Time-varying causal coherence function and its application to renal blood pressure and blood flow data.

This paper describes the development of a model-based approach to estimating both feedforward and feedback paths of causal time-varying coherence functions (TVCF). Theoretical derivations of the coherence bounds of the causal TVCF using the proposed approach are also provided. Both theoretical derivations and simulation results revealed interesting observations, and they were corroborated using experimental renal blood pressure and flow data. Specifically, both theoretical derivations and experimental data showed that in certain cases, the calculation of the traditional TVCF was inappropriate when the system under investigation was a causal system. Moreover, the use of the causal TVCF not only provides quantitative assessment of the coupling between the two signals, but it also provides valuable insights into the composition of the physical structure of the renal autoregulatory system.

Renal damage and salt-dependent hypertension in aged transgenic mice overexpressing endothelin-1.

The recent development of endothelin-1 (ET-1) antagonists and their potential use in the treatment of human disease raises questions as to the role of ET-1 in the pathophysiology of such cardiovascular ailments as hypertension, heart failure, renal failure and atherosclerosis. It is still unclear, for example, whether activation of an endogenous ET-1 system is itself the primary cause of any of these ailments. In that context, the phenotypic manifestations of chronic ET-1 overproduction may provide clues about the tissues and systems affected by ET-1. We therefore established two lines of transgenic mice overexpressing the ET-1 gene under the direction of its own promoter. These mice exhibited low body weight, diminished fur density and two- to fourfold increases in the ET-1 levels measured in plasma, heart, kidney and aorta. There were no apparent histological abnormalities in the visceral organs of young (8 weeks old) transgenic mice, nor was their blood pressure elevated. In aged (12 months old) transgenic mice, however, renal manifestations, including prominent interstitial fibrosis, renal cysts, glomerulosclerosis and narrowing of arterioles, were detected. These pathological changes were accompanied by decreased creatinine clearance, elevated urinary protein excretion and salt-dependent hypertension. It thus appears that mild, chronic overproduction of ET-1 does not primarily cause hypertension but triggers damaging changes in the kidney which lead to the susceptibility to salt-induced hypertension.

A new algorithm for linear and nonlinear ARMA model parameter estimation using affine geometry.

A linear and nonlinear autoregressive (AR) moving average (MA) (ARMA) identification algorithm is developed for modeling time series data. The new algorithm is based on the concepts of affine geometry in which the salient feature of the algorithm is to remove the linearly dependent ARMA vectors from the pool of candidate ARMA vectors. For noiseless time series data with a priori incorrect model-order selection, computer simulations show that accurate linear and nonlinear ARMA model parameters can be obtained with the new algorithm. Many algorithms, including the fast orthogonal search (FOS) algorithm, are not able to obtain correct parameter estimates in every case, even with noiseless time series data, because their model-order search criteria are suboptimal. For data contaminated with noise, computer simulations show that the new algorithm performs better than the FOS algorithm for MA processes, and similarly to the FOS algorithm for ARMA processes. However, the computational time to obtain the parameter estimates with the new algorithm is faster than with FOS. Application of the new algorithm to experimentally obtained renal blood flow and pressure data show that the new algorithm is reliable in obtaining physiologically understandable transfer function relations between blood pressure and flow signals.

Vascular abnormalities and elevated blood pressure in mice lacking adrenomedullin gene.

BACKGROUND: Adrenomedullin (AM) is a vasodilating peptide involved in the regulation of circulatory homeostasis and in the pathophysiology of certain cardiovascular diseases. Levels of AM are markedly increased in the fetoplacental circulation during pregnancy, although its function there remains unknown. To clarify the physiological functions of AM, we chose a gene-targeting strategy in mice. METHODS AND RESULTS: Targeted null mutation of the AM gene is lethal in utero: the mortality rate among AM(-/-) embryos was >80% at E13.5. The most apparent abnormality in surviving AM(-/-) embryos at E13.5 to E14.0 was severe hemorrhage, readily observable under the skin and in visceral organs. Hemorrhage was not detectable at E12.5 to E13.0, although the yolk sac lacked well-developed vessels. Electron microscopic examination showed endothelial cells to be partially detached from the basement structure at E12.5 in vitelline vessels and hepatic capillaries, which allowed efflux of protoerythrocytes through the disrupted barrier. The basement membrane was not clearly recognizable in the aorta and cervical artery, and the endothelial cells stood out from the wall of the lumen, only partially adhering to the basement structure. AM(+/-) mice survived to adulthood but exhibited elevated blood pressures with diminished nitric oxide production. CONCLUSIONS: AM is indispensable for the vascular morphogenesis during embryonic development and for postnatal regulation of blood pressure by stimulating nitric oxide production.

Hypotension and resistance to lipopolysaccharide-induced shock in transgenic mice overexpressing adrenomedullin in their vasculature.

BACKGROUND: Adrenomedullin (AM) is a vasodilating peptide involved in the regulation of circulatory homeostasis and in the pathophysiology of certain cardiovascular diseases. To determine the extent to which chronic AM overproduction affects circulatory physiology under normal and pathological conditions, we used a preproendothelin-1 promoter to establish transgenic mouse lines overexpressing AM in their vasculature. METHODS AND RESULTS: Transgenic mice overexpressing AM mainly in vascular endothelial and smooth muscle cells exhibited significantly lower blood pressure (BP) and higher plasma cGMP levels than their wild-type littermates. Blockade of NO synthase with N(G)-monomethyl-L-arginine elevated BP to a greater degree in AM transgenic mice, offsetting the BP difference between the 2 groups. Despite their lower basal BP, administration of bacterial lipopolysaccharide elicited smaller declines in BP and less severe organ damage in AM transgenic mice than in wild-type mice. Furthermore, the 24-hour survival rate after induction of lipopolysaccharide shock was significantly higher in the transgenic mice. CONCLUSIONS: A chronic increase in vascular AM production reduces BP at least in part via an NO-dependent pathway. In addition, smaller responses to LPS in transgenic mice suggest that AM is protective against the circulatory collapse, organ damage, and mortality characteristic of endotoxic shock.

Elevation of blood pressure by genetic and pharmacological disruption of the ETB receptor in mice.

Exogenously administered endothelin (ET) elicits both pressor and depressor responses through the ETA and/or the ETB receptor on vascular smooth muscle cells and ETB on endothelial cells. To test whether ETB has pressor or depressor effects under basal physiological conditions, we determined arterial blood pressure (BP) in ETB-deficient mice obtained by crossing inbred mice heterozygous for targeted disruption of the ETB gene with mice homozygous for the piebald (s) mutation of the ETB gene (ETBs/s). F1 ETB-/s and ETB+/s progeny share an identical genetic background but have ETB levels that are approximately (1)/(8) and (5)/(8), respectively, of wild-type mice (ETB+/+). BP in ETB-/s mice was significantly higher, by approximately 20 mmHg, than that in ETB+/s or ETB+/+ mice. Immunoreactive ET-1 concentration in plasma as well as respiratory parameters was not different between ETB-/s and ETB+/s mice. A selective ETB antagonist, BQ-788, increased BP in ETB+/s and ETB+/+ but not in ETB-/s mice. Pretreatment with indomethacin, but not with NG-monomethyl-L-arginine, can attenuate the observed pressor response to BQ-788. The selective ETA antagonist BQ-123 did not ameliorate the increased BP in ETB-/s mice. Moreover, BP in mice heterozygous for targeted disruption of the ETA gene was not different from that in wild-type controls. These results suggest that endogenous ET elicits a depressor effect through ETB under basal conditions, in part through tonic production of prostaglandins, and not through secondary mechanisms involving respiratory control or clearance of circulating ET.

Endothelin in the central control of cardiovascular and respiratory functions.

1. Exogenously administered endothelin (ET) modulates the activity of cardiovascular and respiratory neurons in the central nervous system (CNS) and, thus, affects arterial blood pressure (ABP) and ventilation. However, a physiological role(s) for endogenous ET in the CNS has not been elucidated. To address this question, we examined ABP and ventilation in mutant mice deficient in ET-1, ETA and ETB receptors and endothelin-converting enzyme-1, which were made by gene targeting. 2. Respiratory frequency and volume was measured in mice by whole body plethysmography when animals breathed normal room air and hypoxic and hypercapnic gas mixtures. A few days after respiratory measurements, a catheter was implanted into the femoral artery under halothane anaesthesia. On the following day, the ABP of awake mice was measured through the indwelling catheter and heart rate was calculated from the ABP signal. After 2 h ABP measurement, arterial blood was collected through the catheter and pH and the partial pressures of O2 and CO2 were measured by a blood gas analyser. 3. Compared with corresponding controls, the mean (+/- SEM) ABP in ET-1+/- and ETB-deficient mice was significantly higher (118 +/- 2 vs 106 +/- 3 mmHg for ET-1+/- (n = 22) and ET-1+/+ (n = 17) mice, respectively; 127 +/- 3 vs 109 +/- 4 mmHg for ETB-/s (n = 9) and ETB+/s (n = 9) mice, respectively; P < 0.05 for both). In ET-1+/- mice, PCO2 tended to be higher and PO2 was significantly lower than corresponding values in ET-1+/+ mice. Under resting conditions, there was no significant difference in respiratory parameters between mutants and their corresponding controls. However, reflex increases of ventilation to hypoxia and hypercapnia were significantly attenuated in ET-1+/-, ET-1-/- and ETA-/- mice. 4. In another series of experiments in ET-1+/- mice, we found that sympathetic nerve activity (SNA) was augmented and reflex excitation of phrenic nerve activity (PNA) in response to hypoxia and hypercapnia was blunted. Attenuation of the reflex PNA response to hypercapnia was also observed in the medulla-spinal cord preparation from ET-1-/- mice. 5. Elevation of ABP in ETB-deficient mice was most likely due to a peripheral mechanism, because SNA and respiratory reflexes were not different from those in control animals. 6. We conclude that endogenous ET-1 plays an important role in the central neural control of circulation and respiration and that ETA receptors mediate this mechanism.

Renal sympathetic nerve activity in mice: comparison between mice and rats and between normal and endothelin-1 deficient mice.

Recently generated knockout mice with disrupted genes encoding endothelin (ET)-1 showed an elevation of arterial blood pressure (AP) and supplied an evidence for intrinsic ET-1 as one of the physiological regulators of systemic AP. Little is yet known, however, why deficiency of ET-1, which was originally found as a potent vasoconstrictor, led to higher AP in these mice. To address this apparent paradox, we first developed a method to measure renal sympathetic nerve activity (RSNA) in mice using rats as reference and successively compared it between normal and ET-1 deficient mice. RSNA was successfully recorded in urethane-anesthetized and artificially ventilated mice by a slight modification of the method used for rats. At basal condition, mean AP (MAP) and RSNA in ET-1 deficient mice (105+/-2 mmHg and 9.71+/-1.49 muVs, n=20) were significantly higher than those in wild-type mice (96+/-2 mmHg and 5. 07+/-0.70 muVs, n=25). Basal heart rate (HR) and baroreflex-control of HR was not significantly different between the two. On the other hand, resting RSNA, RSNA range, and maximum RSNA were significantly greater in ET-1 deficient mice, and thus MAP-RSNA relationship was upwards reset. Hypoxia-induced increase in RSNA was not different between ET-1 deficient (73.4+/-9.4%) and wild-type mice (91.2+/-12.0%), while hypercapnia-induced one was significantly attenuated in ET-1 deficient mice (18.8+/-3.6 vs. 39.1+/-5.2% at 10% CO2). These results indicate that endogenous ET-1 participates in the central chemoreception of CO2 and reflex control of the RSNA. Baroreceptor resetting and normally preserved hypoxia-induced chemoreflex may explain a part of the elevation of AP in ET-1 deficient mice.

[Central autonomic control in human hypertension].

Sympathetic nerve fibers innervating the heart and blood vessels exhibit tonic activity which originates from vasomotor neurons in the rostral ventrolateral medulla (RVLM). Whether sympathetic nerve activity (SNA) is elevated in humans with essential hypertension is still unsettled. Recording of muscle SNA (MSNA) by the microneurographic method mostly points to elevated SNA in hypertensives including borderline and mildly hypertensive patients. Exaggerated MSNA is most conspicuous in the accelerated hypertensives. Power spectral analyses of heart rate and blood pressure variability provide controversial results on this issue. Nevertheless, most studies demonstrate altered sympathetic cardiovascular control in hypertensive subjects. In young borderline hypertensives, the sympathetic response to mental stress or passive tilting is exaggerated, whereas in elderly subjects it is attenuated.

Impaired ventilatory responses to hypoxia and hypercapnia in mutant mice deficient in endothelin-1.

We studied respiratory functions in mutant mice deficient in endothelin-1 (ET-1) generated by gene targeting. In conscious adult mice heterozygous for ET-1 gene mutation (ET+/- heterozygous mice), arterial PO2 was significantly lower, PCO2 tended to be higher, and pH tended to be lower than in wild-type littermates. When these conscious mice breathed room air, respiratory minute volume and rate, determined by body plethysmography, were not significantly different between the two groups. However, when ET+/- heterozygous mice were subjected to systemic hypoxia (1:1 air-N2) or hypercapnia (5% CO2-95% O2), increases in respiratory minute volume were significantly attenuated. In conscious newborn ET-/- homozygous mice delivered by cesarean section and tracheotomized, ventilatory responses to systemic hypoxia and hypercapnia, regularly observed in newborn wild-type mice, were almost totally absent. In urethan-anesthetized adult ET+/- heterozygous mice, increases in phrenic nerve discharges in response to hypoxia and hypercapnia were significantly attenuated. Our results demonstrate that ventilatory responses to hypoxia and hypercapnia are impaired in ET-1-deficient mice and suggest that endogenous ET-1 participates in the physiological control of ventilation.