Glucocorticoids act in the dorsal hindbrain to modulate baroreflex control of heart rate

Systemic corticosterone (Cort) modulates arterial baroreflex control of both heart rate and renal sympathetic nerve activity. Because baroreceptor afferents terminate in the dorsal hindbrain (DHB), an area with dense corticosteroid receptor expression, we tested the hypothesis that prolonged activation of DHB Cort receptors increases the midpoint and reduces the gain of arterial baroreflex control of heart rate in conscious rats. Small (3-4 mg) pellets of Cort (DHB Cort) or Silastic (DHB Sham) were placed on the surface of the DHB, or Cort was administered systemically by placing a Cort pellet on the surface of the dura (Dura Cort). Baroreflex control of heart rate was determined in conscious male Sprague Dawley rats on each of 4 days after initiation of treatment. Plots of arterial pressure vs. heart rate were analyzed using a four-parameter logistic function. After 3 days of treatment, the arterial pressure midpoint for baroreflex control of heart rate was increased in DHB Cort rats (123 +/- 2 mmHg) relative to both DHB Sham (108 +/- 3 mmHg) and Dura Cort rats (109 +/- 2 mmHg, P < 0.05). On day 4, baseline arterial pressure was greater in DHB Cort (112 +/- 2 mmHg) compared with DHB Sham (105 +/- 2 mmHg) and Dura Cort animals (106 +/- 2 mmHg, P < 0.05), and the arterial pressure midpoint was significantly greater than mean arterial pressure in the DHB Cort group only. Also on day 4, maximum baroreflex gain was reduced in DHB Cort (2.72 +/- 0.12 beats x min(-1) x mmHg(-1)) relative to DHB Sham and Dura Cort rats (3.51 +/- 0.28 and 3.37 +/- 0.27 beats x min(-1) x mmHg(-1), P < 0.05). We conclude that Cort acts in the DHB to increase the midpoint and reduce the gain of the heart rate baroreflex function.     

Catecholamines in fetal pig plasma and the response to acute hypoxia and chronic fetal decapitation

Summary Dopamine, norepinephrine and epinephrine were measured by radioenzymatic assay in blood plasma samples drawn from the umbilical arteries of 30 anaesthetised Landrace pig fetuses. Just prior to term, the concentrations of dopamine (0.46±0.14 ng·ml^–1) and norepinephrine (1.74±0.60 ng·mg^–1) were lower than earlier in gestation, whereas epinephrine concentrations at term (0.80±0.31 ng·ml^–1) were similar to those at mid-gestation, intervening stages of gestation having higher levels of plasma epinephrine. Fetal hypoxia was induced by clamping the umbilical cord for 2 min and the catecholamines determined in arterial blood samples immediately thereafter, then again 3 min after removal of the clamp. Inconsistent effects of cord clamping on catecholamine levels were seen at 55 days, but thereafter, in all but one instance, the hormone levels were increased. Fetuses near term tended to respond less than fetuses at 75 and 96 days gestation (term=114±1 day). Catecholamines were also present in the circulation of fetuses decapitated at 42 days gestation and studied at 109±1 days. The average concentrations of dopamine (1.12±0.27 ng·ml^–1) and norepinephrine (8.23±3.04 ng·ml^–1) were greater than in intact fetuses, the plasma epinephrine levels being comparable to, or slightly higher than, those in intact fetuses. The results demonstrate that catecholamines are present in the circulation of the intact and decapitated pig fetus and that the actual concentrations and the type of response to umbilical cord clamping are dependent on gestation age.     

Exogenous stimuli maintain intraepithelial lymphocytes via aryl hydrocarbon receptor activation

The body's surfaces form the interface with the external environment, protecting the host. These epithelial barriers are also colonized by a controlled diversity of microorganisms, disturbances of which can give rise to disease. Specialized intraepithelial lymphocytes (IELs), which reside at these sites, are important as a first line of defense as well as in epithelial barrier organization and wound repair. We show here that the aryl hydrocarbon receptor (AhR) is a crucial regulator in maintaining IEL numbers in both the skin and the intestine. In the intestine, AhR deficiency or the lack of AhR ligands compromises the maintenance of IELs and the control of the microbial load and composition, resulting in heightened immune activation and increased vulnerability to epithelial damage. AhR activity can be regulated by dietary components, such as those present in cruciferous vegetables, providing a mechanistic link between dietary compounds, the intestinal immune system, and the microbiota.     

Catecholamines help snakes have a change of heart

JGP study on python snakes reveals that the regulation of ventricular repolarization by the sympathetic nervous system is evolutionarily conserved.

The T-wave of an electrocardiogram (ECG) arises from local differences in ventricular repolarization and represents a vulnerable period for the generation of arrythmias when some, but not all, of the myocardium is still refractory and unable to generate a new action potential. In mammals, ventricular repolarization is regulated by catecholamines released by the autonomic nervous system. In this issue of JGP, Boukens et al. show that this mode of regulation is conserved in the ball python, Python regius (1).Bas Boukens (left), Bjarke Jensen (center), and colleagues reveal that, similar to mammals, catecholamines released by the autonomic nervous system regulate ventricular repolarization in ball pythons. An ECG (right) shows that, by altering the pattern of ventricular repolarization, adrenaline treatment causes an inversion of the T-wave (red arrowhead). A similar phenomenon is observed in snakes undergoing a rise in body temperature, when autonomic tone increases.Working together at Amsterdam UMC, Bas Boukens and Bjarke Jensen are interested in the electrophysiological adaptations that have occurred during cardiac evolution. Cold-blooded reptiles have a much longer ventricular repolarization phase than warm-blooded mammals, even at 37°C (2). Moreover, the T-wave is typically negative in reptiles, whereas in mammals it is usually positive. Anecdotal observations, however, suggest that, in some reptiles, the T-wave can invert and become positive at higher body temperatures (3, 4). “We were curious about what might underlie these observations,” Jensen says.The researchers therefore recorded ECGs in living ball pythons as their body temperatures were increased (1). The ball python’s heart is unique in having functionally distinct ventricles, with a high-pressure left side and a low-pressure right side, even though, as in other snakes, the two sides are not anatomically separated. Though results varied across individual pythons and ECG leads, raising body temperature from 25 to 35°C caused an inversion of the snakes’ T-wave, reflecting temperature-dependent changes in the pattern of ventricular repolarization.In 1880, Burdon-Sanderson and Page (5) showed in their classic experiments that local differences in temperature change T-wave polarity in the ectothermic heart, presumably due to a direct effect of temperature on the activity of cardiac ion channels. However, when Boukens et al. recorded an ECG from a decapitated python, they found that raising temperature did not cause T-wave inversion (1). “So, we realized that it might not be a direct effect of temperature but might involve another factor, namely catecholamines released by the autonomic nervous system,” says Boukens. Autonomic activity increases at higher temperatures, but the ability of catecholamines to modulate ventricular repolarization would be blunted in decapitated snakes lacking a functional nervous system.Sure enough, the researchers found that stimulating the β-adrenergic receptor induced T-wave inversion in pythons maintained at a stable temperature. In contrast, the β blocker propranolol largely prevented higher temperatures from inducing T-wave inversion.Thus, similar to mammals, catecholamines regulate ventricular repolarization in ball pythons, and the increase in autonomic tone at higher temperatures alters the pattern of repolarization and changes T-wave shape. “The T-wave inversion suggests that certain regions of the python heart respond more strongly to adrenergic stimulation than other regions,” Boukens says.To test this idea, the researchers performed RNA sequencing of tissue samples taken from different regions of the python heart. “Catecholamine-associated genes exhibited differential expression between the left and right sides of the ventricle, consistent with the repolarization of these regions being differentially modulated by adrenergic signaling,” says Jensen.This may provide some sort of advantage to pythons as their body temperature rises, though the resulting changes in repolarization pattern could also leave them vulnerable to developing arrythmias. Boukens and Jensen are now extending their studies to a different branch of the evolutionary tree, examining repolarization and arrhythmogenesis in zebra finches (6).     

Effect of a beta-adrenergic antagonist on blood pressure, heart rate and beta-adrenoceptors in turkey poults

An irreversible beta-adrenergic blocker, bromoacetylalprenololmenthane (BAAM), was administered both peripherally and centrally to turkey poults, Meleagris gallopavo. Peripheral administration of BAAM (60 mg/kg body weight) effected a significant reduction in blood pressure and heart rate. Twenty minutes postinjection, mean blood pressure and heart rate were reduced 34.5 and 24.2%, respectively. Two days later, mean blood pressure values remained significantly depressed at 17.3% below preinjection determinations. Biochemical analysis of heart tissue following peripheral (intraperitoneal) injections of BAAM (60 mg/kg body weight) showed a significant decrease in beta-adrenergic receptors (BAR). Little or no change in the number of BAR in brain tissue was observed. Central (intraventricular) administration of BAAM (0.72 mg/g brain weight) resulted in no change in mean blood pressure or heart rate during a 20 min postinjection period. Biochemical analysis of heart tissue following central injections of BAAM showed little or no change in the number of BAR. There was, however, a significant decrease in the number of BAR in brain tissue.     

心脏中不同β肾上腺素受体亚型的信号体系及其病理生理意义

生理情况下,β肾上腺素受体(βAR)对心肌收缩和舒张活动起至关重要的作用;病理情况下,长期激动βAR可以诱发心肌细胞肥大、凋亡以及细胞坏死等心肌重塑性活动,从而参与了慢性心衰的发病过程。近十年以来,许多资料表明β1和β2肾上腺素受体亚型(β1AR和β2AR)共存于心脏中,且激动不同信号系统。短时间激动β1AR,使Gs蛋白-腺苷酸环化酶-环苷腺酸-蛋白激酶A(Gs-adenyly cyclase-cAMP-PKA)信号体系激活并广布于细胞内,而激动βAR则同时激活G1蛋白而产生空间及功能局限的cAMP信号;长时间激动β1AR和β2AR则对心肌细胞的命运产生不同影响：β1AR诱导细胞肥大和凋亡,β2AR促使细胞存活。β2AR的心肌保护作用是通过激活Gi蛋白-Gβγ-PI3K-Akt途径介导。但出乎意料,β1AR的心肌肥厚和凋亡效应并不依赖于经典的cAMP/PKA信号途径,而是激活钙,钙调素依赖性蛋白激酶Ⅱ(caMK Ⅱ)途径。用心肌特异性表达βAR亚型的转基因小鼠进行实验,进一步证实不同βAR亚型在调节心肌重塑和功能方面作用各异。βAR亚型作用不同的新观点不仅为β阻滞剂治疗慢性心衰提供了分子和细胞机制的依据,而且提出了选择性β1AR阻滞和β2AR激动联合治疗慢性心衰的新的治疗思路。     

M-mode ultrasonic assessment of equine fetal heart rate

Embryonic heart rate was studied during 87 examinations in 56 mares on Days 25 to 174 post ovulation. Real-time B-mode using a stopwatch and M-mode ultrasonography were compared. For analyses of heart rates, data were grouped into weeks or months. For the stopwatch and M-mode techniques, respectively, heart rates averaged 123 and 133 beats per minute during Month 1, peaked during Month 3 at 172 or 196 beats, then gradually decreased over the duration of the study to 126 and 130 beats during Month 6. There was a significant increase in heart rate from Months 1 to 2 and Months 2 to 3 and a significant decrease from Months 3 to 4 and Months 5 to 6 for both techniques. There was a tendency for a difference (P<0.08) between techniques during Month 1 and a significant difference between techniques for each of Months 2 and 3. The lower heart rate values for the stopwatch technique during Months 1, 2 and 3 may have reflected the difficulties associated with maintaining the transducer field on the fetal heart continuously for an adequate period of time (Months 1 and 2) and difficulty counting fast enough to keep up with the fetal heart (Months 2 and 3). When data were grouped weekly, the largest increase in fetal heart rate occurred between Weeks 4 and 5.     

Using nonlinear features for fetal heart rate classification

Fetal heart rate (FHR) is used to evaluate fetal well-being and enables clinicians to detect ongoing hypoxia during delivery. Routine clinical evaluation of intrapartum FHR is based on macroscopic morphological features visible to the naked eye. In this paper we evaluated conventional features and compared them to the nonlinear ones in the task of intrapartum FHR classification. The experiments were performed using a database of 217 FHR records with objective annotations, i.e. pH measurement. We have proven that the addition of nonlinear features improves accuracy of classification. The best classification results were achieved using a combination of conventional and nonlinear features with sensitivity of 73.4%, specificity of 76.3%, and F-measure of 71.9%. The best selected nonlinear features were: Lempel Ziv complexity, Sample entropy, and fractal dimension estimated by Higuchi method. Since the results of automatic signal evaluation are easily reproducible, the process of FHR evaluation can become more objective and may enable clinicians to focus on additional non-cardiotocography parameters influencing the fetus during delivery.     

Engrailed and Fgf8 act synergistically to maintain the boundary between diencephalon and mesencephalon

Specification of the forebrain, midbrain and hindbrain primordia occurs during gastrulation in response to signals that pattern the gastrula embryo. Following establishment of the primordia, each brain part is thought to develop largely independently from the others under the influence of local organizing centers like the midbrain-hindbrain boundary (MHB, or isthmic) organizer. Mechanisms that maintain the integrity of brain subdivisions at later stages are not yet known. To examine such mechanisms in the anterior neural tube, we have studied the establishment and maintenance of the diencephalic-mesencephalic boundary (DMB). We show that maintenance of the DMB requires both the presence of a specified midbrain and a functional MHB organizer. Expression of pax6.1, a key regulator of forebrain development, is posteriorly suppressed by the Engrailed proteins, Eng2 and Eng3. Mis-expression of eng3 in the forebrain primordium causes downregulation of pax6.1, and forebrain cells correspondingly change their fate and acquire midbrain identity. Conversely, in embryos lacking both eng2 and eng3, the DMB shifts caudally into the midbrain territory. However, a patch of midbrain tissue remains between the forebrain and the hindbrain primordia in such embryos. This suggests that an additional factor maintains midbrain cell fate. We find that Fgf8 is a candidate for this signal, as it is both necessary and sufficient to repress pax6.1 and hence to shift the DMB anteriorly independently of the expression status of eng2/eng3. By examining small cell clones that are unable to receive an Fgf signal, we show that cells in the presumptive midbrain neural plate require an Fgf signal to keep them from following a forebrain fate. Combined loss of both Eng2/Eng3 and Fgf8 leads to complete loss of midbrain identity, resulting in fusion of the forebrain and the hindbrain primordia. Thus, Eng2/Eng3 and Fgf8 are necessary to maintain midbrain identity in the neural plate and thereby position the DMB. This provides an example of a mechanism needed to maintain the subdivision of the anterior neural plate into forebrain and midbrain.     

Central muscarinic modulation of fetal blood pressure and heart rate

It has previously been demonstrated that the fetal lamb cardiovascular system can respond to peripheral muscarinic stimulation. However the role of central muscarinic mechanisms in modulating fetal cardiovascular function has not been described. Pilocarpine is a muscarinic agonist that readily crosses the blood-brain barrier and was therefore employed to examine both central and peripheral muscarinic mechanisms in modulating fetal cardiovascular function. Fetal lambs were prepared for chronic intrauterine recording of fetal blood pressure (FBP) and heart rate (FHR). Direct administration of pilocarpine to the fetus resulted in an immediate dose-dependent decrease in both systolic and diastolic blood pressure and a rapid fall in FHR. The initial phase of hypotension was very short-lived (1-2 min) and was subsequently followed by a significant increase in systolic, diastolic and pulse pressures (30-60 min). Fetal heart rate gradually returned to control levels by 30 min after pilocarpine administration. Atropine pretreatment was effective in completely blocking the cardiovascular actions of pilocarpine, while methylatropine was only able to block the initial hypotensive and bradycardiac response. A prolonged tachycardia was also unmasked by methylatropine pretreatment. These data suggest that the initial hypotension and bradycardia in response to pilocarpine administration are mediated via peripheral muscarinic receptors, while stimulation of central muscarinic receptors result in hypertension and tachycardia. These data confirm that, as in the adult, central cholinergic mechanisms are involved in the modulation of cardiovascular function in the developing fetus.     

Catecholamines, heart rate, and oxygen uptake during exercise in persons with spinal cord injury

The purpose ofthis study was to investigate the influence of different injury levelsin persons with spinal cord injury (SCI) on epinephrine (Epi) andnorepinephrine (NE) at rest and during graded wheelchair exercise andthe related changes in heart rate andO₂ uptake(O₂). Twenty tetraplegics(Tetra), 10 high-lesion paraplegics (HLPara), 20 paraplegics with SCIbelow T₅ (MLPara), and 18 able-bodied, nonhandicapped persons (AB) were examined. Because of thehigher level of interruption of the sympathetic pathways, Tetra personsshowed lower Epi and NE at rest and only slight increases duringexercise compared with all other groups; the Tetra subjects' impairedcardiac sympathetic innervation caused restricted cardioaccelerationand strongly reduced maximalO₂. Whencompared with AB persons, HLPara had comparable NE but lower Epi levelsas a result of partial innervation of the noradrenergic system anddenervation of the adrenal medulla. MLPara subjects showed an augmentedbasal and exercise-induced upper spinal thoracic sympathetic activitycompared with AB subjects. The increase in heart rate in relation toO₂ was higher in HLParabecause of a smaller stroke volume as a result of venous blood pooling.The different exercise response in persons with SCI is a result of theinterruption of pathways in the spinal cord to the peripheral sympathetic nervous system in addition to the motor paralysis.

     

Securin and separase phosphorylation act redundantly to maintain sister chromatid cohesion in mammalian cells

The spindle assembly checkpoint monitors the integrity of the spindle microtubules, which attach to sister chromatids at kinetochores and play a vital role in preserving genome stability by preventing missegregation. A key target of the spindle assembly checkpoint is securin, the separase inhibitor. In budding yeast, loss of securin results in precocious sister chromatid separation when the microtubule spindle is disrupted. However, in contrast to budding yeast, mammalian securin is not required for spindle checkpoint, suggesting that there are redundant mechanisms controlling the dissolution of sister chromatid cohesion in the absence of securin. One candidate mechanism is the inhibitory phosphorylation of separase. We generated a nonphosphorylable point mutant (S1121A) separase allele in securin-/- mouse embryonic stem cells. Securin(-/-)separase(+/S1121A) cells are viable but fail to maintain sister chromatid cohesion in response to the disruption of spindle microtubules, show enhanced sensitivity to nocodazole, and cannot recover from prometaphase arrest.     

Chicken embryos can maintain heart rate during hypoxia on day 4 of incubation

Journal of Comparative Physiology B - Acute exposure to hypoxic conditions is a frequent natural event during the development of bird eggs. However, little is known about the effect of such...     

Phosphorylation of chick heart muscarinic cholinergic receptors by the beta-adrenergic receptor kinase

Previous studies have demonstrated that muscarinic cholinergic receptors (mAChR) become markedly phosphorylated when intact cardiac cells are stimulated with a muscarinic agonist. This process appears to be related to the process of receptor desensitization. However, the mechanism of agonist-induced phosphorylation of mAChR is not known. In situ phosphorylation studies suggested that agonist-induced phosphorylation of mAChR may involve the participation of a receptor-specific kinase and/or require agonist occupancy. These observations regarding phosphorylation and desensitization of mAChR are similar to observations made for beta-adrenergic receptors. Recent studies have indicated that homologous desensitization of beta-adrenergic receptors may be due to the phosphorylation of these receptors by a novel protein kinase that only recognizes the agonist-occupied form of the receptors. As muscarinic receptors are structurally homologous to beta-adrenergic receptors, we have initiated studies to identify the protein kinase responsible for the phosphorylation of muscarinic receptors by determining whether the chick heart muscarinic receptor would serve as a substrate for the beta-adrenergic receptor kinase (beta-AR kinase). We report that the purified and reconstituted chick heart muscarinic receptor serves as an excellent substrate in vitro for the beta-AR kinase. Phosphorylation of mAChR receptors by the beta-AR kinase was only observed in the presence of a muscarinic receptor agonist and was prevented in the presence of antagonist. Both the extent of phosphorylation (3-4 mol of P/mol of receptor) and the phosphoamino acid composition of the mAChR after incubation in vitro with beta-AR kinase were similar to the characteristics of agonist-induced phosphorylation of mAChR in situ.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium dynamics in the failing heart: restoration by beta-adrenergic receptor blockade

Changes in calcium (Ca2+) regulation contribute to loss of contractile function in dilated cardiomyopathy. Clinical treatment using beta-adrenergic receptor antagonists (beta-blockers) slows deterioration of cardiac function in end-stage heart failure patients; however, the effects of beta-blocker treatment on Ca2+ dynamics in the failing heart are unknown. To address this issue, tropomodulin-overexpressing transgenic (TOT) mice, which suffer from dilated cardiomyopathy, were treated with a nonselective beta-receptor blocker (5 mg. kg-1. day-1 propranolol) for 2 wk. Ca2+ dynamics in isolated cardiomyocytes of TOT mice significantly improved after treatment compared with untreated TOT mice. Frequency-dependent diastolic and Ca2+ transient amplitudes were returned to normal in propranolol-treated TOT mice and but not in untreated TOT mice. Ca2+ kinetic measurements of time to peak and time decay of the caffeine-induced Ca2+ transient to 50% relaxation were also normalized. Immunoblot analysis of untreated TOT heart samples showed a 3.6-fold reduction of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA), whereas Na+/Ca2+ exchanger (NCX) concentrations were increased 2.6-fold relative to nontransgenic samples. Propranolol treatment of TOT mice reversed the alterations in SERCA and NCX protein levels but not potassium channels. Although restoration of Ca2+ dynamics occurred within 2 wk of beta-blockade treatment, evidence of functional improvement in cardiac contractility assessed by echocardiography took 10 wk to materialize. These results demonstrate that beta-adrenergic blockade restores Ca2+ dynamics and normalizes expression of Ca2+-handling proteins, eventually leading to improved hemodynamic function in cardiomyopathic hearts.