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Biomedical subjects

R Spector

Publications and source records attributed to R Spector.

At least 181 records · Page 10Linked to original sources
PubMed1977

Model for transport in the central nervous system.

A pharmacokinetic model relating the flux of substances between blood, the compartment consisting of the cerebrospinal fluid (CSF)-extracellular space (ECS) of brain, and brain was developed. Transport equations for diffusion, active transport, and bulk flow of CSF between these three compartments were postulated, and kinetic constants were experimentally obtained. The appropriate differential equations were solved on a digital computer to predict concentrations in CSF and brain as a function of the plasma concentration. The ability of the model to predict correctly CSF and brain concentrations of ascorbic acid and mannitol with only knowledge of the plasma concentrations in both steady-state and nonsteady-state conditions was experimentally tested in rabbits. Ascorbic acid was chosen as a model substance that is actively transported into the CSF-ECS of brain and also into brain cells whereas mannitol enters CSF and brain by diffusion. The model made accurate predictions when the assumptions were not violated. Second, the model showed that the experimentally determined Michaelis-Menten transport constant (KT = 0.8 mg=dl-1) for ascorbate transport from blood into the CSF-ECS of brain optimizes ascorbate homeostasis in brain.

Animals↗
PubMed1976

Thiamine transport in the central nervous system.

Total thiamine (free thiamine and thiamine phosphates) transport into the cerebrospinal fluid (CSF), brain, and choroid plexus and out of the CSF was measured in rabbits. In vivo, total thiamine transport into CSF, choroid plexus, and brain was saturable. At the normal plasma total thiamine concentration, less than 5% of total thiamine entry into CSF, choroid plexus, and brain was by simple diffusion. The relative turnovers of total thiamine in choroid plexus, whole brain, and CSF were 5, 2, and 14% per h, respectively, when measured by the penetration of 35S-labeled thiamine injected into blood. From the CSF, clearance of [35S]thiamine relative to mannitol was not saturable after the intraventricular injection of various concentrations of thiamine. However, a portion of the [35S]thiamine cleared from the CSF entered brain by a saturable mechanism. In vitro, choroid plexuses, isolated from rabbits and incubated in artificial CSF, accumulated [35S]thiamine against a concentration gradient by an active saturable process that did not depend on pyrophosphorylation of the [35S]thiamine. The [35S]thiamine accumulated within the choroid plexus in vitro was readily released. These results were interpreted as showing that the entry of total thiamine into the brain and CSF from blood is regulated by a saturable transport system, and that the locus of this system may be, in part, in the choroid plexus.

Animals↗
PubMed1976

Inhibition of methotrexate transport from cerebrospinal fluid by probenecid.

In rabbit, the efflux of intraventricularly injected methotrexate from the cerebrospinal fluid was retarded by pretreatment with 200 mg/kg of ip probenecid. In vitro, the ability of the isolated choroid plexus to concentrate methotrexate was depressed by the inclusion of probenecid in the incubation medium. These experimental results are consistent with the hypothesis that probenecid depresses the clearance of methotrexate from the cerebrospinal fluid by blocking the transport of methotrexate from cerebrospinal fluid to blood via the choroid plexus.

Animals↗
PubMed1976

The effect of uremia on penicillin flux between blood and cerebrospinal fluid.

The effect of uremia on the choroid plexus system responsible for transport of penicillin from cerebrospinal fluid (CSF) to blood was studied in vitro and in vivo. Uremia was induced in rabbits by injecting toxic doses of cephaloridine or by obstructing urine flow. Three days after the induction of uremia, isolated choroid plexuses from normal rabbits were unable to concentrate penicillin 14C normally when incubated in either CSF or ultrafiltrates of plasma from uremic rabbits. In vivo, a decrease in the disappearance of penicillin 14C from CSF was observed in uremic rabbits. However, the choroid plexus transport system for penicillin was only partially and reversibly depressed in uremia. The increased CSF levels of penicillin uremia are due to: decreased excretion of penicillin by the kidney, depression of active efflux of penicillin from the CSF, and decreased plasma binding of penicillin.

Animals↗
PubMed1975

Medical care by nurses in an internal medicine clinic. Analysis of quality and its cost.

The quality and cost of medical care provided by specially trained nurses in an internal medicine clinic was evaluated prospectively during a three-month period. Clinic physicians referred patients to these nurses when they believed that the nurse could contribute to the patient's overall health care. Nurse care was judged to be adequate in dealing with 98% of old problems (defined by the physician) and 85% of new problems (detected by nurse). Scheduled visits to the physician and unscheduled visits decreased significantly (P less than .05) during the period of nurse care vs a control period, but the overall cost of health care per patient was increased significantly during the nurse care period (P less than .01) because of a disproportionate increase in visits to the nurse. We conclude that nurse care was feasible and of adequate quality. However, it was not cost-effective.

Boston↗
PubMed1975

Folate transport by the choroid plexus in vitro.

Reduced folates are transported from blood into cerebrospinal fluid against a concentration gradient. In vitro, folates were transported into and released by isolated rabbit choroid plexuses. The choroid plexus uptake mechanism was specific for folates, energy dependent, and depressed by cold temperatures. In vivo, the choroid plexus may transport folates from blood to cerebrospinal fluid.

Animals↗
PubMed1975

Myo-inositol transport in the central nervous system.

Free myo-inositol (inositol) transport into the cerebrospinal fluid (CSF), brain, and choroid plexus and out of the cerebrospinal fluid was measured in rabbits. In vivo, inositol transport from blood into choroid plexus, CSF, and brain was saturable with an apparent affinity constant (K-t) of approximately 0.1 mM. The relative turnover of free inositol in choroid plexus (16 percent/h) was higher than in CSF 4percent/h) and brain (0.3percent/h) when meausred by tissue penetration of tracer [3-H]-labeled inositol injected into blood. However, the passage of tracer inositol was not greater than the passage of mannitol into brain when measured 15 s after a rapid injection of inositol and mannitol into the left common carotid artery. From the CSF, the clearance of inositol relative to inulin was saturable after the intraventricular injection of various concentrations of inositol and inulin. Moreover, a portion of the inositol cleared from the CSF entered brain by a saturable mechanism. In vitro, choroid plexuses, isolated from rabbits and incubated in artificial CSF, accumulated [3-H-labeled myo-inositol against a concentration gradient by a specific, active, saturable process with a K-t of 0.2 mM inositol. These results were interpreted as showing that the entry of inositol into the central nervous system from blood is regulated by a saturable transport system, and that the locus of this system may be, in part, in the choroid plexus.

Animals↗
PubMed1975

Folate transport in the central nervous system.

Methyltetrahydrofolic acid or folic acid was infused intravenously at a constant rate into conscious untreated or methotrexate-pretreated rabbits. After 150 min, at equivalent plasma concentrations, folic acid or methyltetrahydrofolic acid readily entered the cerebrospinal fluid and probably brain by a saturable transport system. In contrast, after intraventricular injections, folic acid but not methyltetrahydrofolic acid was cleared from cerebrospinal fluid to blood by a saturable system. Intraventribular injection of folic acid at concentrations that saturated folic acid clearance from cerebrospinal fluid did not affect the transport of methyltetrahydrofolic acid from blood into cerebrospinal fluid. These results suggest that the transport system for mehtyltetrahydrofolic acid, which is about half-saturated at normal plasma concentrations, helps maintain the cerebrospinal fluid and probably brain methyltetrahydrofolic acid concentrations within relatively narrow limits. Moreover, folic acid, which the brain cannot utilize, is transported from cerebrospinal fluid. A possible locus for the systems that transport folic acid from and methyltetrahydrofolic acid into the cerebrospinal fluid is the choroid plexus.

Animals↗
PubMed1975

The transport of gentamicin in the choroid plexus and cerebrospinal fluid.

Gentamicin often attains inadequate therapeutic levels in cerebrospinal fluid (CSF). In the present study, the possibility that gentamicin might be transported from CSF to blood by a mechanism in the choroid plexus was investigated. Rabbit choroid plexuses were incubated 15 to 120 minutes in artificial CSF containing 14C-gentamicin and 3H-inulin. The choroid plexuses accumulated gentamicin linearly up to 120 minutes where a tissue/medium ratio of 5.4 was attained. Uptake of gentamicin was inhibited by: 1) increasing the gentamicin concentration or adding kanamycin; 2) omitting oxygen and glucose; 3) adding dinitrophenol to the medium; 4) incubating at 4 degrees C; OR 5) incubating in rabbit CSF. However, uptake was not inhibited by probenecid or ammonium ion. Kinetic analysis revealed that gentamicin was transported into choroid plexus by a process that satisfied Michaelis-Menten kinetics. A Lineweaver-Burk transformation of the transport values for the saturable component yielded a Kt of 51.8 mug/ml and a Vmax = 1.8 mug/ml/min. In vivo, gentamicin clearance from CSF, although saturable, was only 1.4 times greater than inulin because of the relatively low Kt and Vmax of the choroid plexus transport system for gentamicin and the presence of inhibitors of gentamicin transport in rabbit CSF. We conclude that the saturable transport of gentamicin from CSF, although quantitatively not large, is a factor in determining gentamicin levels in CSF.

Ammonium Sulfate↗
PubMed1975

Should tonometry screening be done by technicians instead of physicians?

A random sample of 768 clinic patients 40 years or older were screened by a technician utilizing Schlotz tonometry. All patients with an intraocular pressure of 20 mm Hg or more were referred for ophthalmological evaluation. The prevalence of frank glaucoma and suspected glaucoma in this infirm population was 1.8% and 2.5%, respectively. The cost of detecting frank or suspected glaucoma decreased in the second half of the study due to a decrease in the number of false-positive tests. These findings suggest that all clinic patients 40 years or older should be screened for glaucoma. Because many internists are reluctant to screen for glaucoma and because there are large number of false-positive tests by those who do tonometry episodically, we suggest that technicians do glaucoma screening.

Adult↗