Harald M. Stauss

Cellular Distribution of Angiotensin-Converting Enzyme After Myocardial Infarction

We studied the cellular distribution of angiotensin-converting enzyme (ACE) in the heart related to the cell types involved in left ventricular repair and remodeling before and after myocardial infarction by immunohistochemical techniques using monoclonal and polyclonal antibodies. In noninfarcted myocardium of both human and rat, ACE expression was confined to endothelial cells and subendocardial cell layers of the aortic valve. ACE was prominent in endothelia of small arteries and arterioles, whereas only half the coronary capillaries were immunoreactive and venous vessels were almost completely devoid of the enzyme. In a rat model of myocardial infarction, ACE distribution was determined 1, 3, and 7 days and 2, 3, and 6 weeks after coronary occlusion. Three and 7 days after infarction, endothelial cells of sprouting capillaries and macrophages in the marginal zone of necrosis revealed ACE expression. In both human and rat with the onset of fibrosis, intense staining of the enzyme was found in the marginal zone of the repair tissue. In situ hybridization for collagen type I in the rat revealed that zones with high collagen content had almost no ACE immunoreactivity. Vascular smooth muscle cells and cardiomyocytes revealed no ACE expression throughout the study. We conclude that endothelial cells are the principal source for the expression of ACE after myocardial infarction. The observed induction of ACE with the onset of fibrosis suggests a role of this enzyme that is related to tissue repair and remodeling.

Identification of blood pressure control mechanisms by power spectral analysis.

1 Blood pressure and organ perfusion are controlled by a variety of cardiovascular control systems, such as the baroreceptor reflex and the renin–angiotensin system (RAS), and by local vascular mechanisms, such as shear stress‐induced release of nitric oxide (NO) from the endothelium and the myogenic vascular response. Deviations in arterial blood pressure from its set point activate these mechanisms in an attempt to restore blood pressure and/or secure organ perfusion. However, the response times at which different cardiovascular mechanisms operate differ considerably (e.g. blood pressure control by the RAS is slower than blood pressure control via the baroreceptor reflex). 2 Owing to these different response times, some cardiovascular control systems affect blood pressure more rapidly and others more slowly. Thus, identifying the frequency components of blood pressure variability (BPV) by power spectral analysis can potentially provide important information on individual blood pressure control mechanisms. 3 Evidence is presented that the RAS, catecholamines, endothelial‐derived NO and myogenic vascular function affect BPV at very low frequencies (0.02–0.2 Hz) and that low‐frequency (LF) BPV (0.2–0.6 Hz) is affected by sympathetic modulation of vascular tone and endothelial‐derived NO in rats. In humans, LF BPV (0.075–0.15 Hz) is affected by sympathetic modulation of vascular tone and myogenic vascular function. The impact of the RAS and endothelial‐derived NO on BPV in humans requires further investigation. 4 In conclusion, power spectral analysis is a powerful diagnostic tool that allows identification of pathophysiological mechanisms contributing to cardiovascular diseases, such as hypertension, heart failure and stroke, because it can separate slow from fast cardiovascular control mechanisms. The limitation that some cardiovascular control mechanisms affect the same frequency components of BPV requires the combination of blood pressure spectral analysis with other techniques.

Frequency response characteristics of sympathetically mediated vasomotor waves in humans

In a recent study, we demonstrated that transmission from peripheral sympathetic nerves to vascular smooth muscles is strongest in the frequency band from 0.2 to 0.5 Hz in conscious rats. In contrast, sympathetic modulation of vasomotor tone in humans is suggested to be reflected in the power spectrum of arterial blood pressure in a frequency range centered around ∼0.1 Hz. Therefore, we addressed whether frequency response characteristics of sympathetic transmission from peripheral sympathetic nerves to vascular smooth muscles in humans differ from those in rats. In 12 male subjects, skin-sympathetic fibers of the left median nerve were electrically stimulated via microneurography needles with stimulation frequencies ranging from 0.01 to 0.5 Hz. Simultaneously, blood flow in the innervated skin area at the palm of the ipsilateral hand was recorded by a laser-Doppler device. The skin blood flow in the same area of the contralateral hand was recorded as a control. Median nerve stimulation produced transient decreases in skin blood flow in the ipsilateral hand. At frequencies ranging from 0.025 to 0.10 Hz, median nerve stimulation evoked high-power peaks at the same frequencies in the skin blood flow power spectra of the ipsilateral but not of the contralateral hand. The greatest responses were found in the frequency range from 0.075 to 0.10 Hz. Therefore, these data indicate that the transmission from peripheral sympathetic nerves to cutaneous vascular smooth muscles in humans is slower than in rats. In addition, the frequency range believed to be most important in sympathetic modulation of vasomotor activity in humans corresponds to the frequency band of the greatest response of cutaneous vascular smooth muscle contraction to sympathetic nerve stimulation.

Does low frequency power of arterial blood pressure reflect sympathetic tone

We tested whether power spectral analysis of arterial blood pressure (ABP) is a feasible tool to detect differences in peripheral sympathetic nerve activity in normotensive and hypertensive rats with differing basal sympathetic tones. Nine Wistar Kyoto rats (WKY), 10 Sprague-Dawley rats (SD), 10 spontaneously hypertensive rats (SHR) and 9 hypertensive transgenic rats harbouring the mouse Ren-2 gene (TGR) were chronically instrumented with femoral artery catheters and nerve electrodes around the sympathetic major splanchnic nerve. Two days after surgery ABP and splanchnic nerve activity (SpNA) were recorded in the conscious state during basal conditions as well as during alpha 1-adrenergic receptor blockade. Power spectra and squared coherence in the low (LF, 0.02-0.20 Hz), mid (MF, 0.20-0.80 Hz) and high (HF, respiration peak +/- 0.3 Hz) frequency bands were calculated for ABP and SpNA. Mean blood pressure in SHR (133 +/- 8 mmHg) and TGR (142 +/- 8 mmHg) was significantly higher (P < 0.05) than in WKY (115 +/- 3 mmHg) and SD (95 +/- 4 mmHg). SpNA in SHR was higher than in WKY (23.4 +/- 6.4 microV vs. 11.6 +/- 0.8 microV, P < 0.05) while SpNA in TGR was lower than in SD (20.1 +/- 3.9 microV vs. 28.8 +/- 4.2 microV, P < 0.05). LF and MF components of ABP variability were not significantly higher in those rats with high sympathetic tones. However, alpha 1-adrenergic receptor blockade reduced LF and MF components of ABP and SpNA in all strains except SHR. LF and MF coherence was not greater in rats with high sympathetic tones than in those with low sympathetic tones. The reduction of LF and MF components of ABP variability by alpha 1-adrenergic receptor blockade indicates an important contribution of peripheral sympathetic nerve activity to LF and MF blood pressure variability on an acute basis. However, the lack of higher LF and MF power in the ABP spectra of those rats with high SpNA together with the finding that LF and MF coherence was not higher in those rats with high SpNA led to the conclusion that LF and MF spectral components of ABP do not appear to be suitable markers for the prevailing sympathetic nerve activity.

Sympathetic modulation of blood pressure variability.

Although sympathetic nervous activity (SNA) displays oscillations synchronous with the heart beat and respiration, and between 0.1–0.4 Hz, it is apparent that each of these frequencies does not have the same effect on the vasculature. Frequencies above 1 Hz do not produce oscillations in the vasculature but instead contribute to the mean level of vasoconstriction. Slower oscillations in SNA result in a cycle of vasoconstriction and vasodilation within the vasculature, the amplitude of which, generally decreases with increasing frequency. Some studies indicate that, within the same species, differences exist in the frequency responses between vascular beds, such as the skin and gut. This differential responsiveness is also found between the medullary and cortical vasculature regions of the rabbit kidney. Low-pass filter properties have been described in the iliac circulation of rats, and evidence has been provided that noradrenaline reuptake mechanisms are not the frequency limiting step of the vasculature response. Recent studies on isolated rat vascular smooth muscle cells suggest that sympathetic modulation of vascular tone is limited by the α-adrenoceptor signal transduction into the cells and not by an intrinsic inability of the cells to contract and relax at higher rates.

Angiotensin Type 1a Receptors in the Subfornical Organ Are Required for Deoxycorticosterone Acetate-Salt Hypertension

Although elevated renin–angiotensin system activity and angiotensinergic signaling within the brain are required for hypertension, polydipsia, and increased metabolic rate induced by deoxycorticosterone acetate (DOCA)-salt, the contribution of specific receptor subtypes and brain nuclei mediating these responses remains poorly defined. We hypothesized that angiotensin type 1a receptors (AT1aR) within the subfornical organ (SFO) mediate these responses. Transgenic mice carrying a conditional allele of the endogenous AT1aR (AT1aRflox) were administered an adenovirus encoding Cre-recombinase and enhanced green fluorescent protein (eGFP) or adenovirus encoding eGFP alone into the lateral cerebral ventricle. Adenovirus encoding Cre-recombinase reduced AT1aR mRNA and induced recombination in AT1aRflox genomic DNA specifically in the SFO, without significant effect in the paraventricular or arcuate nuclei, and also induced SFO-specific recombination in ROSATdTomato reporter mice. The effect of SFO-targeted ablation of endogenous AT1aR was evaluated in AT1aRflox mice at 3 time points: (1) baseline, (2) 1 week after virus injection but before DOCA-salt, and (3) after 3 weeks of DOCA-salt. DOCA-salt–treated mice with deletion of AT1aR in SFO exhibited a blunted increase in arterial pressure. Increased sympathetic cardiac modulation and urine copeptin, a marker of vasopressin release, were both significantly reduced in DOCA-salt mice when AT1aR was deleted in the SFO. Additionally, deletion of AT1aR in the SFO significantly attenuated the polydipsia, polyuria, and sodium intake in response to DOCA-salt. Together, these data highlight the contribution of AT1aR in the SFO to arterial pressure regulation potentially through changes on sympathetic cardiac modulation, vasopressin release, and hydromineral balance in the DOCA-salt model of hypertension.

Effect of the data sampling rate on accuracy of indices for heart rate and blood pressure variability and baroreflex function in resting rats and mice

The aim of this study was to determine the minimal sampling rate (SR) required for blood pressure (BP) waveform recordings to accurately determine BP and heart rate (HR) variability indices and baroreceptor reflex sensitivity in rats and mice. We also determined if an 8-bit (versus 12-bit) analog-to-digital converter (ADC) resolution is sufficient to accurately determine these hemodynamic parameters and if spline interpolation to 1000 Hz of BP waveforms sampled at lower SRs can improve accuracy. BP and ECG recordings (1000 Hz SR, 12-bit ADC resolution) from two strains of rats and BP recordings (1000 Hz SR, 12-bit ADC resolution) from two strains of mice were mathematically converted to lower SRs and/or 8-bit ADC resolution. Time-domain HR variability and frequency-domain HR and BP variability indices and baroreflex sensitivity (using the sequence technique) were determined and the results obtained from the original files were compared to the results obtained from the mathematically altered files. Our results demonstrate that an ADC resolution of 8 bit is not sufficient to determine HR and BP variability in rats and mice and baroreceptor reflex sensitivity in mice. Average values for systolic, mean and diastolic BP and HR can be accurately derived from BP waveforms recorded at a minimal SR of 200 Hz in rats and mice. Spline interpolation of BP waveforms to 1000 Hz prior to extracting derived parameters reduces this minimal SR to 50 Hz in rats but still requires 200 Hz in mice. Frequency-domain BP variability (very low and low frequency spectral powers) can be estimated accurately at a minimum SR of 100 Hz in rats and mice and spline interpolation of BP waveforms to 1000 Hz reduces this minimal SR to 50 Hz in rats but does not reduce the minimal SR in mice. Time- and frequency-domain HR variability parameters require at least a SR of 1000 Hz in rats and mice. Spline interpolation of BP waveforms to 1000 Hz reduces this minimal SR to 100 Hz in rats and to 200 Hz in mice. Estimation of baroreflex sensitivity using the sequence technique requires a SR of at least 1000 Hz in rats and mice. Spline interpolation of BP waveforms to 1000 Hz reduces this minimal SR to 100 Hz in rats but does not reduce the minimum SR in mice. Finally, our results indicate that HR time series derived from BP waveforms are not totally consistent with HR time series derived from the ECG in rats. In conclusion, accurate assessment of HR variability and baroreflex sensitivity from BP waveform recordings requires a SR of at least 1000 Hz in rats and mice. If lower SRs are used for BP waveform recordings, a cubic spline interpolation to 1000 Hz (or an even higher SR) prior to extracting derived parameters significantly improves accuracy.

Upregulation of the cardiac bradykinin B2 receptors after myocardial infarction.

An increase in myocardial bradykinin (BK) might be a mechanism to protect the heart during acute myocardial infarction (MI). To characterize the regulation of the myocardial B2 receptor during MI, we studied the expression of this BK receptor in the right ventricle (RV), left ventricle (LV) and myocardial septum (S) 24 h after left coronary ligation. Experiments were performed in male Wistar Kyoto rats (n = 10) and compared with sham operated animals (n = 6). After total RNA extraction, the myocardial B2-receptor expression was analyzed by a RNase protection assay (n = 6), using a specific probe from the coding region of the receptor gene. After 24 h, rats with MI were normotensive and showed an impaired left ventricular function. The B2-receptor expression of the LV of these rats was significantly elevated (2.3-fold) compared to sham operated rats. Furthermore, we found a dramatic upregulation of the B2 receptor in the RV (7.8-fold) and a dramatic expression of B2 receptor mRNA in S of infarcted hearts, whereas in the S of sham operated rats no B2 receptor expression could be detected. Our data show clearly that the described increase in BK during myocardial ischemia is accompanied by an elevated B2-receptor expression in the infarcted and non-infarcted parts of cardiac ventricles.

Heart Rate Variability Just a Surrogate for Mean Heart Rate

See related article, pp 1334–1343 In this issue of Hypertension , Monfredi et al1 address a phenomenon that has been repeatedly described in the literature2–5 but, nevertheless, went largely unnoticed by the scientific community using heart rate variability (HRV) to assess cardiac autonomic control or to predict cardiovascular health. The phenomenon studied by Monfredi et al1 is that most HRV parameters are inversely related to the actual level of HR, such that HRV is usually lower if HR is high and vice versa. The importance of the work by Monfredi et al1 is that it explores the physiological mechanisms underlying the relationship between HR and HRV using mathematical and biophysical models applied to data from various species and even in vitro preparations with differing levels of mean HR. Furthermore, using the standard deviation (SD) of normal to normal intervals6 as an example of one of the more frequently used HRV parameters; they provide a simple equation that can be used to normalize this HRV parameter so that it is independent of the actual level of HR. By convention, HRV is typically calculated from RR-interval and not from HR time series.6 As pointed out in a recent review article by Sacha,4 the relationship between HRV (calculated from RR-intervals) and HR is partly because of the inverse relationship between HR and RR-interval (eg, if HR fluctuates by ±10 bpm, the corresponding fluctuations in RR-intervals are ±250 ms at an average HR of 50 bpm but only ±60 ms at an average HR of 100 bpm). Thus, if HRV is calculated from RR-intervals, a …

Effect of chronic volume overload on baroreflex control of heart rate and sympathetic nerve activity

Baroreceptor-heart rate reflex sensitivity is decreased in congestive heart failure. The reflex control of heart rate and sympathetic nerve activity in rats with chronic volume overload, an established model for moderate heart failure, is still unknown. Therefore, we investigated the regulation of humoral and neuronal sympathetic activity and the baroreflex control of heart rate and sympathetic nerve activity in conscious, unrestrained rats with aortocaval shunt. Rats with aortocaval shunts had larger hearts (388 +/- 11 vs. 277 +/- 4 mg/100 g body wt), elevated central venous pressures (14 +/- 4 vs. 4 +/- 3 mmHg), and higher atrial natriuretic peptide plasma levels (87 +/- 16 vs. 25 +/- 3 pmol/l) than controls but had similar systemic blood pressure and heart rate values. Plasma epinephrine (0.63 +/- 0.16 vs. 0.21 +/- 0.08 pmol/l, P < 0.05) and norepinephrine concentrations (0.27 +/- 0.03 vs. 0.16 +/- 0.02 pmol/l, P < 0.05) were elevated in shunted rats compared with controls. Nitroprusside-induced hypotension led to a significantly greater increase in efferent splanchnic sympathetic nerve activity in shunted rats than in controls (0.9 +/- 0.1 vs. 2.6 +/- 0.6 microV, P < 0.05), whereas the heart rate responses were not different between the groups. These results indicate that the regulation of the autonomic nervous system is altered in chronically volume-overloaded rats. The arterial baroreflex control of efferent splanchnic sympathetic nerve activity was dissociated from the control of heart rate. Therefore, analysis of the activation of sympathetic nervous system assessed by direct measurements of efferent sympathetic nerve activity appears to be more sensitive for the detection of altered autonomic nervous system function than the analysis of baroreflex control of heart rate.Baroreceptor-heart rate reflex sensitivity is decreased in congestive heart failure. The reflex control of heart rate and sympathetic nerve activity in rats with chronic volume overload, an established model for moderate heart failure, is still unknown. Therefore, we investigated the regulation of humoral and neuronal sympathetic activity and the baroreflex control of heart rate and sympathetic nerve activity in conscious, unrestrained rats with aortocaval shunt. Rats with aortocaval shunts had larger hearts (388 ± 11 vs. 277 ± 4 mg/100 g body wt), elevated central venous pressures (14 ± 4 vs. 4 ± 3 mmHg), and higher atrial natriuretic peptide plasma levels (87 ± 16 vs. 25 ± 3 pmol/l) than controls but had similar systemic blood pressure and heart rate values. Plasma epinephrine (0.63 ± 0.16 vs. 0.21 ± 0.08 pmol/l, P < 0.05) and norepinephrine concentrations (0.27 ± 0.03 vs. 0.16 ± 0.02 pmol/l, P < 0.05) were elevated in shunted rats compared with controls. Nitroprusside-induced hypotension led to a significantly greater increase in efferent splanchnic sympathetic nerve activity in shunted rats than in controls (0.9 ± 0.1 vs. 2.6 ± 0.6 μV, P < 0.05), whereas the heart rate responses were not different between the groups. These results indicate that the regulation of the autonomic nervous system is altered in chronically volume-overloaded rats. The arterial baroreflex control of efferent splanchnic sympathetic nerve activity was dissociated from the control of heart rate. Therefore, analysis of the activation of sympathetic nervous system assessed by direct measurements of efferent sympathetic nerve activity appears to be more sensitive for the detection of altered autonomic nervous system function than the analysis of baroreflex control of heart rate.