Distribution and characterization of natriuretic peptide receptors in the gills of the spiny dogfish, Squalus acanthias

The distribution and nature of natriuretic peptide receptors (NPR) in the gills of dogfish, Squalus acanthias, were examined by tissue section autoradiography, competition analysis, protein electrophoresis, guanylate cyclase (GC) assays, and molecular cloning. Specific NP binding occurred on the gill filaments, but not on the interbranchial septum or gill arch. The binding was densest on the efferent edge of the gills. Higher resolution light-microscopic examination of emulsion-coated sections showed that specific binding occurred mainly on the secondary lamellae and filament body and not on the arterial circulation. At least two types of NPR were revealed. One is linked to GC since NP binding stimulates the production of cGMP. The GC receptor may be similar to the NPR-B mammalian receptor since only pCNP stimulated cGMP production. The second receptor is not linked to GC and binds the specific ligand C-ANF [rat des(Gln18, Ser19, Gly20, Leu21, Gly22)]. The sequence of a cDNA generated using primers based on conserved regions of vertebrate NPR-C had considerable homology with mammalian and eel NPR-C and eel NPR-D. The presence of GC-linked NPR and NPR-C/ NPR-D suggests that the gills are an important target organ for NP action.

Spectral pattern of neonatal cerebral blood flow velocity: comparison with spectra from blood pressure and heart rate

The cerebral blood flow velocity (CBFV) frequency spectra were studied in 106 premature and term newborns (gestational age range. 24-42 wk) and compared with the heart rate (HR) and mean arterial blood pressure (BP) spectra over the 0.005-0.5 Hz range. CBFV, BP, and HR were shown to have similar but not identical spectral patterns. Adjustment of a l/f model to these spectra produced highly significant fittings, but the residuals were not independent. This condition was met only by the CBFV and BP spectra over a limited frequency range (0.005-0.06 Hz). These results provide a characterization of the CBFV spectra for a much larger population of newborns than hitherto available, indicating that under certain conditions CBFV and BP might show the properties of chaotic systems. In infants without major complications, gestational age (GA) did not have a significant influence on the CBFV spectrum, whereas the spectral power to 0.5 Hz of both BP and HR was found to increase with GA. The spectral power increased over the first 24 h of postnatal life for all three variables: only CBFV showed a significant spectral change in the low frequency (LF, 0.02-0.08 Hz) range. A matched group comparison, adjusted for GA and postnatal age, indicated a reduction in CBFV LF power for term infants with birth asphyxia when compared with normal infants, which was not reproduced in the HR spectra.

Variability in cerebral blood flow velocity: observations over one minute in preterm babies

Cerebral blood flow velocity (CBFV) was measured using Doppler ultrasound on the first day of life in a consecutively admitted cohort of 52 very low birthweight infants. Recordings were made for a period of 1 min. The coefficient of variation for the area under the curve of 20 successive cardiac cycles was calculated, and a 20 cycle moving window then applied to the whole recording. This showed that the coefficient of variation varied widely during 1 min in any individual, the range being from 2% to 28% (median 8%). There was a strong correlation between the variability in CBFV and that in systemic blood pressure (BP). Variability in CBFV was significantly higher in babies with hypotensive episodes (P = 0.026). Babies who died had a higher maximum coefficient of variation than those who survived (P = 0.05), but we were unable to confirm any association with brain injury or patent ductus arteriosus.

A study of the spectral broadening of simulated Doppler signals using FFT and AR modelling

Doppler ultrasound is used clinically to detect stenosis in the carotid artery. The presence of stenosis may be identified by disturbed flow patterns distal to the stenosis that cause spectral broadening in the spectrum of the Doppler signal around peak systole. This paper investigates the behaviour of the spectral broadening index (SBI) derived from wide-band spectra obtained using autoregressive modelling (AR), compared with the SBI based on the fast-Fourier transform (FFT) spectra. Simulated Doppler signals were created using white noise and shaped filters to analyse spectra typically found around the systolic peak and to assess the magnitude and variance of AR and FFT-SBI for a range of signal-to-noise ratios. The results of the analysis show a strong correlation between the indices calculated using the FFT and AR algorithms. Despite the qualitative improvement of the AR spectra over the FFT, the estimation of SBI for short data frames is not significantly improved using AR.

Signals from dual gated TCD systems: curious observations and possible explanations

Dual gated transcranial Doppler (TCD) systems have the potential to distinguish definitively embolic events in the cerebral circulation from artefact. While developing such a system, three curious observations were made and are the subject of this article. The first was that the velocity of propagation of an embolus in an artery as calculated from the ultrasonic Doppler shift was, in many cases, dramatically different from the velocity calculated using a time of flight method. The second observation was that the durations of embolic signals measured in two gates were often very different, despite the received gates being identical in length. The third observation was that some emboli detected in the deeper gate did not appear in the more superficial gate, and, more surprisingly (as has recently been reported by another group), some emboli that had not previously appeared in the deeper gate appeared in the more superficial gate. It is hypothesised that all of these effects can be explained in terms of the geometry of the middle cerebral artery in relation to the interrogating ultrasound beam. None of these effects should detract from the usefulness of TCD, but will need to be widely appreciated if dual gated measurements of cerebral emboli are to be interpreted correctly.

Analysis of the frequency modulation present in Doppler ultrasound signals may allow differentiation between particulate and gaseous cerebral emboli

The purpose of this study was to investigate the incidence of frequency modulation in Doppler signals from cerebral emboli and to seek possible explanations for its occurrence. Signals from 200 particulate emboli and 200 presumed gaseous emboli were studies. The Doppler signals were visualised in the time domain and were classified into three main types. Type I signals contained no modulation, type II signals showed gradual frequency changes and type III showed a rapid change evident in only a small percentage of the entire signal. Type I signals were observed from 71.5% of particulate emboli but only 19% of gaseous emboli (X(2)) = 111, p < 0.001). Type II signals were found in 28.5% of particulate emboli and 38% of gaseous emboli (X(2)) = 4.06, p < 0.05). The most surprising and significant finding was that 43% of gaseous embolic signals were categorised as type III signals compared with 0% of signals from particulate emboli (X(2)) = 109, p < 0.001). The finding that known particulate emboli appear never to produce rapid frequency modulation may provide a basis for differentiating between gaseous and particulate emboli.

An evaluation of the potential and limitations of three-dimensional reconstructions from intravascular ultrasound images

Three-dimensional (3D) reconstructions of arteries can be produced using two-dimensional (2D) intravascular ultrasound (IVUS) images. Any artefact that affects 2D images has the potential to limit the quality of a 3D reconstruction. Using a catheter withdrawal technique, a range of test rigs were used to assess: (i) the effect of rotation of the probe orientation; (ii) the ability to reconstruct the true path of a tortuous vessel; (iii) the effect of image distortion on diameter measurements; (iv) the number of images per unit length used to produce a 3D reconstruction; and (v) the quality of the IVUS 3D reconstruction of a stent. These investigations show that 3D IVUS imaging is prone to artefacts. For 3D IVUS images to be used to quantify the vessel path or to make accurate measurements of vessel dimensions, more information about the catheter tip position and orientation is required than is currently available with the pullback technique.

Genotoxin resistance properties of transgenic tobacco plants expressing bacteriophage T4 DenV and Saccharomyces cerevisiae Apn1 proteins

We have examined whether the resistance to genotoxic agents can be altered in transgenic plants by introducing heterologous DNA repair enzymes. Transformation of tobacco tissue produced two lines of plants, one expressing bacteriophage T4 UV endonuclease (DenV) and the other expressing Saccharomyces cerevisiae apurinic-apyrimidinic endonuclease and 3'-diesterase (Apn1). Some of the transformants were subsequently crossed, with the expectation that Apn1 activity might complement DenV activity in hybrid plants. Apn1 transgenotes behaved similarly to control plants upon exposure to UV-C light, oxidizing agents, or alkylating agents, as measured by chlorophyll bleaching. This is in contrast to plants expressing DenV activity, which have been previously shown to exhibit varying degrees of sensitivity to UV-C light and the alkylating agent dimethyl sulfate. APN1/denV hybrid plants were more sensitive to UV-C light than were parental lines, but reproducibly displayed enhanced resistance to dimethyl sulfate. These data indicate that repair processes are an important component of natural protective systems in tobacco, because exogenous repair genes compromised the natural resistance of denV-transformed plants. In the hybrid plants, the two proteins appeared to act in concert, potentiating the effects of UV damage but enhancing the resistance to alkylation damage.

Analysis of cerebral blood flow autoregulation in neonates

The dynamic response of cerebral autoregulation to spontaneous changes in arterial blood pressure (ABP) is described by the relationship between cerebral blood flow velocity (CBFV) and resistance-area product (RAP). CBFV was measured with Doppler ultrasound in the middle cerebral artery and ABP with an intra-arterial catheter in 66 neonates. Spontaneous changes in mean ABP were automatically detected and the maximum derivative was used to synchronize the coherent averaging of corresponding CBFV and RAP transients. These were classified into two groups corresponding to intact (group A) or impaired (group B) autoregulation. The cross correlation between RAP and CBFV indicates a significant relationship with a time delay of 5 s for group A. The frequency response of RAP was estimated by the cross spectra with CBFV. Groups A and B present a similar amplitude spectra but the phase spectra of group A lags that of group B. The impulse responses of the two groups are also markedly different and were used to simulate the velocity response to a 5% step change in ABP. Impulse responses were also obtained for four different levels of pCO2 showing that hypercapnia leads to an impulse response similar to that of group B (impaired autoregulation). This method can be used to extend the usual dichotomic classification adopted in clinical studies of autoregulation.

SFV topoisomerase: sequence specificity in a genetically mapped interval

Poxviral DNA topoisomerases are sequence-specific enzymes whose activities are thought to influence such diverse processes as transcription, DNA replication, and genetic recombination. To obtain further insights into the relatedness of these enzymes, and their influence on virus-mediated recombination, we have determined the target-specificity and other catalytic properties of the Shope fibroma virus (SFV) topoisomerase. SFV topoisomerase was expressed in Escherichia coli and purified as a glutathione S-transferase (GST) or (his)6-tagged fusion protein. The recombinant Leporipox-virus (SFV) enzyme displayed catalytic properties very similar to vaccinia topoisomerase. In particular SFV topoisomerase recognizes the same pentanucleotide motif [5'-(C/T)CCTT-3'] and promotes the same DNA relaxation, strand transfer, and strand cleavage reactions catalyzed by the Orthopoxviral (vaccinia) enzyme. The SFV enzyme can also efficiently cleave DNA 3' of the variant site 5'-CCCTG-3' in certain sequence contexts. These studies identified several sites where SFV topoisomerases interact with a recombinational substrate and permitted a comparison of recombination frequencies across intervals which did, or did not, span these sites. We failed to detect any effect of topoisomerase-recognition sites on recombination frequencies, except for a small (< 2-fold) stimulation seen when the substrates encoded a nearby poxviral promoter. This and other work shows that poxviral topoisomerases from several genera share common target specificities, but other enzymatic systems probably catalyze the high-frequency recombination seen in poxvirus-infected cells.

Tobacco plants expressing T4 endonuclease V show enhanced sensitivity to ultraviolet light and DNA alkylating agents

DNA repair processes and UV-filtering pigments protect organisms from the cytotoxicity of UV light and endow plants with a high degree of natural UV resistance. In an attempt to further enhance this UV resistance we have constructed transgenic tobacco lines that express a DNA repair enzyme encoded by the bacteriophage T4 denV gene. The denV gene encodes endonuclease V, an enzyme which initiates base excision repair of cyclobutane pyrimidine dimers. Its presence is expected to provide transgenotes with a repair pathway complementary to, but likely distinct from, the repair pathways found in tobacco. The denV gene, flanked by a CaMV 35S promoter and poly(A) addition site, was introduced into tobacco and mature plants regenerated. The transgenotes expressed high levels of a UV-specific endonuclease and no such activity was found in control plants. Curiously, assays which detected several different biological endpoints showed that the denV+ transgenotes were also hypersensitive to UV-C light. This hypersensitivity segregated with the denV gene and was not caused by altered concentrations of UV-filtering pigments. Moreover, the denV+ transgenotes were also hypersensitive to high levels of baseless lesions that would be generated by a transgenically expressed beta-eliminating lyase such as endonuclease V.

ETB-type receptors mediate endothelin-stimulated contraction in the aortic vascular smooth muscle of the spiny dogfish shark, Squalus acanthias

Using various agonists, and the specific antagonist BQ-123, we have examined the sensitivity to endothelin of the vascular smooth muscle of the ventral aorta of the spiny dogfish shark, Squalus acanthias. Human endothelin-1 produced significant contraction of isolated vascular smooth muscle rings, with an EC50 of 10 nmol.l-1. The presence of an intact endothelium did not alter this response but the magnitude of the contraction was greater in rings with an intact endothelium. The response to 0.2 mumol.l-1 endothelin-1 was equivalent to that of 0.1 mmol.l-1 acetylcholine, and significantly greater than that to 80 mmol.l-1 KCl, suggesting high sensitivity even to the heterologous, mammalian peptide. The Hill plot of the contractile response was a straight line with a slope of 1.12, indicating that a single receptor was mediating the response. Endothelin-1, endothelin-3, and sarafotoxin S6c produced similar concentration-response curves, and the response to endothelin-1 was insensitive to the ETA-specific inhibitor BQ-123. Our data are consistent with the hypothesis that the receptor involved in the contractile response to endothelin of shark aortic vascular smooth muscle is of the ETB-rather than the ETA-type.

The effect of geometrical spectral broadening on the estimation of mean blood velocity using wide and narrow ultrasound beams

If an ultrasound beam uniformly insonates the cross section of a blood vessel then the Doppler signal can be analyzed to give a frequency proportional to the spatial mean blood velocity. This is also possible if the beam can be thought of as negligibly thin compared to the blood vessel radius, centrally placed, and the blood velocity profile is axisymmetric and monotonic, although the analysis takes a different form. The immunity of these mean velocity estimators to broadening of the ideal frequency spectrum is studied. If the broadening of a frequency component is such that its mean frequency, weighted by intensity, is unchanged then the analysis with a uniformly insonating beam still leads to the correct mean velocity. In contrast, for any such broadening, the analysis if the beam is negligibly thin produces an underestimate of the mean velocity. Error expressions are derived for idealized cases and some practical results given.

The effect of noise and high-pass filtering on the estimation of mean blood velocity using wide and narrow ultrasound beams

Recently, a new method of analysis has been proposed for the calculation of a Doppler frequency proportional to mean blood velocity for the case where the Doppler beam is assumed to be of negligible thickness compared to the vessel diameter, and the velocity profile is axisymmetric and monotonic increasing from the vessel wall to the vessel center. Such analysis of the Doppler signal is an alternative to that commonly performed under the assumption that the beam insonates the vessel uniformly. Errors in each method are found and compared for the case where the Doppler signal is contaminated by noise, and for the case where the signal is subjected to an ideal high-pass filter. The frequency resulting from the new method of analysis is affected by low-frequency perturbations approximately twice as much as that resulting from the standard method. However, the new method is much more immune to high frequency perturbations. If each method is used with the beam shape for which it is appropriate then, for a given velocity profile, each method is equally affected by the use of the same ideal high-pass filter.

The effect of transcription on genetic recombination in poxvirus-infected cells

We have examined the effects of transcription on recombination frequencies in poxvirus-infected cells. A synthetic poxviral promoter was shown to function as a hybrid early/late transcription element when fused to a luciferase reporter gene, and then cloned into genetically-marked recombination substrates. These lambda DNA substrates were transfected into cells infected with Shope fibroma virus (SFV) and the recombinants detected by recovering the transfected DNA, packaging it in vitro into infectious particles, and then assaying the yield of recombinants on Escherichia coli. Controls showed that the poxviral promoter conferred no replicative advantage, or disadvantage, on molecules encoding the promoter. Furthermore, the promoter had no detectable effect on the recombination frequency when recombination was measured in the interval immediately adjacent to the promoter-insertion site. However, the promoter did appear to stimulate recombination at a distance, in a manner that appeared to be dependent on the level of transcription, and the effect was observed regardless of whether or not the promoter was present on one or both of the recombinational substrates. The peak of recombinational enhancement was centered about 500 bp away from the promoter element, where the frequency of recombination was 30-50% higher than that seen when the recombinational substrates lacked the promoter. Possible explanations for these observations are discussed.

Compensation for phase and amplitude imbalance in quadrature Doppler signals

Quadrature phase detection is widely used in ultrasound Doppler systems to obtain directional information about flow velocities in blood vessels. The presence of phase and/or gain imbalance between the quadrature outputs produces crosstalk between the forward and reverse channels which may introduce errors into mean and maximum velocity estimates and hamper the interpretation of sonograms. This technical note describes the assessment of the crosstalk in a commercial pulsed-wave Doppler system, and suggests methods of compensation for the causative imbalance which can be implemented using digital signal processing techniques. Examples of applying these methods to the outputs of the commercial system are given.

Power Doppler: an advance over colour Doppler for transcranial imaging?

The aim of this study is to compare the ability of mean frequency-based colour Doppler (CD) and power Doppler (PD) to identify the basal cerebral arteries in normal subjects. Fifty volunteers were recruited into the study. The temporal acoustic window was used to image the basal cerebral arteries bilaterally. The number of positively identified vessels was recorded, first using CD and then with PD. Our results show that both modalities were equally effective at demonstrating the anterior cerebral artery (ACA), the middle cerebral artery mainstream (MCA M1) and the precommunicating segment of the posterior cerebral artery (PCA P1; ACA: 82% and 83%; MCA M1: 84% and 89%; PCA P1: 81% and 83%; CD and PD, respectively). However, the M2 portion of the MCA (CD: 25%, PD: 54%; difference 29% [95% CI 20-38%]), the P2 portion of the PCA (CD: 67%, PD: 75%; difference 8% [95% CI 3-13%]) and the posterior communicating artery (CD: 15%, PD 32%; difference 17% [95% CI 10-24%]) were all identified in a significantly greater proportion of subjects using PD. Although its clinical value remains undetermined, PD offers significant advantages over colour-coded sonography in imaging small-calibre and low-flow arteries and those which run at unfavourable angles to the ultrasound beam.

Differentiation between emboli and artefacts using dual-gated transcranial Doppler ultrasound

It is well documented that transcranial Doppler ultrasound has the ability to detect cerebral emboli. During intraoperative patient monitoring studies, many signals due to artefact (probe motion, patient movement or surgical manipulation) are also detected and can be difficult to distinguish from genuine embolic events. We have constructed a Doppler system that can simultaneously range-gate at two separate depths, in order to test the hypothesis that it should be possible to distinguish between emboli and artefact by comparing the signal from the two separate regions within the vessel. The classification algorithm is based on the principle that emboli propagate with blood motion (whereas artefacts do not) and thus will be detected sequentially at different depths along the insonated cerebral artery. One hundred thirty-eight (presumed) embolic and 170 artefact signals were analysed. The median (interquartile range) gate separation was 10.01 mm (7.41-10.78 mm). The time delay between detection of embolic signals in the two channels was 11.04 ms (6.24-16.41 ms, but was only 0.08 ms (-0.48(-)+0.64 ms) for artefact (p < 0.0001). Dual-gated Doppler ultrasound is a conclusive and independent method that differentiates emboli from artefact. Incorporation of this system for long-term monitoring may eliminate the need for an experienced observer to be present.

Interpretation of embolic phenomena during carotid endarterectomy

BACKGROUND AND PURPOSE

Air and particulate emboli are a major source of morbidity during carotid endarterectomy (CEA); however, amplitude overload and poor time resolution have restricted the ability of transcranial Doppler ultrasound to differentiate between the two.

METHODS

We have now overcome these two limitations by (1) rerouting embolic signals away from the audio frequency amplifier to avoid amplitude overload and (2) substituting the Wigner distribution function for the fast Fourier transform to improve time and frequency resolution. Thus, we can now accurately determine embolic duration and embolic velocity, the product of which is the sample volume length (SVL). This measurement represents the physical distance over which an embolic signal can be detected. The underlying hypothesis was that air reflected more ultrasound and would therefore be detected over a greater SVL.

RESULTS

The median SVL (interquartile range) for 75 in vitro air emboli was 1.97 cm (range, 1.70 to 2.35) compared with 0.27 cm (range, 0.16 to 0.43) for 185 particulate emboli detected during the dissection phase of CEA. Off-line analysis on an additional 560 embolic signals detected during different phases of CEA suggested that 46 of 143 (32%) of emboli immediately after shunt insertion were particulate, as were 19 of 33 (58%) occurring during shunting, 28 of 78 (36%) after restoration of flow in the external carotid artery, 23 of 251 (9%) after restoration of flow in the internal carotid artery, and 55 of 55 (100%) of those emboli detected during the early recovery phase.

CONCLUSIONS

This development provides objective physical criteria upon which embolus characterization (particulate/air) can be based. This could have major implications for future patient monitoring with respect to modification of surgical technique and pharmacological intervention.

DNA strand transfer catalyzed by the 5'-3' exonuclease domain of Escherichia coli DNA polymerase I

A protein which promotes DNA strand transfer between linear double-stranded M13mp19 DNA and single-stranded viral M13mp19 DNA has been isolated from recA- E.coli. The protein is DNA polymerase I. Strand transfer activity residues in the small fragment encoding the 5'-3' exonuclease and can be detected using a recombinant protein comprising the first 324 amino acids encoded by polA. Either the recombinant 5'-3' exonuclease or intact DNA polymerase I can catalyze joint molecule formation, in reactions requiring only Mg2+ and homologous DNA substrates. Both kinds of reactions are unaffected by added ATP. Electron microscopy shows that the joint molecules formed in these reactions bear displaced single strands and therefore this reaction is not simply promoted by annealing of exonuclease-gapped molecules. The pairing reaction is also polar and displaces the 5'-end of the non-complementary strand, extending the heteroduplex joint in a 5'-3' direction relative to the displaced strand. Thus strand transfer occurs with the same polarity as nick translation. These results show that E.coli, like many eukaryotes, possesses a protein which can promote ATP-independent strand-transfer reactions and raises questions concerning the possible biological role of this function.

Circulatory effects of ventilator rate and end-expiratory pressure in unparalysed preterm infants

Impairment of cerebrovascular autoregulation may be important in the pathogenesis of ischaemic brain injury in preterm infants. A previous study in ventilated preterm infants paralysed with pancuronium showed that changes in cerebral blood flow velocity (CBFV) were related to concomitant changes in arterial blood pressure. In a similar study in unparalysed infants, changes in CBFV in response to changes in ventilator rate or end-expiratory pressure were independent of associated changes in the arterial blood pressure. These results emphasize the importance of avoiding large swings in blood pressure in paralysed infants. Whether alternative paralysing agents have similar effects warrants further study.

A microcomputer-based system for coronary artery Doppler studies

Intra-vascular Doppler catheters are becoming popular as research tools for studies of the coronary circulation. Velocity estimates can be obtained from Doppler signals sampled with such catheters, and used to calculate various derived parameters which can then be correlated with the haemodynamic situation under investigation. This note discusses the requirements necessary for a system to perform the acquisition of the high-frequency Doppler signals obtained from catheter-tipped transducers, and a system developed to capture, process and store Doppler, ECG and pressure signals is described. Software used to calculate parameters from the stored waveforms is discussed and examples of the system outputs presented.

Electron spectroscopic imaging of encapsidated DNA in vaccinia virus

We have used electron spectroscopic imaging to locate the phosphorus in vaccinia DNA in situ in unstained, ultrathin sections of virions. The phosphorus of the DNA backbone appeared to form a halo on the core periphery surrounding a phosphorus-impoverished central element. These results constrain models for how DNA could be packaged into mature vaccinia particles.

Natriuretic peptide receptors in the kidney and the ventral and dorsal aortae of the Atlantic hagfish Myxine glutinosa (Agnatha)

The character of natriuretic peptide receptors (NPRs) in the kidney and aortae of the Atlantic hagfish Myxine glutinosa was determined and compared with that of NPRs in hagfish gills. The relationship of hagfish kidney and aortic NPRs with NPRs from higher vertebrates was also examined. Iodinated atrial and C-type natriuretic peptides (NPs) (125I-ANP, 125I-CNP) were used in tissue section autoradiography, competition studies and guanylate cyclase (GC) assays. Rat atrial and porcine C-type NPs (rANP, pCNP) and rat des[Gln18, Ser19, Gly20, Leu21 Gly22]ANP-(4-23)-NH2 (C-ANF, which binds to the mammalian and teleost 'clearance' receptor, NPR-C), were used as competing ligands. 125I-ANP binding sites were observed on both aortae and on the glomeruli, neck segments and archinephric ducts of the kidney. 4.0 nmol l-1 rANP competed for 50% of 125I-ANP glomerular sites. 125I-CNP did not visibly bind to any of the tissues, but 300 nmol l-1 pCNP competed for 50% of 125I-ANP glomerular sites. C-ANF failed to compete for 125I-ANP sites. rANP and pCNP stimulated cyclic GMP production in kidney membrane preparations, but C-ANF did not, demonstrating that the hagfish kidney NPR is GC-linked. This study suggests that a predominant population of ANP-like receptors, similar to the mammalian NPR-A, exists in the myxinoid aortae and kidney tissue. However, no detectable population of a receptor that binds all NPs, such as is present in the hagfish gill, nor an NPR similar to the NPR-C of higher vertebrates was discovered.

Estimation of critical closing pressure in the cerebral circulation of newborns

We have developed a method to estimate the critical closing pressure (CrCP) of the cerebral circulation based on the intrinsic variability of arterial blood pressure (BP) around stable values of mean arterial pressure (MAP). A consecutive cohort of 33 premature newborns was studied at 6, 12, 24, 48 and 72 hours of age. Cerebral blood flow velocity (CBFV) was measured with Doppler ultrasound in the middle cerebral artery and BP was recorded in the abdominal aorta or in a peripheral artery. Continuous measurements lasting five minutes were recorded on digital magnetic tape and signals were digitized at a rate of 200 samples/seconds for processing on a digital computer. Mean values of BP (mBP) and CBFV (mBV) were computed for each cardiac cycle and CrCP was determined as the pressure axis intercept of the regression line of mBV as a function of mBP using 100 sequential heart beats. The resistance-area product (RAP) was obtained from the slope of the regression line. For 57 records (30 patients) the mean +/- SD values of CrCP and RAP were 23.9 +/- 11.6 mmHg and 4.07 +/- 1.83 x 10(4) kg.m-2.s-1, respectively. CrCP has a highly significant correlation with mean arterial pressure (p < 0.001) but RAP has not. Neither CrCP nor RAP are significantly correlated (p > 0.05) with PO2, PCO2, pH, haematocrit, gestational age, birthweight, postnatal age, heart rate on Pourcelot's resistance index. Our results suggest that cerebral perfusion pressure should be defined as MAP-CrCP for normal values of intracranial pressure.