Biomedical subjects

H K Richards

Publications and source records attributed to H K Richards.

At least 19 recordsLinked to original sources

PubMed1996

Relationship between transcranial Doppler-determined pulsatility index and cerebrovascular resistance: an experimental study.

Clinical studies with transcranial Doppler suggest that the pulsatility of the flow velocity (FV) waveform increases when the distal cerebrovascular resistance (CVR) increases. To clarify this relationship, the authors studied animal models in which the resistance may be decreased in a controlled manner by an increase in arterial CO2 tension, or by a decrease in cerebral perfusion pressure (CPP) in autoregulating animals. Twelve New Zealand white rabbits were anesthetized, paralyzed, and ventilated. Transcranial Doppler basilar artery FV, laser Doppler cortical blood flow, arterial pressure, intracranial pressure, and end-tidal CO2 concentration were measured continuously. Cerebrovascular resistance (CPP divided by laser Doppler cortical flux) and Gosling Pulsatility Index (PI, defined as an FV pulse amplitude divided by a timed average FV) were calculated as time-dependent variables for each animal. Four groups of animals undergoing controlled manipulations of CVR were analyzed. In Group I, arterial CO2 concentration was changed gradually from hypocapnia to hypercapnia. In Group II, gradual hemorrhagic hypotension was used to reduce CPP. In Group III, the short-acting ganglion blocking drug trimetaphan was injected intravenously to induce transient hypotension. Intracranial hypertension was produced by subarachnoid saline infusion in Group IV. During the hypercapnic challenge the correlation between the cortical resistance and Doppler flow pulsatility was positive (r = 0.77, p<0.001). In all three groups in which cerebral perfusion pressure was reduced a negative correlation between pulsatility index and cerebrovascular resistance was found (r = -0.84, p<0.001). The authors conclude that PI cannot be interpreted simply as an index of CVR in all circumstances.

Animals↗

PubMed1995

Uncoupling of LCBF and LCGU in two different models of hydrocephalus: a review.

We have used two different experimental models to examine the relationship between local cerebral blood flow and metabolism in hydrocephalus. In our first experiments local cerebral blood flow (LCBF) and local cerebral glucose utilization (LCGU) were measured by quantitative autoradiographic methods in adult rats rendered hydrocephalic, though asymptomatic, by the injection of kaolin intracisternally at 3 weeks of age and in control animals. There were no significant differences in LCGU or LCBF in any of the 29 areas of grey matter examined, including layer IV of the cerebral cortex. Scanning across the cerebral cortex revealed an appreciable fall in LCGU and LCBF towards the inside and the outside of the mantle in control animals. Hydrocephalus had no significant effect on this "transmantle" pattern of reduction in cortical metabolism towards the periphery, but in contrast, significantly enhanced the reduction in cortical blood flow in 7 out of the 10 cortical regions examined. Hence, in this model of asymptomatic hydrocephalus there is relative uncoupling of LCBF and LCGU in the inner and outer layers of the cerebral mantle. In a study performed in congenitally hydrocephalic H-Tx rats at 10, 20 and 28 days we found that uptake of deoxyglucose was impaired in hydrocephalic rats compared with their non-hydrocephalic siblings. Small changes were seen at 10 and 21 days, but statistically significant changes were seen only at 28 days. A small reduction in LCBF was observed in all regions at 10 days, with statistically significant differences between control and hydrocephalic rats in auditory and parietal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Early metabolic changes during m-Dinitrobenzene neurotoxicity and the possible role of oxidative stress.

m-Dinitrobenzene (m-DNB) is an industrial chemical causing gliovascular lesions in the brain stem similar to those produced by nitroimidazoles and by thiamine deficiency. To identify early preneuropathic indices of toxicity we examined the action of m-DNB on glycolysis and on measures of oxidative stress in the brain both in vivo and in vitro. Significant increases in local cerebral glucose utilization were seen in 14 of 30 brain regions prior to development of lesions. Rat brain astrocyte cultures also showed increases in both glucose consumption and lactic acid formation in the first 24 h following exposure to 0.5 mM m-DNB and prior to the development of cytotoxicity. The concentration of reduced glutathione in these cultures was decreased to about half of control values over a 2-h incubation period, indicating an early disturbance of redox balance. The rate of reduction of nitroblue tetrazolium increased eightfold during a 1-h incubation period, suggesting a free radical-mediated process. Superoxide dismutase partially prevented this increase, although other protective agents failed to do so possibly due to lack of cellular penetration. These observations show that m-DNB neurotoxicity involves early metabolic stimulation and redox disruption that may be causally associated with the production of free radicals.

Animals↗

PubMed1995

Estimation of laser-Doppler flux biological zero using basilar artery flow velocity in the rabbit.

Laser-Doppler flowmetry has potential for continuous cerebral blood flow (CBF) measurement in man and experimental animals. However, laser-Doppler flux (LDFx) measured when perfusion is absent (the biological zero, 0biol) does not necessarily coincide with the instrument's electrical zero. To evaluate laser-Doppler flowmetry further we have compared LDFx in rabbits with continuous measurement of the maximum flow velocity (FVx) in the basilar artery using Doppler ultrasonography. Arterial blood pressure (ABP), FVx, and LDFx were measured continuously in anesthetized New Zealand White rabbits. ABP was altered by controlled hemorrhage with subsequent reinfusion. 0biol was estimated from regression analysis of FVx vs. LDFx and compared with 0biol obtained after death. There was a strong linear relationship between LDFx and FVx (r = 0.94). The absolute difference between estimated 0biol and true 0biol was 5.24% of control prehemorrhage LDFx. Variations in 0biol (range 4-409) suggest that percent changes in LDFx must be related to 0biol if results between individual animals are to be compared.

Animals↗

PubMed1994

Changes in neuronal metabolism in hypothalamic periventricular regions following acute alteration of cerebrospinal fluid composition in conscious rats.

Quantitative 2-deoxyglucose (2-DG) autoradiography was employed to determine the effect of elevating cerebrospinal fluid osmolality on neuronal metabolism of hypothalamic periventricular regions in conscious rats. Injection of hypertonic mannitol solution into the dorsal third ventricle had no significant effect on local cerebral glucose utilization, whereas administration into the ventral third ventricle resulted in a significant elevation in 2-DG uptake in only the posterior pituitary, median preoptic nucleus (MnPo), median eminence and suprachiasmatic nucleus. These results indicate the contribution of a synaptic connection in the MnPo in the activation of the osmoreceptor complex.

Animals↗

PubMed1993

Local cerebral blood flow in rats with congenital hydrocephalus.

Local cerebral blood flow (LCBF) has been measured by [14C]iodoantipyrine quantitative autoradiography in H-Tx rats with inherited congenital hydrocephalus at 10, 21, and 30 days after birth. LCBF at 10 days was uniformly low in all ten brain regions studied and not significantly different between hydrocephalic and age-matched control rats. By 21 days LCBF had increased significantly in control rats. LCBF was significantly lower (< 69%) in the hydrocephalics in all cortical regions and in the inferior colliculus LCBF at both 21 and 30 days. The cerebellar cortex, pons, and caudate were not significantly affected. At 30 days LCBF ranged from 55 to 115 ml 100 g-1 min-1 for hydrocephalics and from 100 to 183 ml 100 g-1 min-1 for controls.

Animals↗

PubMed1992

Directional and compartmentalised drainage of interstitial fluid and cerebrospinal fluid from the rat brain.

Pathways for drainage of interstitial fluid and cerebrospinal fluid from the rat brain were investigated by the injection of 2-5 microliters Indian ink into cerebral white and grey matter and into the subarachnoid space over the vertex of the left frontal lobe. Animals were killed by formalin or glutaraldehyde perfusion 5 min-2 years after injection, and the distribution of ink over the surface of the brain, in 2-mm slices of brain cleared in cedar wood oil, in paraffin sections and by electron microscopy was documented. These investigations showed that carbon particles were distributed diffusely through the interstitial spaces of the white matter whereas they spread selectively along perivascular spaces in the grey matter outlining both arteries and veins and extending to surround capillaries within 1 h. Carbon particles were rapidly ingested by perivascular cells and, to some extent, by meningeal cells surrounding the larger vessels. Very little movement of carbon-labelled perivascular cells and perivascular macrophages was seen after 2 years. Carbon particles entering the subarachnoid space over the vertex of the cerebral hemispheres drained along selected paravascular and subfrontal pathways in the subarachnoid space to the cribriform plate and thence into nasal lymphatics and cervical lymph nodes. These studies demonstrate the diffuse spread of fluidborne tracers through cerebral white matter in the rat, the perivascular spread of tracer in grey matter and the compartmentalised directional flow or tracer through the subarachnoid space to the cribriform plate and nasal lymphatics. Furthermore, particulate matter selectively injected into perivascular spaces in rat grey matter is rapidly and efficiently ingested by perivascular cells.

Animals↗

PubMed1992

Blood flow in portal systems with special reference to the rat pituitary gland.

Regional pituitary blood flow has been studied in adult female Fischer 344 rats by [14C]iodoantipyrine autoradiography. A general mathematical solution has been derived to allow the calculation of blood flow in the second compartment of a portal system and the proportion of blood "shunted" through the first compartment without exposure to tissue uptake from a knowledge of (a) the volume ratios of the two compartments, (b) the tissue tracer uptakes of the two compartments, and (c) the arterial tracer concentration with respect to time of a freely diffusible tracer. Significant diffusion limitation and/or arteriovenous shunting has been demonstrated in the neurohypophysis, suggesting that the majority of incoming blood is "shunted" unchanged to the adenohypophysis. The mean value of the shunt is 89% (range of 84-93%) for the median eminence and lies between 72% (range of 52-82%) and 73% (range of 59-81%) for the posterior pituitary. Neurohypophysial flow rates of 1.20 (range of 0.99-1.55) ml g-1 min-1 for the median eminence and 1.68 (range of 0.83-3.53) ml g-1 min-1 for the posterior pituitary were measured. These values represent "tissue-available" (nonshunted) flow; estimated mean total (shunted plus nonshunted) neurohypophysial flow rates were 11.7 (range of 9.5-17.5) ml g-1 min-1 for the median eminence and 6.1 (range of 3.1-8.9) ml g-1 min-1 (minimum) for the posterior pituitary. Adenohypophysial blood flow is heterogeneous. In the long portal territory, the flow rate was 1.18 (range of 0.95-1.75) ml g-1 min-1 but short portal territory flow calculation is complicated by an unquantifiable nonportal venous drainage; using the natural limits of zero and 100% gives a minimum adenohypophysial flow rate of 1.42 (range of 0.76-2.07) ml g-1 min-1 and a maximum value of 1.97 (range of 1.03-2.82) ml g-1 min-1.

Algorithms↗

PubMed1992

Functional/metabolic modulation of the brain stem lesions caused by 1,3-dinitrobenzene in the rat.

To determine whether neuronal activity plays a role in the localisation of brain stem lesions in 1,3-dinitrobenzene intoxication we produced asymmetrical changes in auditory input by rupturing the left tympanic membrane in Fischer rats. This raised the auditory threshold on that side from 57-63 dB to 104-122 dB. It also decreased glucose utilisation in the ipsilateral cochlear nucleus and significantly increased utilisation in the contralateral nucleus, resulting in a relative deficit of 72 +/- 6%. Similarly, tympanic membrane rupture led to decreased glucose utilisation in the contralateral and increased utilisation in the ipsilateral inferior colliculus. Additional exposure to "white noise" prevented the decrease in glucose utilisation in the contralateral inferior colliculus. Dosing with dinitrobenzene (10 mg/kg in 4 doses over 48 hr) to otherwise normal rats produces symmetrical vasculonecrotic lesions in these regions, but in animals with left tympanic membrane rupture the severity of morphological changes in the ipsilateral cochlear nucleus and the contralateral inferior colliculus were substantially reduced. Additional exposure to "white noise" increased the degree of damage in the ipsilateral cochlear nucleus and contralateral inferior colliculus. These findings indicate that altered auditory function in rats, with its associated metabolic consequences exercises a significant role in the development of brain stem damage in auditory pathways following dinitrobenzene intoxication.

Acoustic Stimulation↗

PubMed1991

Cerebral blood flow in the anaesthetized immature sheep fetus and the response to hypercapnia.

Regional cerebral blood flow (CBF) has been measured in eight anaesthetized, exteriorized, fetal sheep between 58 and 62 days gestation; four were controls, four were hypercapnic (PaCO2 = 78 +/- 5 mmHg, mean +/- S.E.M.). Blood flow values were calculated from quantitative autoradiography following the infusion of [14C]iodoantipyrine into a cannulated fetal placental vein, for the cerebellum, medulla, and five layers of the developing neocortex: cortical plate (CP), subplate zone (SP), intermediate zone (IZ), subventricular zone (SV), and the ventricular zone (VZ). The highest control CBF rates were recorded in the cortical plate (49.3 +/- 7.4 ml min-1 (100 g)-1, mean +/- S.E.M., posterior cortex) and in the ventricular zone (40.5 +/- 4.8, posterior cortex), which at this stage of development are the regions of greatest cell density. The lowest CBF rates were recorded in the subplate zone (23.8 +/- 6.8, anterior cortex) and in the intermediate zone (23.4 +/- 7.6, anterior cortex), which are the regions of lowest cell density. Experimentally induced hypercapnia increased CBF in all brain regions and enhanced the regional pattern of flow. The results provide evidence that CBF in the immature fetal sheep brain (at 58-62 days gestation) is heterogeneous under both control and hypercapnic conditions (especially in the neocortex). Blood vessels of the fetal sheep brain at this early stage of development are clearly responsive to CO2.

Animals↗

PubMed1991

Plasma active and acid-activatable renin during the development of one-kidney, one-clip and two-kidney, one-clip hypertension in the rabbit.

Systolic blood pressure in the central ear artery of eight rabbits increased by 21 mmHg (1 mmHg = 133.32 Pa) over 40 days following renal artery clipping and contralateral nephrectomy (one-kidney, one-clip). Plasma active and acid-activatable (pH 2.8) renin did not change significantly. Similar data were obtained from a group of 12 rabbits following renal artery clipping alone (two-kidney, one-clip) except that blood pressure in this group increased for 26 days but then declined until 40 days. Two animals with one-kidney, one-clip hypertension and three rabbits with two-kidney, one-clip hypertension had large increases in plasma active and inactive renin levels, which followed a more exaggerated rise in blood pressure than in the previous two groups. Forty days after unilateral renal artery clipping, the unclipped kidney was removed in 10 animals with two-kidney, one-clip hypertension. A further increase in blood pressure (+29%) occurred in seven of the animals but no change in plasma active or inactive renin. Results were compared with two groups of control animals, a unilateral nephrectomy group and a laparotomy group. None of the surgical procedures used produced a consistent pattern of change in the relative amounts of active and inactive renin in plasma. No marked changes in sodium, potassium, or water balance occurred in any group of animals.

Animals↗

PubMed1989

The uptake of [14C]deoxyglucose into brain of young rats with inherited hydrocephalus.

The effect of hydrocephalus on cerebral glucose utilization as reflected by deoxyglucose uptake has been examined in rats with inherited hydrocephalus at 10, 20, and 28 days after birth using a semiquantitative method. Injection of [14C]deoxyglucose intraperitoneally was followed by freezing the brain, sectioning, and quantitative autoradiography of 10 brain regions. Brain [14C] concentration, cortical thickness, and plasma glucose concentrations were measured. Maximal thinning of the cerebral cortex had already occurred by 10 days after birth, although obvious symptoms such as gait disturbance developed after 20 days. In control rats, the cerebral isotope concentration was lower and more homogeneous at 10 days than at 20 or 28 days, which may be a reflection of the use of metabolic substrates other than glucose in younger animals. In order to make comparisons between control and hydrocephalic groups, tissue isotope concentrations were normalized to cerebellar cortex which was not affected by the hydrocephalus at any age. In hydrocephalic rats at 10 and 20 days, the concentration of [14C] was lower in all areas except the inferior colliculi and pons but the reduction was only significant in the sensory-motor cortex at 10 days and in the caudate nuclei at 20 days. By 28 days after birth, all areas except the cerebellum (six cortical regions, inferior colliculi, pons, and caudate) had significantly lower isotope concentrations in the hydrocephalic group. It is concluded that cerebral glucose metabolism is significantly reduced by 28 days after birth in H-Tx rats with congenital hydrocephalus and that less marked reductions occur prior to 28 days.

Aging↗

PubMed1987

A modification of the method for the measurement of cerebral blood flow using [14C]iodoantipyrine in small animals.

We have modified the method for the measurement of cerebral blood flow using [14C]iodoantipyrine in small animals to overcome problems caused by rapid sequential arterial sampling. A second isotope, gamma-emitting [99mTc]pertechnetate, is infused together with [14C]iodoantipyrine. The 99mTc is counted externally by a detector placed over the chest. The arterial curve for [14C]iodoantipyrine can be reconstructed from a single arterial sample collected from the severed neck and the shape of the 99mTc chest curve. Because of the short half-life of 99mTc (6 h), all 14C measurements, including preparation of autoradiograms, may be made 3 days later.

Animals↗

PubMed1987

Temporary blood flow stasis with degradable starch microspheres (DSM) for liver metastases in a rat model.

A reliable liver metastasis model using intraportal injections of sarcoma cells was established in syngeneic hooded Lister rats to study the blood supply of the tumours and to evaluate the role of degradable starch microspheres (DSM) in conjunction with selective hepatic arterial and portal venous chemotherapy. The tumour/normal liver (T/L) ratio after intra-arterial and intraportal injection of 113Sn microspheres was 1.04 (range: 0.38-1.15) and 0.03 (range: 0.006-0.22), respectively. After intravenous 14C-iodoantipyrine quantitative autoradiography of tumour and normal regions demonstrated a mean T/L ratio of 0.74 +/- 0.05. After hepatic artery ligation (HAL) and portal vein ligation (PVL) the values were 0.32 +/- 0.05 and 0.42 +/- 0.05, respectively. These results confirm that the vascularity of the tumours in this model is similar to human colorectal cancer metastases. Radiolabelled 14C 5-Fluorouracil (5-FU) was given intravenously, via the hepatic artery and via the portal vein (the latter two routes with and without DSM). Quantitative autoradiography of tumour regions showed that selective hepatic arterial administration with DSM resulted in a significantly increased concentration of 14C-5-FU within the tumours. These results suggest that DSM may enhance the therapeutic benefit of hepatic arterial 5-FU by increasing its uptake into tumours.

Animals↗

PubMed1985

Low-cost microcomputer-based densitometer.

We have constructed low-cost equipment for the measurement of the optical density of x-ray films. This equipment is based on a video system connected via an analog to digital converter to an Apple microcomputer. The optical density of an x-ray film can thus be accessed by commands in BASIC or machine-code programs. By using a calibrated step-wedge film, we have found a stable linear response for optical densities of up to 0.8.

Computers↗

PubMed1985

Local cerebral glucose utilisation in experimental chronic hydrocephalus in the rat.

Many of the clinical features of normal pressure hydrocephalus have been ascribed to stretching of the periventricular nerve fibres. However, many patients may be asymptomatic despite considerable ventriculomegaly. To define the periventricular changes in cerebral energy metabolism we have measured local cerebral glucose utilisation in hydrocephalic and control Sprague-Dawley rats. Experimental hydrocephalus was induced by injection of kaolin into the cisterna magna of 3-week-old rats. After 3 months, the animals fed and walked normally. LCGU was measured using the 14C-deoxyglucose method of Sokoloff. Autoradiographs were prepared and isotope concentration determined by densitometry. Glucose utilisation was measured in 39 anatomically defined areas, widely distributed throughout the brain, and comparisons between hydrocephalic animals and controls (given intracisternal saline) were made by analysis of variance. It was found that there was no significant difference in glucose utilisation between the two groups of rats in any of the areas measured. Hence ventriculomegaly in the young rat does not impair LCGU in surviving periventricular tissue.

Animals↗

PubMed1982

Active and inactive renin release from rabbit kidney cortex slices: effect of sodium concentration and of furosemide.

1. Active and inactive renin release by rabbit kidney cortex slices was investigated. Inactive renin was estimated as the increase in renin activity after acidification (pH 2 . 8) of slice supernatant solutions. 2. Active renin release was increased when incubation medium [Na+] was reduced. This relationship was linear (r2 = 0 . 96) over the range [Na+] = 23-133 mM. 3. For the same range of [Na+] inactive renin secretion decreased when medium [Na+] was reduced (r2 = 0 . 92). Therefore, the proportion of total renin which was in the inactive form decreased linearly as [Na+] was reduced (r2 = 0 . 97). 4. Chloride ions did not appear to be important in altering the secretion of either active or inactive renin. 5. Adding furosemide to the incubation medium in concentrations up to 40 micrograms/ml. did not change secretion of either form of renin. The action of furosemide on secretion of active and inactive renin in vivo is therefore secondary to altered renal function. 6. Regulation of the relative amount of active and inactive renin in plasma could be entirely an intrarenal event. It is not essential to invoke a plasma activating enzyme for inactive renin in order to explain changes in plasma levels of the two forms of renin. 7. This paper supports the hypothesis that release of inactive renin by the kidney is controlled by a sodium-sensitive mechanism.

Animals↗

PubMed1982

Secretion control for active and inactive renin: interactions with calcium and sodium ions.

Excitation-secretion coupling for active and inactive renin was investigated by manipulating the ionic composition of media in which rabbit kidney cortex slices were incubated. Alterations in [Ca++] and [K+] produced qualitatively similar, but quantitatively distinct, changes in the release of the two forms of renin. Under control conditions, the slices secreted 15-20% of total renin in the inactive form and exposure to Ca++-free or low [K+] media increased this to 30-40%, a similar proportion to that stored within the kidney. Decreasing [Na+] stimulated release of active renin but reduced inactive renin. There appeared to be antagonism between the effects of Na+ and Ca++ ions in respect of active renin but synergism for release of the inactive form. It is suggested that the characteristics of a single renin release mechanism in the rat kidney resemble those for secretion of inactive, rather than active, renin in other species.

Animals↗