Modeling Fatty Acid Transfer from Artery to Cardiomyocyte.

Despite the importance of oxidation of blood-borne long-chain fatty acids (Fa) in the cardiomyocytes for contractile energy of the heart, the mechanisms underlying the transfer of Fa from the coronary plasma to the cardiomyocyte is still incompletely understood. To obtain detailed insight into this transfer process, we designed a novel model of Fa transfer dynamics from coronary plasma through the endothelial cells and interstitium to the cardiomyocyte, applying standard physicochemical principles on diffusion and on the chemical equilibrium of Fa binding to carrier proteins Cp, like albumin in plasma and interstitium and Fatty Acid-Binding Proteins within endothelium and cardiomyocytes. Applying these principles, the present model strongly suggests that in the heart, binding and release of Fa to and from Cp in the aqueous border zones on both sides of the cell membranes form the major hindrance to Fa transfer. Although often considered, the membrane itself appears not to be a significant hindrance to diffusion of Fa. Proteins, residing in the cellular membrane, may facilitate transfer of Fa between Cp and membrane. The model is suited to simulate multiple tracer dilution experiments performed on isolated rabbit hearts administrating albumin and Fa as tracer substances into the coronary arterial perfusion line. Using parameter values on myocardial ultrastructure and physicochemical properties of Fa and Cp as reported in literature, simulated washout curves appear to be similar to the experimentally determined ones. We conclude therefore that the model is realistic and, hence, can be considered as a useful tool to better understand Fa transfer by evaluation of experimentally determined tracer washout curves.

Echogenic perfluorohexane-loaded macrophages adhere in vivo to activated vascular endothelium in mice, an explorative study.

BACKGROUND: Macrophages may concentrate ultrasound contrast agents and exhibit selective adhesion to activated endothelium. The present study investigates in mice the potential of perfluorohexane (PFH) loaded macrophages to act as ultrasound contrast agent with high reflectivity and specifically targeted at (atherosclerotic) vascular lesions. METHODS: Lung passage was evaluated with a mouse echo scanner after injection, at a slow pace or as a bolus, of varying doses of PFH-loaded and unloaded bone marrow macrophages (BMM) into the jugular vein. The interaction of PFH-loaded and unloaded BMM with TNF-α stimulated carotid artery endothelium after tail vein injection was assessed by means of intravital microscopy. RESULTS: High doses of jugular vein injected PFH-loaded BMM were visible with ultrasound in the pulmonary artery and detectable in the carotid artery. At intravital microscopy, tail vein injected BMM exhibited rolling and adhesion behavior at the TNF-α stimulated carotid endothelium, similar to that of native blood leukocytes. Rolling behavior was not different between PFH-loaded and unloaded BMM (p = 0.38). CONCLUSION: In vivo, perfluorohexane loaded macrophages pass the pulmonary circulation and appear on the arterial side. Moreover, they roll and adhere selectively to activated endothelium under physiological flow conditions. These findings indicate that perfluorohexane loaded BMM could be used to study processes in vivo where endothelial activation plays a role, such as atherosclerosis.

Intra-cardiac transfer of fatty acids from capillary to cardiomyocyte.

Blood-borne fatty acids (Fa) are important substrates for energy conversion in the mammalian heart. After release from plasma albumin, Fa traverse the endothelium and the interstitial compartment to cross the sarcolemma prior to oxidation in the cardiomyocytal mitochondria. The aims of the present study were to elucidate the site with lowest Fa permeability (i.e., highest Fa resistance) in the overall Fa trajectory from capillary to cardiomyocyte and the relative contribution of unbound Fa (detach pathway, characterized by the dissociation time constant τAlbFa) and albumin-bound Fa (contact pathway, characterized by the membrane reaction rate parameter dAlb) in delivering Fa to the cellular membranes. In this study, an extensive set of 34 multiple indicator dilution experiments with radiolabeled albumin and palmitate on isolated rabbit hearts was analysed by means of a previously developed mathematical model of Fa transfer dynamics. In these experiments, the ratio of the concentration of palmitate to albumin was set at 0.91. The analysis shows that total cardiac Fa permeability, Ptot, is indeed related to the albumin concentration in the extracellular compartment as predicted by the mathematical model. The analysis also reveals that the lowest permeability may reside in the boundary zones containing albumin in the microvascular and interstitial compartment. However, the permeability of the endothelial cytoplasm, Pec, may influence overall Fa permeability, Ptot, as well. The model analysis predicts that the most likely value of τAlbFa ranges from about 200 to 400 ms. In case τAlbFa is fast, i.e., about 200 ms, the extracellular contact pathway appears to be of minor importance in delivering Fa to the cell membrane. If Fa dissociation from albumin is slower, e.g. τAlbFa equals 400 ms, the contribution of the contact pathway may vary from minimal (dAlb≤5 nm) to substantial (dAlb about 100 nm). In the latter case, the permeability of the endothelial cytoplasm varies from infinite (no hindrance) to low (substantial hindrance) to keep the overall Fa flux at a fixed level. Definitive estimation of the impact of endothelial permeability on Ptot and the precise contribution of the contact pathway to overall transfer of Fa in boundary zones containing albumin requires adequate physicochemical experimentation to delineate the true value of, among others, τAlbFa, under physiologically relevant circumstances. Our analysis also implies that concentration differences of unbound Fa are the driving force of intra-cardiac Fa transfer; an active, energy requiring transport mechanism is not necessarily involved. Membrane-associated proteins may facilitate Fa transfer in the boundary zones containing albumin by modulating the membrane reaction rate parameter, dAlb, and, hence, the contribution of the contact pathway to intra-cardiac Fa transfer.

Spatial distension variations are associated with focal atherosclerotic plaques.

BACKGROUND: Alterations in arterial absolute distension (AD) and relative distension (RD) in the common carotid artery (CCA), as observed by ultrasound, may relate to atherosclerotic processes in a nearby vascular segment. The main objective of the study was to establish to what extent these mechanical vascular alterations relate to carotid bulb stenosis. METHODS: The AD, RD and their interrecording variation (Delta AD and Delta RD), representing the spatial inhomogeneity of AD and RD, were evaluated in 308 CCA and associated with the degree of bulb stenosis. RESULTS: The mechanical CCA parameters were marginally correlated to the degree of ipsilateral (r = -0.14 for AD and r = -0.28 for RD) and contralateral stenosis (r = -0.21 for AD and r = -0.27 for RD). However, Delta AD (r = 0.54; p < 0.001) and Delta RD (r = 0.59; p < 0.001) were significantly correlated to ipsilateral stenosis, independent of age, gender and pulse pressure. CONCLUSIONS: The indices of CCA arterial stiffness are weakly associated with ipsilateral bulb stenosis, but their interrecording variation is a good indicator for atherosclerotic alterations in the carotid bulb. The results indicate that a focal atherosclerotic lesion is associated with modifications in the dynamic characteristics of a nearby vascular segment.

Contribution of vascular and neural segments to baroreflex sensitivity in response to postural stress.

BACKGROUND/AIMS: The baroreflex pathway has a vascular and a neural segment, both being modulated by variations in peripheral blood pressure (BP). Besides overall baroreceptor sensitivity (BRS), defined as the spectral relationship between changes in peripheral BP and R-R interval within the frequency band of 0.05-0.15 Hz, vascular and neural segment contributions to the overall BRS can be distinguished. We test the hypothesis that changes in overall BRS following a postural maneuver mainly originate from the vascular (peripheral pressure to carotid artery diameter) rather than the neural segment (carotid artery diameter to R-R interval). METHODS: Peripheral pressure (Finapress), carotid artery diameter (ultrasound in B-/M-mode) and electrocardiogram values of 20 young subjects in supine and upright-seated postures were recorded simultaneously. Transfer gains were computed for the segmental and overall responses. RESULTS: Postural change significantly increases peripheral BP and carotid artery diameter. The vascular segment has a uniform spectral distribution. Statistical analyses revealed that postural change decreased overall (p < 0.004) and vascular (p < 0.0001) transfer gains, but did not modify neural gain. CONCLUSIONS: Unlike the neural segment, the vascular segment is frequency non-specific. The decrease in overall BRS due to a postural change is mainly explained by the reduced transfer gain of the vascular segment.

Genetic variation in the renin-angiotensin system and arterial stiffness. The Rotterdam Study.

We studied the associations of three renin-angiotensin system polymorphisms, angiotensin-converting enzyme (ACE) I/D, angiotensinogen 235 M/T, and angiotensin II receptor type I 573 C/T, with arterial stiffness. The study was embedded in the Rotterdam Study, a population-based study older adults. The association of the polymorphisms with pulse wave velocity, the carotid distensibility, and pulse pressure was investigated in 3706 subjects. We found no association of the ACE I/D polymorphism with pulse wave velocity, but the D-allele was associated with a lower distensibility coefficient (p = 0.05) and higher pulse pressure (p = 0.01). For the angiotensinogen 235 M/T polymorphism, no significant associations with either pulse wave velocity (p = 0.71), the distensibility coefficient (p = 0.16) or pulse pressure (p = 0.34) were found. Also, we found no significant associations of pulse wave velocity (PWV) (p = 0.32), the distensibility coefficient (p = 0.08), and pulse pressure (p = 0.09) with the angiotensin II receptor type 1 573 C/T polymorphism. No epistatic effects were observed between the three renin-angiotensin system (RAS) genes with arterial stiffness. Our findings suggest that genetic variation in the renin-angiotensin system may play a role in determining carotid distensibility and pulse pressure.

Confluence of incident and reflected waves interferes with systolic foot detection of the carotid artery distension waveform.

OBJECTIVES: Local pulse wave velocity, a direct measure of arterial stiffness, can be measured using the systolic foot of the pressure waveform as the time reference point. The accuracy and precision of systolic foot identification, which may be disturbed by early wave reflections, heavily affects pulse wave transit time measurements. We investigated within subjects the existence of early wave reflections and their interference with systolic foot identification. METHODS: Fourteen ultrasound-derived distension waveforms, spaced over 16.4 mm, were simultaneously recorded in the CCA 3 cm proximal of the bifurcation of 12 young subjects. The second derivatives of the distension waveforms were calculated to identify the systolic foot and an inflection point preceding systolic peak distension. Pulse wave transit time was calculated as the time difference between the most proximal and most distal time-point, using either the systolic foot or the inflection point. The time to reflection (DeltaTSF_IP) was defined as the time difference between the systolic foot and the inflection point. RESULTS: Both transit times (TT SF and TT IP) could be determined with good intrasubject precision: 0.7 and 1.4 ms, respectively. The systolic foot is running forward, TT SF = 3.1 +/- 0.9 ms, whereas the inflection point appears to run backward, TT IP = -3.9 +/- 1.4 ms. DeltaTSF_IP was 44.3 +/- 8.8 ms. CONCLUSION: Despite the good intrasubject reproducibility, confluence of incident and reflected waves disturbs identification and discrimination of the systolic foot and the inflection point, resulting in biased estimates. Therefore both points are unsuitable for local pulse transit time measurements in the common carotid artery.

Perfluorohexane-loaded macrophages as a novel ultrasound contrast agent: a feasibility study.

PURPOSE: We investigated in vitro the potential of macrophages to act as targeted vehicle for ultrasound molecular imaging. PROCEDURES: Murine bone marrow-derived macrophages (BMM), incubated for 3 h with different concentrations of perfluorohexane (PFH) emulsions, were monitored by microscopy, flow cytometry, and ultrasound. Effects of PFH loading on BMM adhesion molecule (PSGL-1, VLA-4, Mac-1, LFA-1) expression were analyzed by flow cytometry. Static adhesion of PFH loaded BMM to unstimulated and TNF-alpha stimulated b.End5 endothelial cells was assessed by microscopy. RESULTS: Incubation of BMM with PFH emulsions resulted in dose-dependent uptake and increased echogenicity (max. 17 dB). Flow cytometry analyses revealed no down-regulation related to PFH loading of BMM adhesion molecule expression. Endothelial adhesion remained functional, even after 24 h, although PFH loading dose-dependently attenuated static adhesion. CONCLUSION: PFH loaded BMM may potentially serve as ultrasound contrast agent for noninvasive detection of atherogenic hotspots in arteries.

Wall shear stress as measured in vivo: consequences for the design of the arterial system.

Based upon theory, wall shear stress (WSS), an important determinant of endothelial function and gene expression, has been assumed to be constant along the arterial tree and the same in a particular artery across species. In vivo measurements of WSS, however, have shown that these assumptions are far from valid. In this survey we will discuss the assessment of WSS in the arterial system in vivo and present the results obtained in large arteries and arterioles. In vivo WSS can be estimated from wall shear rate, as derived from non-invasively recorded velocity profiles, and whole blood viscosity in large arteries and plasma viscosity in arterioles, avoiding theoretical assumptions. In large arteries velocity profiles can be recorded by means of a specially designed ultrasound system and in arterioles via optical techniques using fluorescent flow velocity tracers. It is shown that in humans mean WSS is substantially higher in the carotid artery (1.1-1.3 Pa) than in the brachial (0.4-0.5 Pa) and femoral (0.3-0.5 Pa) arteries. Also in animals mean WSS varies substantially along the arterial tree. Mean WSS in arterioles varies between about 1.0 and 5.0 Pa in the various studies and is dependent on the site of measurement in these vessels. Across species mean WSS in a particular artery decreases linearly with body mass, e.g., in the infra-renal aorta from 8.8 Pa in mice to 0.5 Pa in humans. The observation that mean WSS is far from constant along the arterial tree implies that Murray's cube law on flow-diameter relations cannot be applied to the whole arterial system. Because blood flow velocity is not constant along the arterial tree either, a square law also does not hold. The exponent in the power law likely varies along the arterial system, probably from 2 in large arteries near the heart to 3 in arterioles. The in vivo findings also imply that in in vitro studies no average shear stress value can be taken to study effects on endothelial cells derived from different vascular areas or from the same artery in different species. The cells have to be studied under the shear stress conditions they are exposed to in real life.

The interleukin-6-174 G/C promoter polymorphism and arterial stiffness; the Rotterdam Study.

Arterial stiffness normally increases with age and has been established as a precursor of cardiovascular disease. Interleukin-6 is a pleiotropic inflammatory cytokine with an important role in the inflammatory cascade, such as up-regulation of C-reactive protein (CRP). The interleukin-6-174-G/C promoter polymorphism appears to influence levels of inflammatory markers, which have been shown to be associated with arterial stiffness. We studied the association of this polymorphism with levels of interleukin-6 and CRP and with arterial stiffness. The study (n=3849) was embedded in the Rotterdam Study, a prospective, population-based study. Analyses on the association between the -174-G/C polymorphism and pulse wave velocity, distensibility coefficient, and pulse pressure were performed using analyses of variance. Analyses on the levels of inflammatory markers and arterial stiffness were performed using linear regression analyses. Analyses were adjusted for age, sex, mean arterial pressure, heart rate, known cardiovascular risk factors, and atherosclerosis. We found pulse wave velocity to be 0.35 m/s higher for CC-homozygotes vs. wildtype GG-homozygotes (p = 0.018) with evidence for an allele-dose effect (p trend = 0.013), and a similar pattern for pulse pressure (p trend = 0.041). No apparent consistent association with the distensibility coefficient was found. CRP levels were associated with pulse wave velocity (p = 0.007). In conclusion, the interleukin-6-174 G/C polymorphism is associated with increased arterial stiffness and pulse pressure.

Arterial stiffness and risk of coronary heart disease and stroke: the Rotterdam Study.

BACKGROUND: Arterial stiffness has been associated with the risk of cardiovascular disease in selected groups of patients. We evaluated whether arterial stiffness is a predictor of coronary heart disease and stroke in a population-based study among apparently healthy subjects. METHODS AND RESULTS: The present study included 2835 subjects participating in the third examination phase of the Rotterdam Study. Arterial stiffness was measured as aortic pulse wave velocity and carotid distensibility. Cox proportional hazards regression analysis was performed to compute hazard ratios. During follow-up, 101 subjects developed coronary heart disease (mean follow-up period, 4.1 years), and 63 subjects developed a stroke (mean follow-up period, 3.2 years). The risk of cardiovascular disease increased with increasing aortic pulse wave velocity index. Hazard ratios and corresponding 95% CIs of coronary heart disease for subjects in the second and third tertiles of the aortic pulse wave velocity index compared with subjects in the reference category were 1.72 (0.91 to 3.24) and 2.45 (1.29 to 4.66), respectively, after adjustment for age, gender, mean arterial pressure, and heart rate. Corresponding estimates for stroke were 1.22 (0.55 to 2.70) and 2.28 (1.05 to 4.96). Estimates decreased only slightly after adjustment for cardiovascular risk factors, carotid intima-media thickness, the ankle-arm index, and pulse pressure. The aortic pulse wave velocity index provided additional predictive value above cardiovascular risk factors, measures of atherosclerosis, and pulse pressure. Carotid distensibility as measured in this study was not independently associated with cardiovascular disease. CONCLUSIONS: Aortic pulse wave velocity is an independent predictor of coronary heart disease and stroke in apparently healthy subjects.

Arterial stiffness, cardiovagal baroreflex sensitivity and postural blood pressure changes in older adults: the Rotterdam Study.

OBJECTIVE: Arterial stiffness may be involved in the impairment of the arterial baroreflex. In the present study the associations between arterial stiffness and cardiovagal baroreflex sensitivity (BRS) and between BRS and postural blood pressure (BP) changes were investigated within the framework of the Rotterdam Study. METHODS: Arterial stiffness was determined by aortic pulse wave velocity and the carotid distensibility coefficient. Continuous recording of the R-R interval and finger BP was performed with the subject resting supine, and BRS was estimated from the spontaneous changes in systolic BP and corresponding interbeat intervals. Measures of aortic stiffness or carotid distensibility and BRS were available in 2490 and 2083 subjects, respectively. The association between arterial stiffness and ln BRS was investigated by multivariate linear regression analysis and then by analysis of covariance, comparing BRS by quartiles of arterial stiffness. RESULTS: The mean age of the subjects was 71.7 +/- 6.6 (41.7% men). Aortic stiffness was negatively associated [beta = -0.029; 95% confidence interval (CI): -0.040, -0.019] and the carotid distensibility coefficient positively associated with BRS (beta = 0.017; 95% CI: 0.010, 0.024). An orthostatic decrease in systolic BP was absent in 1609 subjects, between 1 and 10 mmHg in 502 and >10 mmHg in 269 subjects, with corresponding mean values (95% CI) of ln BRS of 1.47 (1.44-1.51), 1.43 (1.37-1.49) and 1.36 (1.28-1.44) ms/mmHg (test for trend P < 0.019). An orthostatic decrease in diastolic BP was absent in 1123 subjects, 1-10 mmHg in 1057 and >10 mmHg in 209 subjects, with corresponding mean values of ln BRS of 1.49 (1.45-1.53), 1.41 (1.37-1.45) and 1.45 (1.36-1.54) ms/mmHg (P < 0.04). CONCLUSION: In a large population of older subjects, arterial stiffness appears to be an independent determinant of impaired BRS. Within the same population, impaired BRS was associated with orthostatic BP changes.

Carotid artery pulse wave time characteristics to quantify ventriculoarterial responses to orthostatic challenge.

Central blood pressure waveforms contain specific features related to cardiac and arterial function. We investigated posture-related changes in ventriculoarterial hemodynamics by means of carotid artery (CA) pulse wave analysis. ECG, brachial cuff pressure, and common CA diameter waveforms (by M-mode ultrasound) were obtained in 21 healthy volunteers (19-30 yr of age, 10 men and 11 women) in supine and sitting positions. Pulse wave analysis was based on a timing extraction algorithm that automatically detects acceleration maxima in the second derivative of the CA pulse waveform. The algorithm enabled determination of isovolumic contraction period (ICP) and ejection period (EP): ICP=43+/-8 (SD) ms (4-ms precision), and EP=302+/-16 (SD) ms (5-ms precision). Compared with the supine position, in the sitting position diastolic blood pressure (DBP) increased by 7+/-4 mmHg (P<0.001) and R-R interval decreased by 49+/-82 ms (P=0.013), reflecting normal baroreflex response, whereas EP decreased to 267+/-19 ms (P<0.001). Shortening of EP was significantly correlated to earlier arrival of the lower body peripheral reflection wave (r2=0.46, P<0.001). ICP increased by 7+/-7 ms (P<0.001), the ICP-to-EP ratio increased from 14+/-3% (supine) to 19+/-3% (P<0.001) and the DBP-to-ICP ratio decreased by 7% (P=0.023). These results suggest that orthostasis decreases left ventricular output as a result of arterial wave reflections and, presumably, reduced cardiac preload. We conclude that CA ultrasound and pulse wave analysis enable noninvasive quantification of ventriculoarterial responses to changes in posture.

Measurement of local pulse wave velocity: effects of signal processing on precision.

Pulse wave velocity (PWV) provides information about the mechanical properties of the vessel: the stiffer the artery is, the higher the PWV will be. PWV measured over a short arterial segment facilitates direct characterization of local wall properties corrected for prevailing pressure without the necessity of measuring pulse pressure locally. Current methods for local PWV assessment have a poor precision, but it can be improved by applying linear regression to a characteristic time-point in distension waveforms as recorded simultaneously by multiple M-line ultrasounds. We investigated the precision of this method in a phantom scaled according to realistic in vivo conditions. Special attention was paid to the identification of the foot of the wave, using the maximum of the second derivative, the intersecting tangent and the 20% threshold method. Before foot detection, the distension waveforms were subjected to preprocessing with various filters. The precision of the maximum of the second derivative had a coefficient of variation (CV) of 0.45% and 10.45% for an eighth and second order low pass filter, respectively. The intersecting tangent and the threshold method were less sensitive to filtering; the CVs were 0.66% and 0.68% for the high order filter and 2.36% and 1.43% for the low order filter, respectively. We conclude that foot detection by a threshold of 20% or by the tangent method are more suitable to identify the foot of the wave to measure local PWV. Both methods are less sensitive to (phase) noise than the maximum of the second derivative method and exhibit good precision with a CV of less than 1%.

The onset of ventricular isovolumic contraction as reflected in the carotid artery distension waveform.

The blood pressure waveform carries information about the cardiac contraction and the impedance characteristics of the vascular bed. Here, we demonstrate that the start of isovolumic ventricular contraction is persistently reflected as an inflection point in the pressure wave as recorded in the aortic root (TP(IC)) as well as in the carotid artery distension waveform (TD(IC)) as it travels down the arterial tree. In a group of six patients with normal pressure gradients across the aortic valve after valve replacement, the TP(IC) had a small delay with respect to the onset of isovolumic ventricular contraction (<10 ms). In a group (n = 21) of young presumably healthy volunteers, the inflection point occurred persistently in the carotid distension waveform, as recorded by means of ultrasound, before the systolic foot (intersubject delay between inflection point and systolic foot: mean +/- SD = 40.0 +/- 9.4 ms, intrasubject SD 4.6 ms). Retrograde coronary blood flow during isovolumic ventricular contraction may be the origin of the persistent end-diastolic pressure and distension perturbation. This study shows that the duration of the isovolumic contraction can be reliably extracted from the carotid artery distension waveform.

Impaired fasting glucose is associated with increased arterial stiffness in elderly people without diabetes mellitus: the Rotterdam Study.

OBJECTIVES: To study the association between impaired fasting glucose (IFG) and arterial stiffness in older adults. DESIGN: Cross-sectional population-based study. SETTING: The Rotterdam Study, a Dutch population-based cohort study. PARTICIPANTS: Two thousand nine hundred eighty-seven subjects aged 60 and older. MEASUREMENTS: Arterial stiffness assessed by measuring common carotid arterial distensibility and glucose status classified into three categories: normal fasting glucose (NFG) (fasting glucose <6.1 mmol/L), IFG (fasting glucose 6.1-6.9 mmol/L), and diabetes mellitus (DM). RESULTS: In the total cohort, common carotid distensibility decreased with increasing impairment of glucose metabolism. Subjects younger than 75 with IFG were comparable with subjects with NFG with respect to arterial stiffness. Subjects aged 75 and older with IFG had stiffer arteries than subjects with NFG, reaching the same arterial stiffness as subjects with DM. For subjects younger than 75, mean difference in distensibility coefficient between subjects with NFG and with IFG was 0.1 (95% confidence interval (CI)=-0.04-0.05, P=.88) and between subjects with NFG and with DM was 1.2 (95% CI=0.7-1.7, P<.001). For subjects aged 75 and older, the mean difference between these groups was 0.7 (95% CI=0.2-1.2, P=.007) and 0.8 (0.3-1.4; P=.002), respectively. In the total cohort, fasting glucose was strongly associated with carotid distensibility (beta-coefficient=-0.29, P<.001). CONCLUSION: IFG is related to arterial stiffness in elderly subjects. An advanced stage of arterial stiffness, comparable with that of subjects with DM, is only reached at the age of 75.

Dimensions of compartments and membrane surfaces in the intact rabbit heart of importance in studies on intramyocardial transfer of blood-borne substances.

Cardiac studies on the uptake, storage and intramyocardial transfer of blood-borne substances require detailed information on the geometric ultrastructural dimensions of myocardial compartments and parts thereof, and the membranes separating these compartments. Such a specific ultrastructural set of data of the heart is yet lacking. In the present study, we quantitatively assessed these dimensions in glutaraldehyde-perfusion fixed rabbit hearts by means of histological and tailored mathematical techniques. We showed the true ellipsoid nature of the myocardial capillary cross section and estimated the mean capillary diameter dcap. After correction for the ellipsoid shape, dcap was found to be 5.21±1.41 µm. Effective widths of the endothelial cell and the pericapillary interstitium (is1), dimensions of importance in diffusion, amounted to 187±7 and 160±10 nm, respectively. The fractional volume of the large vessels (arteries and veins larger than 10 µm), capillaries, endothelium, is1, cardiomyocytes, non-pericapillary interstitium is2, t-tubular compartment and interstitial cells amounted on average to 5.92%, 9.36%, 1.83%, 1.94%, 73.07%, 5.97%, 0.95% and 0.96%, respectively, of total myocardial volume, defined as the cardiac tissue volume, the large blood vessels included. Normalized to total myocardial volume, the surface area of the luminal and abluminal endothelial membranes and of the cardiomyocyte membrane opposing the endothelial cells amounted to 75.2±5.5·10³, 82.2±6.0·10³ and 89.1±6.5·10³ m²/m³, respectively. The present study provides quantitative information about ultrastructural dimensions of the adult rabbit heart, among others, of importance for studies on cardiac uptake, and intramyocardial transfer and storage of blood-supplied substances.

Left ventricular septal and apex pacing for optimal pump function in canine hearts.

OBJECTIVES: The goal of this study was to test the hypothesis that left ventricular (LV) pump function is optimal when pacing is performed at the LV near the sites where the impulses exit the Purkinje system. BACKGROUND: Pacing at the conventional site, the right ventricular (RV) apex, adversely affects hemodynamics. During normal sinus rhythm (SR), electrical activation of the working myocardium starts at the LV septal endocardium and spreads from apex to base. METHODS: Experiments were conducted in anesthetized open-chest dogs with normal ventricular conduction to investigate hemodynamic effects of pacing at various epicardial LV sites, the RV apex, and combinations of these sites (n = 11) and of RV and LV septal pacing (n = 8). The LV septal endocardium was reached via the RV by puncturing through the septum with a barbed electrode. Left ventricular systolic (LVdP/dtpos and stroke work) and diastolic (LVdP/dtneg and Tau) function were assessed using pressure-volume relations (conductance catheter technique). RESULTS: Left ventricular systolic and diastolic function were highly dependent on the site of pacing, but not on QRS duration. Left ventricular function was maintained at SR level during LV septal, LV apex, and multisite pacing, was moderately depressed during pacing at epicardial LV free wall sites, and was most severely depressed during RV apex pacing. On average, RV septal pacing did not improve LV function, compared with RV apex pacing, but in each experiment one (variable) RV pacing site was found, which only moderately reduced LV function. CONCLUSIONS: During ventricular pacing, LV pump function is maintained best (i.e., at SR level) when pacing at the LV septum or LV apex, potentially because pacing from these sites creates a physiological propagation of electrical conduction.

Neural activity of the cardiac baroreflex decreases with age in normotensive and hypertensive subjects.

OBJECTIVES: Baroreflex control of the heart rate is impaired in hypertensive subjects and decreases with age. The decrease in cardiac baroreflex sensitivity (BRS) is often ascribed to decreased distension of the pressure-sensing arterial wall segments. However, alterations in the sensing and processing of neural signals may be involved as well. DESIGN: Conventionally, both vessel wall stiffness and the sensing and neural processing of the baroreflex are incorporated in the measure of pressure-derived BRS. We introduce stretch-derived BRS, which only considers the sensing and neural components of the baroreflex. METHODS: To determine stretch-derived BRS in a non-invasive manner, we measured the spontaneously occurring low-frequency variations (range, 0.06-0.12 Hz) in the carotid artery diameter and the corresponding R-R interval fluctuations, and determined the associated transfer function. The stretch-derived BRS in a group of age-matched (age range, 25-72 years) normotensive (n = 20) and hypertensive (n = 21) subjects was compared. RESULTS: In both subject groups the stretch-derived BRS decreased significantly with age. Moreover, the stretch-derived BRS of both groups was only different below 50 years of age. CONCLUSIONS: The analysis of low-frequency fluctuations in the carotid artery diameter demonstrates that aging as well as hypertension are associated with impaired neural control of the baroreflex. Beyond 50 years of age the effect of hypertension cannot be distinguished from the effect of aging.

Pentobarbital fails to reduce cerebral oxygen consumption early after non-hemorrhagic closed head injury in rats.

It is unknown whether barbiturates suppress cerebral oxygen metabolism after cerebral trauma as they do in normal individuals. We evaluated the influence of pentobarbital on cerebral oxygen handling of normal rats and rats subjected to non-hemorrhagic closed head injury (CHI). Oxygen delivery was assessed by measuring cerebral perfusion and oxygen extraction, enabling the calculation of cerebral metabolic rate of oxygen (CMRO2). Mitochondrial function was assessed by studying changes in the oxidized cytochrome oxidase concentration. CHI caused changes in both systemic and cerebral hemodynamics. Cerebral blood flow was reduced to 66% of its control value, but the cerebral metabolic rate of oxygen remained unchanged. Pentobarbital administration induced a significant lowering of the cerebral oxygen consumption in normal rats associated with a secondary decrease in cerebral perfusion. In rats subjected to CHI, pentobarbital was unable to lower the cerebral metabolic demand and did not cause a further decrease in perfusion. Pentobarbital was unable to significantly modulate mitochondrial function in traumatized rats, whereas it exerted this effect in all control animals. We therefore conclude that, in rats subjected to CHI, pentobarbital is unable to perform its beneficial effects on the cerebral metabolism.