Parabrachial and hypothalamic interaction in sodium appetite

Rats with bilateral lesions of the lateral hypothalamus (LH) fail to exhibit sodium appetite. Lesions of the parabrachial nuclei (PBN) also block salt appetite. The PBN projection to the LH is largely ipsilateral. If these deficits are functionally dependent, damaging the PBN on one side and the LH on the other should also block Na appetite. First, bilateral ibotenic acid lesions of the LH were needed because the electrolytic damage used previously destroyed both cells and axons. The ibotenic LH lesions produced substantial weight loss and eliminated Na appetite. Controls with ipsilateral PBN and LH lesions gained weight and displayed robust sodium appetite. The rats with asymmetric PBN-LH lesions also gained weight, but after sodium depletion consistently failed to increase intake of 0.5 M NaCl. These results dissociate loss of sodium appetite from the classic weight loss after LH damage and prove that Na appetite requires communication between neurons in the LH and the PBN.

Effects of the circadian rhythm of corticosteroids on leukocyte-endothelium interactions in the AM and PM

Circadian rhythms are responsible for a number of key cycles within the body. In vivo microscopy was used to investigate the hypothesis that the circadian rhythm of corticosterone in rats produces different leukocyte-endothelium interactions throughout the day. The data indicate that corticosterone levels range from 12 ng/ml in the AM to 260 ng/ml in the PM. In contrast, the number of circulating polymorphonuclear leukocytes (PMNs) yields peak values in the AM (630 PMNs/microl) and trough values in the PM (262 PMNs/microl). During surgical stress there is a significant increase in the number of circulating PMNs in the PM but little change in the AM. Furthermore, there is significantly greater leukocyte-endothelium adhesion in the PM (5.2 cells/100 microm) than in the AM (2.9 cells/100 microm). Addition of the chemoattractant FMLP increased adhesion 125% in the AM but only 62% in PM. Both exogenous glucocorticoid supplementation for 2 weeks and bilateral adrenalectomy abolished the circadian rhythms of circulating PMNs, the number of sticking white blood cells and the initial stages of an acute inflammatory response. These findings suggest that the circadian rhythm of corticosterone alters leukocyte-endothelium interactions throughout the day.

Ventricular volume, chamber stiffness, and function after anteroapical aneurysm plication in the sheep

OBJECTIVE

The success of left ventricular aneurysm plication depends on how the procedure affects both end-systolic elastance and diastolic compliance and how those changes affect ventricular function (stroke work/end-diastolic volume [PRSW] and stroke volume/end-diastolic pressure [Starling] relationships).

METHODS

Five male Dorsett sheep were surgically instrumented with coronary artery snares, an inferior vena caval occluder, and an ascending aortic ultrasonic flow probe. One week later an anteroapical myocardial infarction was produced by tightening the coronary snares. Ten weeks after myocardial infarction, the left ventricular aneurysm was plicated. Absolute left ventricular volume was measured by long-axis transdiaphragmatic echocardiography, and relative changes in left ventricular volume were measured with a conductance catheter. End-systolic elastance, diastolic compliance, PRSW, and Starling relationships were measured immediately before myocardial infarction, 10 weeks after myocardial infarction (immediately before plication), and immediately after and 6 weeks after aneurysm plication.

RESULTS

After plication, end-diastolic and end-systolic left ventricular volumes return to preinfarction values. The slopes of end-systolic elastance, diastolic compliance, and PRSW decrease 10 weeks after myocardial infarction, increase with aneurysm plication, and then decrease 6 weeks after aneurysm plication. The Starling relationship undergoes a downward parallel shift with aneurysm plication.

CONCLUSION

Aneurysm plication abruptly decreases left ventricular volume and diastolic compliance, increases end-systolic elastance and PRSW, but decreases the Starling relationship. The net effect on left ventricular function is mixed. Furthermore, left ventricular remodeling 6 weeks after aneurysm plication causes left ventricular volume, end-systolic elastance, diastolic compliance, PRSW, and the Starling relationship to return to preplication values.

The effect of ventricular volume reduction surgery in the dilated, poorly contractile left ventricle: a simple finite element analysis

OBJECTIVES

Ventricular volume reduction surgery has been proposed by Batista to improve cardiac function in patients with dilated cardiomyopathy. However, limited clinical data exist to determine the efficacy of this operation. A finite element simulation is therefore used to determine the effect of volume reduction surgery on left ventricular end-systolic elastance, diastolic compliance, stroke work/end-diastolic volume (preload recruitable stroke work), and stroke work/end-diastolic pressure (Starling) relationships.

METHODS

End-diastole and end-systole were represented by elastic finite element models with different unloaded shapes and nonlinear material properties. End-systolic elastance, diastolic compliance, preload recruitable stroke work, and Starling relationships, as well as energy expenditure per gram of unresected myocardium, were calculated. Two different types of volume reduction surgery (apical and lateral) were simulated at 10% and 20% left ventricular mass reduction.

RESULTS

Ventricular volume reduction surgery causes diastolic compliance to shift further to the left on the pressure-volume diagram than end-systolic elastance. Volume reduction surgery increases the slope of the preload recruitable stroke work relationship (dilated cardiomyopathy 0.006 J/mL; 20% lateral volume reduction surgery 0.009 J/mL) but decreases the slope of the Starling relationship (dilated cardiomyopathy 0.028 J/mm Hg; 20% lateral volume reduction 0.023 J/mm Hg). For a given amount of resection, lateral volume reduction has a greater effect than apical volume reduction. Ten-percent and 20% lateral volume reduction reduces energy expenditure by 7% and 17%, respectively.

CONCLUSION

Ventricular volume reduction surgery shifts end-systolic elastance and diastolic compliance to the left on the pressure-volume diagram. The net effect on ventricular function is mixed. Volume reduction surgery increases the slope of preload recruitable stroke work, but increased diastolic compliance causes a small decrease in the Starling relationship (3 mm Hg difference between dilated cardiomyopathy and volume reduction surgery at stroke work = 0.5 J).

Managing an increasingly complex system

A study of more than 170,000 health care workers (including 47,692 registered nurses [RNs]) in 138 acute care health care organizations revealed that the role of the RN is characterized by excessive numbers of activities, a loss of focus on the professional components of nursing and significant activity overlap with other job classes. Additionally, the study found that these characteristics were related to reduced morale, decreased patient and physician satisfaction with care and increased health care costs. The results of this study suggest a need for nursing leaders to develop new methods for controlling the complexity of health care systems, particularly the complexity of the RN role. Controlling complexity requires better tools for identifying system inefficiencies, more advanced skills in cross-functional work process diagnostics and more effective strategies for reducing complexity across health care systems.

Digoxin cardiotoxicity in aging anesthetized F344 rats

It is generally accepted that the sensitivity to toxic effects of digitalis increases with advancing age; however, the relative contribution of pharmacokinetics and pharmacodynamics to this aging-related change is presently unknown. The current study was designed to determine if senescence affects digitalis tolerance in an animal model and if observed changes are mediated by altered cardiac or autonomic nervous system responsiveness. Male, F344 rats of three age groups (4, 14 and 25 months) were anesthetized and infused intravenously with digoxin at a rate of 880 micrograms/kg per min. Two separate anesthetic regimens were employed: (a) an age-adjusted dose of urethane in spontaneously breathing animals (AR1); and (b) a non-age-adjusted dose of alpha-chloralose plus urethane in mechanically ventilated rats (AR2). Heart rate, EKG and arterial blood pressure were monitored continuously; baroreceptor reflex function was estimated before and 10 min following the start of digoxin infusion by examining the response to bilateral carotid occlusion. The infusion time required for digoxin-induced AV-dissociation was significantly reduced by senescence in rats anesthetized by AR1. However, doses of digoxin required to elicit ventricular extrasystoles and death were not significantly different among age groups in this anesthetized model and serum digoxin levels did not differ at the time of cardiac arrest. Similarly, AR2 animals showed a significant aging-related decrease in the time to AV-dissociation. However, in contrast to AR1, animals in AR2 displayed an aging-associated increase in the doses of digoxin required to produce ventricular arrhythmias and cardiac arrest. Thus, results suggest that aging in the F344 rat may, by pharmacodynamic mechanisms, promote the sensitivity to digoxin-induced AV-dissociation but not to ventricular arrhythmias or cardiac arrest.

Aging: inotropic effects of Bay K-8644 and nifedipine on rat cardiac muscle

Inotropic effects of Bay K-8644, nifedipine, isoproterenol and extracellular calcium (Ca2+o) were examined in right ventricular strips, papillary muscle and left atria isolated from 4, 14 and 25 month old, male F344 rats. Under the experimental conditions used (37 degrees C, 1.4 mM Ca2+o and 4 Hz), control developed tension (expressed per mg wet weight) increased with age in right ventricular strips and papillary muscle, but decreased in left atria. The maximal positive inotropic response to Bay K-8644 was diminished with age in right ventricular strips and papillary muscle (but not left atria), while the negative inotropic action of nifedipine was not affected in any of the three tissues. Age decreased the inotropic efficacy of isoproterenol in right ventricular strips and papillary muscle (not left atria), had no effect on the efficacy of Ca2+o in right ventricular strips and left atria, but diminished the maximum response to Ca2+o in papillary muscle. Kd and B(max) values for [3H]nitrendipine binding were not significantly different in the three age groups. These data suggest: (1) that age-related changes in basal contractility and inotropic responsiveness differ in atrial and ventricular muscle; and (2) that these changes may result from alterations in excitation-contraction coupling which are not mediated by changes in Ca2+ channel density.

Aging: stimulation rate on cardiac intracellular Na+ activity and developed tension

Previous reports suggested that Na,K-ATPase activity and Na(+)-pump capacity decrease with senescence in left atrial myocardium of F344 rats. Current experiments were designed to determine if this reduction in the Na(+)-pump affects free intracellular Na+ levels. Mean intracellular Na+ ion activity (aiNa) was measured with Na-selective microelectrodes in left atrial muscle isolated from hearts of 4-, 14- and 25-month-old F344 rats. Preparations were stimulated randomly at frequencies between 0 and 12 h. There were no age-associated differences in aiNa measured at any frequency or in the decay of Na+ activity following discontinuation of electrical stimulation. These data indicate that the aging-related decline in Na,K-ATPase does not result in elevated aiNa even at extremely high stimulation frequencies, thus suggesting that other routes of Na+ influx and efflux are also altered in atrial muscle.

The effect of age on digoxin pharmacokinetics in Fischer-344 rats

Digoxin protein binding and pharmacokinetics were studied in 4-, 14-, and 25-month-old male Fischer-344 rats to determine if there were age-dependent changes in digoxin disposition. Serum protein binding did not differ among age groups. The average percentage unbound digoxin for all animals was 61.3 +/- 5.3% (means +/- SD, n = 15). For pharmacokinetic studies, [3H]digoxin and 1 mg/kg unlabeled digoxin were administered as an intravenous bolus dose to animals from each age group. The [3H]digoxin terminal elimination half-life was 2.0, 2.3, and 2.5 hr, respectively. The steady-state volume of distribution in the three age groups was 1.51, 1.49, and 1.27 liters/kg, respectively. Total body clearance for the three age groups was 14.2, 12.1, and 7.5 ml/min/kg, respectively. Analysis of variance of these data followed by Duncan's multiple range test indicated a significant decrease in clearance in the aged rats (25-month-old, p less than 0.05). This age-dependent decrease in clearance suggested that digoxin pharmacokinetics could be a significant factor in age-related alterations in digoxin cardiotoxicity in the rat, as it is in humans, and that the Fischer-344 rat could be a useful model for studies of digoxin pharmacokinetic changes with age.

Effect of stimulation frequency on intracellular Na+ activity in rat atrial muscle

Mean intracellular Na+ activity (aNai) was measured in rat left atrial muscle stimulated at increasing frequencies between 0 and 12 Hz. Low-pass filtered signals from conventional and ion-selective microelectrodes were used to determine aNai. Preparations were bathed in a low Ca2+ (0.1 mM) Krebs-Henseleit buffer containing 1.0 mM Mn2+ to abolish contractile motion and permit stable impalements. Under these conditions, aNai increased progressively with frequency from 5.8 +/- 1.5 mM at 0 Hz to a maximum of 12.7 +/- 2.1 mM, which was observed at 10, 11, or 12 Hz. Further increases in frequency exceeded the effective refractory period, and aNai tended to decrease. These data suggest that aNai can be approximately doubled in rat atrial muscle by increasing the depolarization rate from 0 to 10-12 Hz, a range that has been shown to elicit a two- to three-fold elevation in Na+-pump activity in similar preparations.

Developmental regulation of chick embryo retina neurite extension by extrinsic factors

Developmental changes in the control of neurite extension by extracellular factors can be examined by utilizing cultures of neurons from various aged embryos. Several conditioned media and tissue extracts were added to cultures of chick embryo retinal neurons on collagen substrates for 1, 3 and 5 days in vitro. Neurite outgrowth, measured as the percentage of neurons with neurites and the length of neurites, was promoted by optic tectal extract and cornea conditioned medium in retina neurons from younger ages (6- to 12-day embryos), but not from older ages (14- and 16-day embryos). The promotion of neurite outgrowth by optic tectal extracts may be mediated by a promotion of glial cell growth. The developmental changes in neurite extension may be due to either an altered sensitivity to the neurite promoting factors or by an altered intrinsic ability of retinal neurons to extend processes.