Young Rat Hearts Research Articles

Basal and stimulated plasma atrial natriuretic peptide (ANP) concentrations and cardiac and contents in old and young rats

To investigate the ability of the ageing heart to release atrial natriuretic peptide (ANP) we compared the response of awake, trained, chronically catheterized old (20–21 months) and young; (4 months) rats to an acute, hypertonic saline challenge. There were no differences between young and old rats in basal plasma concentration of sodium (P Na; old: 141 ± 3 meq/l; young 143 ± 3 meq/l) or ANP (old: 61 ± 5 pg/ml; young: 67 ± 12 pg/ml. Five minutes after acute saline challenge, P Na rose in both groups (old: 146 ± 2 meq/l; young 149 ± 1 meq/l) and approximately 3-fold increases in plasma ANP levels (182 ± 24 pg/ml; young: 179 ± 42 pg/ml). Hearts of old and young rats were assayed for atrial and ventricular ANP content. Atrial ANP levels were similar in old and young rats (13.5 ± 3.6 vs. 24.9 ± 8.7 μg/gl atrial tissue), whereas ventricular ANP content was ∼ 4-fold higher in old vs. young rats (153.7 ± 39.3 vs. 47.5 ± 6.4 ng/g ventricular tissue). Thus, the ageing rat heart responds equally as well as the young rat to an acute NaCl challenge.

Endothelin-1 may act as a negative inotropic agent in cardiac myocytes from young and senescent rats

The contractile response to Endothelin-1 (ET-1) has been investigated in electrically stimulated single cardiac myocytes isolated from hearts of young and senescent rats. ET-1 (0.1-10 nM) exhibited a marked negative inotropic effect in both age groups. ET-1 (1.0 nM) also reduced the occurrence of spontaneous contractile oscillations in young myocytes, suggesting depletion of sarcoplasmic reticulum-Ca(2+) stores. The possibility that ET-1 may have opposite effects on myocardial contractility is discussed.

On the Fibrinolytic System in Aged Rats, and Its Reactivity to Endotoxin and Cytokines

Aged rats are more susceptible to endotoxin-induced effects, including microthrombosis and platelet aggregation, than are young rats. To investigate whether changes in the fibrinolytic system might be involved, we investigated the fibrinolytic activity in plasma euglobulin fractions and tissues (lung and heart) of young (6-months old) and aged (24-months old) rats under baseline conditions and after challenge with endotoxin. Aged rats had lower plasma levels of tissue-type plasminogen activator (t-PA) and of urokinase-type PA (u-PA) activity. PA inhibitor (PAI) activity was higher in the plasma of aged rats, as was t-PA activity in lung and heart. Rats were treated with either a low dose (1 microgram/kg) or a high dose (10 mg/kg) of endotoxin. Both treatments induced a transient phase of increased blood fibrinolytic activity, as evidenced by higher levels of tissue-type plasminogen activator (t-PA) activity and decreased levels of PA inhibitor (PAI) activity. Over time, the fibrinolytic activity decreased, probably due to increased levels of PA inhibitor. Both the early increase in t-PA activity, and the subsequent increase in PAI activity, were more pronounced in the aged rats, as compared with the younger rats, after the high dose of endotoxin. The aged rats also responded to an injection of interleukin-1 beta or tumor necrosis factor-alpha with a larger increase of PAI activity than did the younger rats. Together the data suggest that, compared to young rats, aged rats have a decreased base-line plasma fibrinolytic activity, while their fibrinolytic system is more responsive to challenge by endotoxin and cytokines.

Catecholamine turnover in heart and adrenals of young rats: Effects of rearing temperature

1. 1. A comparative chronological study (from birth to one month of age) was performed to establish the levels and turnover rates of catecholamines in the heart and adrenals of rats born and reared at 28 and 16°C. 2. 2. In the heart, cold induced marked increases of the norepinephrine level and turnover rate only from the second week of age. 3. 3. In the adrenals there was considerable early stimulation of epinephrine production, but only in 28°C rats. 4. 4. The differences observed in the stimulation of the sympathetic nervous system according to the rearing temperature are possibly due to the behavior of the mother in warming the pups.

Prevention of chronic pulmonary oxygen toxicity in young rats with liposome‐encapsulated catalase administered intratracheally

The lungs and hearts of young rats exposed to 100% oxygen (O2) for 8 days (27 to 35 days of age) were studied following recovery in room air at 60 days of age using morphometric, biochemical, and physiological techniques. In an attempt to prevent chronic oxygen toxicity 153 rats had transtracheal catheters surgically implanted and were treated during the O2 exposure with daily intratracheal injections of liposome-encapsulated superoxide dismutase (SOD) and/or catalase (CAT). Oxygen exposure in this model results in chronic cardiopulmonary alterations which include pulmonary hypertension, right ventricular hypertrophy, and a decrease in number of pulmonary arterioles 25 to 50 microns in diameter with increased muscularization of their walls. The volume densities of the parenchyma, parenchymal air space, and the alveolar space are increased, while that of the combined alveolar ductal and respiratory bronchiolar space is decreased. Daily intratracheal administration of liposome-encapsulated CAT (160 U) during the O2 exposure prevented these chronic changes. Liposome-encapsulated SOD (110 U) or SOD (50 U) + CAT (70 U) did not appear to have a preventive effect. During the first 3 to 5 days following oxygen exposure the lung tissue enzymes SOD, CAT, and glutathione peroxidase markedly increased. We conclude that in the young rat animal model liposome-encapsulated CAT (160 U) given intratracheally during the period of O2 exposure is safe and will prevent the chronic vascular and parenchymal damage due to oxygen toxicity.

Physiological expression of neural marker proteins in the heart of young rats

The α-subunit of a GTP-binding protein G o (G oα), is a new marker protein of neurons and neuroendocrine cells. In our previous report, this protein was shown to be also distributed in the cardiac muscle of young rats, although the cardiac muscle is traditionally thought to be a non-neuronal tissue. To evaluate this unusual finding, expression of other neural marker proteins in the heart was examined. Immunohistochemical studies using antibodies against γ-enolase, neurofilament and G oα protein revealed localization of these neural markers only in neurons and neuroendocrine cells in adult rats. In young rats, however, these neural markers were localized not only in these cells but also in the cardiac muscle. An enzyme immunoassay study of enolase isozymes showed that the heart of young rats was rich in neuron-specific γ-enolase rather than in muscle-specific β-enolase, while the heart of adult rats was deprived of the former and rich in the latter. These results suggest that the heart of young rats should be understood as a neuroendocrine tissue as well as a muscular tissue.

Increased myocardial adenosine production and reduction of beta-adrenergic contractile response in aged hearts.

The contractile response of the aged adult heart to beta-adrenergic stimulation is known to be reduced compared with the young adult heart. Since endogenous adenosine exerts an antiadrenergic action in the heart, this study was undertaken to determine if the basal endogenous level of myocardial adenosine increases with age and whether this increase mediates the reduced responsiveness of aged heart to beta-adrenergic stimulation. Young (3-5 months) and aged (12-22 months) Sprague-Dawley adult rat hearts of CD and SD stock were perfused at constant pressure and paced at 270 contractions/min. The two age groups had a similar level of +dP/dtmax (index of contractility) under control conditions. Adenosine release into the coronary effluent was 30 +/- 3 nmol/min/g dry wt from young and 54 +/- 9 nmol/min/g dry wt from aged hearts. Inosine release was also greater from the aged hearts. Isoproterenol (10(-8) M) stimulation increased contractile state by 113% in young hearts and only 69% in aged hearts. Isoproterenol further increased the adenosine and inosine release from both age groups. Theophylline (5 x 10(-5) M), an adenosine antagonist, prevented the difference in the contractile response to isoproterenol stimulation between the young and aged hearts. Elevation of external calcium from 2 to 4 mM increased contractility equally in both age groups without influencing adenosine release. Myocardial oxygen consumption, coronary effluent PO2, oxygen supply-demand ratio, and lactate release were similar for both age groups, indicating that under the conditions studied the elevated release of adenosine by the aged hearts was not due to hypoxia. Aged (10-14 months) adult guinea pig hearts also displayed a reduced responsiveness to the isoproterenol stimulation and released more adenosine compared with young (3-4 months) adult guinea pig hearts. These findings suggest that enhanced adenosine levels that are present in the aged myocardium are responsible, in part, for the reduced contractile responsiveness of the older adult heart to beta-adrenergic stimulation.

The acute and chronic effects of ethanol on cardiac muscle protein synthesis in the rat in vivo

An investigation was made into the acute and chronic effects of ethanol on rates of protein synthesis in the hearts of young rats (80–100 g body weight). Acute ethanol administration (75 mmol/kg body weight, IP) significantly reduced the fractional rate of protein synthesis by 20% after 2.5 hr, compared with saline-treated controls. Chronic ethanol feeding (36% of total calories) for 6 weeks significantly reduced cardiac wet weight by 11%, when compared to rats fed isovolumetric amounts of the same diet in which ethanol was substituted by isocaloric glucose. Neither the concentration nor the content of mixed cardiac proteins relative to body weight were overtly altered by chronic ethanol feeding, although, the total content of mixed cardiac proteins were significantly decreased. RNA concentrations and RNA relative to body weight increased slightly, but total cardiac DNA decreased. Indices for the capacity or potential of the heart to synthesis protein (indicated by the RNA/protein and RNA/DNA ratios) and the “DNA-unit” (protein/DNA ratio) were increased in response to chronic ethanol treatment. The fractional and absolute rates of mixed protein synthesis in the heart were (relatively) unaltered by chronic ethanol treatment, as was RNA efficiency and synthesis relative to DNA. It was concluded that the heart displays contrasting responses to acute and chronic ethanol exposure.

Synthesis of subcellular protein fractions in the rat heart in vivo in response to chronic ethanol feeding.

An investigation was made into the chronic effects of ethanol on the subcellular protein fractions in the hearts of young rats (80-100 g body weight). Rats were fed a nutritionally adequate liquid diet containing ethanol as 36% of total energy. Controls were fed the same diet in which ethanol was substituted by iso-energetic glucose. At the end of 6 weeks, rats were killed and hearts were fractionated into sarcoplasmic, myofibrillar and stromal protein fractions by differential solubilisation. The total myofibrillar protein content was significantly reduced by chronic ethanol feeding, though the contents of other fractions were relatively unaltered. The fractional and absolute rates of myofibrillar protein synthesis were significantly increased, but the synthesis rates of sarcoplasmic and stromal protein fractions were unaffected by ethanol feeding. This suggests independent regulation of myofibrillar protein content in the ethanol exposed heart. These changes may be responsible for alterations in myocardial function in alcoholic cardiomyopathy.

Evidence for thyroxine transport by the lung and heart capillary endothelium

The uptake and transport of carrier-bound thyroxine by the endothelium were investigated by perfusing through the heart and lung of young rats radiolabeled thyroxine bound to prealbumin ([ 125I]T 4Pa) or serum ([ 125I]T 4S). In addition these complexes were tagged to 5-nm gold particles to obtain quantitative (radioassay) and qualitative (autoradiography) data from the same experiment. The complexes (prewarmed at 37°) were perfused in situ at various concentrations (1 to 50 μCi/ml) for time intervals ranging from 5 to 30 min. After thorough washing of the unbound probe, tissue fragments were either measured for total uptake in a gamma counter or processed for electron microscopy autoradiography. The results showed that both the lung and heart take up [ 125I]T 4 complexes by a process that is saturable at low hormone concentration; uptake is competed by free T 4 and Pa. In specimens perfused with double-labeled complexes (iodinated and tagged to gold) auto-radiography revealed that silver grains and gold particles colocalize predominantly on endothelial plasmalemmal vesicles. The probe appeared first in vesicles open to the capillary lumen (5 min) and further on in vesicles apparently free within the cytoplasm or open to the abluminal front. At 30 min, only silver grains seem to be present in the pericapillary space, on the alveolar epithelial cells, as well as on the nucleus and mitochondria of heart myocytes. The findings suggest that (1) T 4Pa uptake by the endothelial cell (EC) is a specific process (possibly via specific binding sites); (2) T 4Pa is taken up and transported across capillary EC by plasmalemmal vesicles; (3) in the pericapillary space T 4 seems to dissociate from its carrier.

Localization of types I, III and IV collagen mRNAs in rat heart cells by in situ hybridization

Previous studies investigating the cellular origins of several collagens in young adult rat hearts (Eghbali et al., 1988) demonstrated that the mRNAs for types I and III collagen occurred in non-myocyte cells, mostly fibroblasts, whereas the mRNA for type IV collagen was observed in both myocytes and non-myocyte cells. In the present study, cellular localization of collagen mRNAs has been achieved by in situ hybridization in rat heart tissue and in isolated heart cells. Frozen tissue sections, isolated cardiomyocytes, cultured neonatal cardiomyocytes and fibroblasts were hybridized with DNA probes for type-specific collagens, actin, and myosin heavy chain. Silver grains were visualized by dark field imaging. In heart sections, types I and III mRNAs were observed predominantly adjacent to myocytes and in the interstitium, where fibroblasts are known to be present. In contrast, type IV collagen mRNA was identified both within the myocytes and the interstitium. In freshly isolated adult cardiomyocytes and in cultured neonatal cardiomyocytes, collagen type IV mRNA was observed but type I collagen mRNA was not. In cultured neonatal fibroblasts, both types IV and I collagen mRNAs were abundant.

The effects of carbon monoxide on persistent changes in young rat heart: Cardiomegaly, tachycardia and altered DNA content

Newborn rat pups that inhale 500 ppm carbon monoxide (CO) for 32 days and develop increased heart mass (i.e. cardiomegaly) show persistent cardiomegaly and elevated resting heart rate as adults. Studies were carried out to explore the relationship of these phenomena to CO concentration and initial cardiomegaly using exposures of 350, 500 and 700 ppm CO. Initial cardiomegaly was greater in the right ventricle free wall (RV) than in the left ventricle plus interventricular septum (LV + S) at all three CO concentrations, and RV mass excess increased with CO concentration. Initial RV cardiomegaly was greater at 350 and 700 ppm CO in females than in males. Persistent cardiomegaly in the RV also increased with CO concentration, and was significantly greater in the females. Persistent cardiomegaly in the RV increased with initial cardiomegaly but at a decreasing rate at the higher CO concentrations, and when expressed as a percentage of initial cardiomegaly (i.e. 'efficiency'), the relative response was greatest at 500 ppm CO. For LV + S, efficiency of development of persistent cardiomegaly was greatest at 350 ppm CO. Persistent tachycardia increased with CO concentration in males but failed to do so in females, and was only weakly correlated with degree of persistent cardiomegaly. Thus, persistent cardiomegaly and persistent tachycardia (in males) are related to CO concentration. Myocardial DNA content of 32-day-old juveniles was significantly increased at 350 and 700 ppm CO. Adult DNA content of the RV was significantly elevated at 350 ppm CO (females) and continued to rise with CO concentration. DNA concentrations of the RV and LV were increased at 700 ppm CO.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related modifications in rat cardiac phosphoglycerate kinase. Rejuvenation of the old enzyme by unfolding-refolding

The occurrence of age-related modifications in functional and structural properties of several enzymes has been documented; however, the molecular basis of this phenomenon is still mostly unexplained. In the present work a comparative study of phosphoglycerate kinase preparations isolated from hearts of young and old rats was undertaken. Marked age-related effects wer revealed in the heat-inactivation kinetics of the enzyme, similar to the ones previously found in purified muscle phosphoglycerate kinase. In view of the previously reported failure of immunotitration to distinguish between phosphoglycerate kinase forms in crude heart extracts from young and old rats, it appears likely that the modifications in old rat heart phosphoglycerate kinase are in a domain which is not involved in antibody binding, and may be localized in the interior of the enzyme. These age-related modifications were completely relieved by extensive unfolding of the enzyme in 2 M guanidine hydrochloride, followed by enzyme reactivation upon dilution of the denaturant. The refolding products of young and old enzymes displayed identical heat-inactivation kinetics as native young phosphoglycerate kinase. It is concluded that the age-related alterations in rat cardiac phosphoglycerate kinase, like those found in the muscle enzyme, are purely conformational and hence develop postsynthetically.

Cardiac sarcoplasmic reticulum characteristics in hypertrophic hearts from spontaneously hypertensive rats.

Sarcoplasmic reticulum (SR) from left ventricles of rats that developed spontaneous hypertension was studied in vitro. Similar increases of left ventricular mass were found when grouping the animals into mild and severe hypertensives (average systolic arterial pressure of 168 +/- 4 and 202 +/- 6 mmHg, respectively). The amount of SR protein (mg/g of left ventricle) was higher when obtained from hypertrophic ventricles of both hypertensive groups than from ventricles of the control group. The result agreed with the enhanced Ca2+ uptake exhibited by left ventricular homogenates of the hypertensive groups. Consequently, Ca2+ uptake in SR microsomes isolated per gram of left ventricle (nmol Ca2+/g muscle) was 51.62 +/- 10.06 and 64.99 +/- 12.84 in mildly and severely hypertensive groups vs. 17.37 +/- 5.79 in the control group (P less than 0.05). The SR microsomes obtained from ventricles of hypertensive rats showed an enhanced Ca2+ activated ATPase activity that was not accompanied by increased Ca2+ uptake at saturating calcium concentrations, but by increased affinity for calcium (K'app. of 1.09 +/- 0.28 and 2.67 +/- 0.16 microM in SR microsomes of hypertrophic and control ventricles respectively; P less than 0.05). The rates of calcium loss, measured in SR vesicles passively loaded with 45Ca, were similar when assayed in SR obtained from ventricles of both hypertensive and normal rats. These results enable us to suggest that in hearts of rats presenting spontaneous hypertension, the function of the SR system could account for a normal handling of cytosolic calcium. They might support the absence of mechanical alterations described in hearts of young rats of the SHR strain.

Neuron-specific proteins in the young rat heart

Neuron-specific proteins in the young rat heart

Contractile function of isolated young and adult rat heart cells.

The stimulated contractile function of aerobic isolated adult rat heart cells was assessed by laser light diffraction. Cells were maintained for up to 8 h by attachment to a cover slip and continuous perfusion in a chamber on the microscope stage. On stimulation such cells beat as though unattached. Cells showed a negative staircase which was reduced by increasing Ca or isoproterenol. Ryanodine caused a positive staircase on stimulation, which was enhanced by increasing Ca or isoproterenol or by using cells from younger rats. For beats of constant contraction duration, there was a linear relationship between the magnitude of cell shortening and the velocity of shortening, independent of the concentration of extracellular Ca. We conclude the following. 1) The beat characteristics of isolated cells are very similar to those of papillary muscle. 2) Attachment of cells need not alter their unloaded shortening characteristics. 3) Cells appear to contract against an internal load, to an extent determined by the degree of myofilament activation. 4) Cells from young rats require less extracellular Ca than those from adult rats for the same beat magnitude.

Enzyme histochemistry of monoamine oxidase in the heart of aged rats

The distribution of monoamine oxidase types A and B in the heart of young (3-month-old) and aged (26-month) Wistar rats was studied by histochemical methods. In young rats, MAO activity was higher in the left ventricle than in the right. Enzyme reactivity was present within both muscle cells and the blood vessel wall. Histochemical staining was abolished by clorgyline administration. In the heart of aged rats, we observed a very remarkable increase of clorgyline sensitive MAO activity primarily at the level of myocardial cells. The right ventricle showed the highest increase of MAO reactivity.

Effects of thyroid hormone on beta-adrenergic responsiveness of aging cardiovascular systems.

We have compared the effects of beta-adrenergic stimulation on the heart and peripheral vasculature of young (2-mo-old) and older (12-mo-old) rats both in the presence and absence of triiodothyronine (T3)-induced hyperthyroidism. The hemodynamic consequences of T3 treatment were less prominent in the aged hyperthyroid rats compared with young hyperthyroid rats (both in intact and pithed rats). There was a decrease in sensitivity of chronotropic responsiveness to isoproterenol in older pithed rats, which was apparently reversed by T3 treatment. The number and affinity of myocardial beta-adrenergic receptor sites measured by [125I]cyanopindolol were not significantly different in young and older control rats; also, beta-receptor density increased to a similar extent in both young and older T3-treated rats. The ability of isoproterenol to relax mesenteric arterial rings, markedly blunted in older rats, was partially restored by T3 treatment without their being any change in isoproterenol-mediated relaxation in the arterial preparations from young rats. The number and affinity of the beta-adrenergic receptors measured in the mesenteric arteries was unaffected by either aging or T3 treatment. The data suggest that effects of thyroid hormone and age-related alterations of cardiovascular responsiveness to beta-adrenergic stimulation are interrelated in a complex fashion with a net result that the hyperkinetic cardiovascular manifestations in hyperthyroidism are attenuated in the older animals.

Muscle myoglobin as determined by electrophoresis in thermally acclimated rat.

Quantitative analysis of myoglobin (Mb) was established using the sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis in order to determine the influence of thermal acclimation on Mb levels in muscles. Myoglobin was measured in the red and white parts of quadriceps, soleus, diaphragm and heart of young adult male rats (11 weeks old). It was higher in the red muscle (0.92 +/- 0.043 mg/g fresh weight) than in the white muscle (0.08 +/- 0.007), and similar in the red muscle and soleus (1.17 +/- 0.117). Heart showed the highest level (1.52 +/- 0.073) among the tissues studied. The level in diaphragm (0.74 +/- 0.039) was intermediate between red muscle and heart. Cold acclimation (5 degrees C for 4 weeks, 11 weeks old) caused significant increases in Mb levels in white muscle (0.27 +/- 0.031, p less than 0.001), heart (1.95 +/- 0.094, p less than 0.01), and diaphragm (1.01 +/- 0.060, p less than 0.01), but not in red muscle and soleus. The rats reared in cold for many generations (20 generations at 5 degrees C, 11 weeks old) manifested significant increases in Mb levels of all tissues (p less than 0.05-0.001) examined. Heat acclimation (33 degrees C for 4 weeks, 11 weeks old) did not influence Mb levels of the tissues. The above findings suggest that skeletal muscle Mb may be partly involved in an enhanced thermogenesis in cold acclimation by favouring an oxidative capacity of muscles.

Isoenzymatic and ultrastructural variations in the heart of young hypo and euthyroid rats

Isoenzymatic and ultrastructural variations in the heart of young hypo and euthyroid rats