State of the capillary network of rat myocardium in the presence of hypertrophy and physical loading

Morphofunctional state of the capillary network in various myocardial parts was studied in white rats under normal conditions, myocardial hypertrophy of different degree and under physical loading. It was demonstrated that density, metabolic surface and capacity of the capillary bed is larger in the right ventricle than in the left one. The capillary blood supply in hypertrophied myocardium, increasing simultaneously with hypertrophy, at the state of rest corresponds to its increasing mass. On the contrary, under maximal physical loading functioning of the capillary part in the myocardium becomes unadequate that is evident from a decreasing activity of the test animals.     

The recovery process in the fetal rabbit myocardium following mechanical trauma]

Healing processes of the myocardium after an injury during the intrauterine period were studied. After the mechanical trauma of the heart the rabbit fetus is kept in the maternal organism and is born in due course. It was found that the healing of the injured part of the cardiac muscle occurred shortly and was due to the high proliferative activity of the cells. Morphological structure of the tissue appearing at the site of injury is very similar to that of the muscle tissue. However, further studies are necessary for final decision.     

Quantification of the mechanical properties of noncontracting canine myocardium under simultaneous biaxial loading

Detailed understanding of cardiac mechanics depends upon accurate and complete characterization of the three-dimensional properties of both normal and diseased myocardial tissue. This, however, can only be obtained by performing multiaxial tests on cardiac tissue. In this study we subjected thin sheets of passive canine left ventricular myocardium to various combinations of simultaneous biaxial stretching. During each stretch the ratio of the orthogonal strains was kept constant and the corresponding stresses remained proportional. We fitted the biaxial stress-strain data both with exponential strain-energy functions with quadratic powers of strains as well as with an alternative function with nonintegral powers of strains. We used our recently developed nonparametric method to assess the reliability of the coefficients for each of these functions. The quadratic strain-energy functions resulted in wide intra- and interspecimen variability in the coefficients. Moreover, both their absolute and relative values demonstrated marked load history dependence such that interpretation of the direction of anisotropy was difficult. Fitting the data with the alternative nonintegral strain-energy function seemed to alleviate these problems. This alternative strain-energy function may provide more self-consistent results than the more commonly used quadratic strain-energy functions.     

In vitro time-dependent response of periodontal ligament to mechanical loading.

This study examined the time-dependent response of bovine periodontal ligament (PDL). Applying linear viscoelastic theory, the objective was 1) to examine the linearity of the PDL's response in terms of its scaling and superposition property and 2) to generate the phase lag-vs.-frequency spectrum graph. PDL specimens were tested under three separate straining conditions: 1) tension ramp tests conducted at different strain rates, 2) pulling step-straining to 0.3 in discrete tests and to 0.3 and 0.6 in one continuous run, and 3) tension-compression sinusoidal oscillations. To this effect, bar-shaped specimens of bovine roots that comprised portions of dentin, PDL tissue, and alveolar bone were produced and strained in a microtensile machine. The experimental data demonstrated that neither the scaling nor the superposition properties were verified and that the viscoelastic response of the PDL was nonlinear. The PDL's elastic response was essentially stiffening, and its viscous component was pseudoplastic. The tangent of the PDL's strain-stress phase lag was in the 0-0.1 range in the tensile direction and in the 0.35-0.45 range in the compressive direction. In line with other biological tissues, the phase lag was largely independent of frequency. By use of the data generated, a mathematical model is outlined that reproduces both the elastic stiffening and viscous thinning of the PDL's response.     

T4-induced cardiac hypertrophy disrupts cyclic GMP mediated responses to brain natriuretic peptide in rabbit myocardium

Brain natriuretic peptide (BNP) affects the regulation of myocardial metabolism through the production of cGMP and these effects may be altered by cardiac hypertrophy. We tested the hypothesis that BNP would cause decreased metabolism and function in the heart and cardiac myocytes by increasing cGMP and that these effects would be disrupted after thyroxine-induced cardiac hypertrophy (T4). Open-chest control and T4 rabbits were instrumented to determine local effects of epicardial BNP (10(-3) M). Function of isolated cardiac myocytes was examined with BNP (10(-8)-10(-7) M) with or without KT5823 (10(-6) M, cGMP protein kinase inhibitor). Cyclic GMP levels were measured in myocytes. In open-chest controls, O2 consumption was reduced in the BNP area of the subepicardium (6.6+/-1.3 ml O2/min/100 g versus 8.9+/-1.4 ml O2/min/100 g) and subendocardium (9.4+/-1.3 versus 11.3+/-0.99). In T4 animals, functional and metabolic rates were higher than controls, but there was no difference between BNP-treated and untreated areas. In isolated control myocytes, BNP (10(-7) M) reduced percent shortening (PSH) from 6.5+/-0.6 to 4.3+/-0.4%. With KT5823 there was no effect of BNP on PSH. In T4 myocytes, BNP had no effect on PSH. In control myocytes, BNP caused cGMP levels to rise from 279+/-8 to 584+/-14 fmol/10(5) cells. In T4 myocytes, baseline cGMP levels were lower (117+/-2 l) and were not significantly increased by BNP. Thus, BNP caused decreased metabolism and function while increasing cGMP in control. These effects were lost after T4 due to lack of cGMP production. These data indicated that the effects of BNP on heart function operated through a cGMP-dependent mechanism, and that this mechanism was disrupted in T4-induced cardiac hypertrophy.     

Long-term mechanical loading of chondrocyte-chitosan biocomposites in vitro enhanced their proteoglycan and collagen content

One major problem in cartilage tissue engineering is the insufficient biochemical composition of the generated biocomposites. The aim of this study was to improve the collagen and proteoglycan deposition in tissue engineering constructs by application of long-term mechanical loading in culture. Chondrocyte-seeded chitosan biocomposites revealed a homogenous cell distribution, high viability (>95%) and maintenance of a rounded cell shape typical for chondrocytes over 3 weeks of load-free culture. Cyclic compression of chitosan biocomposites (0.1 Hz, amplitude 5-15%, 45 min on and 315 min off) was applied after two different preculture times (3, 21 days) for 3 weeks. At day 42 this resulted in enhanced mRNA levels for aggrecan and a significantly higher specific proteoglycan (5-fold, p<0.0002) and collagen type II (2-fold, p<0.02) deposition compared to unloaded controls. In sum, the chitosan scaffold was highly attractive for cartilage tissue engineering approaches and mechanical loading allowed to further improve the biochemical composition of these biocomposites.     

Early ultrastructural changes in the myocardium following thyroxine-induced hypertrophy

The model of myocardial hypertrophy induced by thyroxine was studied with particular regard to the early ultrastructural changes in fractional volume of the mitochondria and myofibrils, and capillary distribution. Following injections of L-thyroxine (25 mg/kg IP) for 9 consecutive days, rats were sacrificed by vascular perfusion and cardiac tissue samples from the mid-wall zone of the left ventricle were processed routinely for electron microscopy. Heart weight/body weight ratios of thyroxine treated (T) rats showed a significant increase (P less than 0.001) over the ratios in control (C) rats. Likewise, the fractional volume of mitochondria (42%) was significantly increased (P less than 0.001) in the myocardium of T rats when compared with C rats (31%). However, the fractional volume of myofibrils was significantly decreased in the myocardium of T rats (P less than 0.001) and there was no significant difference between the hearts of T and C rats with respect to capillary luminal area/myocyte area. The mitochondria/myofibril ratio was increased in the hearts of T rats (0.82) over that found in control hearts (0.52). These results suggest that in the early stages of thyroxine-induced myocardial hypertrophy there is not an immediate increase in capillary area which may account for the ischemia and significant increase in mitochondrial volume which characterized myocardial hypertrophy in this model.     

Rescue of contractile parameters and myocyte hypertrophy in calsequestrin overexpressing myocardium by phospholamban ablation

Cardiac-specific overexpression of murine cardiac calsequestrin results in depressed cardiac contractile parameters, low Ca(2+)-induced Ca(2+) release from sarcoplasmic reticulum (SR) and cardiac hypertrophy in transgenic mice. To test the hypothesis that inhibition of phospholamban activity may rescue some of these phenotypic alterations, the calsequestrin overexpressing mice were cross-bred with phospholamban-knockout mice. Phospholamban ablation in calsequestrin overexpressing mice led to reversal of the depressed cardiac contractile parameters in Langendorff-perfused hearts or in vivo. This was associated with increases of SR Ca(2+) storage, assessed by caffeine-induced Na(+)-Ca(2+) exchanger currents. The inactivation time of the L-type Ca(2+) current (I(Ca)), which has an inverse correlation with Ca(2+)-induced SR Ca(2+) release, and the relation between the peak current density and half-inactivation time were also normalized, indicating a restoration in the ability of I(Ca) to trigger SR Ca(2+) release. The prolonged action potentials in calsequestrin overexpressing cardiomyocytes also reversed to normal upon phospholamban ablation. Furthermore, ablation of phospholamban restored the expression levels of atrial natriuretic factor and alpha-skeletal actin mRNA as well as ventricular myocyte size. These results indicate that attenuation of phospholamban function may prevent or overcome functional and remodeling defects in hypertrophied hearts.     

Induction and prevention of chondrocyte hypertrophy in culture

Primary chondrocytes from whole chick embryo sterna can be maintained in suspension culture stabilized with agarose for extended periods of time. In the absence of FBS, the cells remain viable only when seeded at high densities. They do not proliferate at a high rate but they deposit extracellular matrix with fibrils resembling those of authentic embryonic cartilage in their appearance and collagen composition. The cells exhibit many morphological and biochemical characteristics of resting chondrocytes and they do not produce collagen X, a marker for hypertrophic cartilage undergoing endochondral ossification. At low density, cells survive in culture without FBS when the media are conditioned by chondrocytes grown at high density. Thus, resting cartilage cells in agarose cultures can produce factors required for their own viability. Addition of FBS to the culture media leads to profound changes in the phenotype of chondrocytes seeded at low density. Cells form colonies at a high rate and assume properties of hypertrophic cells, including the synthesis of collagen X. They extensively deposit extracellular matrix resembling more closely that of adult rather than embryonic cartilage.     

Pacing-induced transient depolarizations in rabbit atrial myocardium

Transient afterdepolarizations (TD) were analyzed following a train of paced action potentials in isolated pectinate muscles bordering the crista terminalis in rabbit right atria. After cessation of a rapid drive TD peaked between 400 and 700 ms. The amplitude of TD were found to be pacing-dependent (maximum at 250 ms pre-drive pacing interval 12.9 +/- 2.2 mV, n = 27). At pacing intervals longer than 1000 ms no TD could be observed but pacing intervals shorter than 250 ms provoked a triggered activity. Verapamil (13.2 X 10(-6) mol.1(-1) completely blocked pacing-induced TD. TD could be described quantitatively using a model of the transient inward current.