca 2+ -troponin complexion due to the duplexed muscles interaction offset the effect of mechanical inhomogeneity

Functional heterogeneity arising due to electrical and mechanical interactions between cardial myocytes in mathematical model of homogeneous myocardial fiber

We developed a mathematical model describing heart muscle strand as a one-dimensional continuous medium of cardiomyocytes, through which electrical excitation propagates and excites the cells for contraction. Intracellular excitation-contraction coupling is presented by means of our earlier published model describing mechanical function of the cardiomyocyte evoked by action potential development and calcium activation of cross-bridge formation. The whole strand model simulates also mechanical interaction between the cardiomyocytes in the tissue and accounts for both intracellular and intercellular electro-mechanical coupling and mechano-electric feedback mechanisms. Numerical experiments with the strand formed of initially identical cardiomyocytes revealed that electrical and mechanical interaction between the cells, as well as intracellular mechano-electric feedbacks caused pronounced nonuniformity of their behavior. Model analysis suggests that cooperative mechanisms of myofilament calcium activation play the key role in dynamic adjustment of electrical and mechanical activity of the interacting cardiomyocytes in the tissue. © 2015