A theoretical model of blood-brain exchange is developed and a procedure is derived that can be used for graphing multiple-time tissue uptake data and determining whether a unidirectional transfer process was dominant during part or all of the experimental period. If the graph indicates unidirectionality of uptake, then an influx constant (Ki) can be calculated. The model is general, assumes linear transfer kinetics, and consists of a blood-plasma compartment, a reversible tissue region with an arbitrary number of compartments, and one or more irreversible tissue regions. The solution of the equations for this model shows that a graph of the ratio of the total tissue solute concentration at the times of sampling to the plasma concentration at the respective times (Cp) versus the ratio of the arterial plasma concentration-time integral to Cp should be drawn. If the data are consistent with this model, then this graph will yield a curve that eventually becomes linear, with a slope of Ki and an ordinate intercept less than or equal to the vascular plus steady-state space of the reversible tissue region.

/pdf/graphical-evaluation-of-blood-to-brain-transfer-constants-2e29v1hms2.pdf

Graphical Evaluation of Blood-to-Brain Transfer Constants from Multiple-Time Uptake Data:

We describe a standard set of quantity names and symbols related to the estimation of kinetic parameters from dynamic contrast-enhanced T(1)-weighted magnetic resonance imaging data, using diffusable agents such as gadopentetate dimeglumine (Gd-DTPA). These include a) the volume transfer constant K(trans) (min(-1)); b) the volume of extravascular extracellular space (EES) per unit volume of tissue v(e) (0 < v(e) < 1); and c) the flux rate constant between EES and plasma k(ep) (min(-1)). The rate constant is the ratio of the transfer constant to the EES (k(ep) = K(trans)/v(e)). Under flow-limited conditions K(trans) equals the blood plasma flow per unit volume of tissue; under permeability-limited conditions K(trans) equals the permeability surface area product per unit volume of tissue. We relate these quantities to previously published work from our groups; our future publications will refer to these standardized terms, and we propose that these be adopted as international standards.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291522-2586%28199909%2910%3A3%3C223%3A%3AAID-JMRI2%3E3.0.CO%3B2-S

Estimating kinetic parameters from dynamic contrast-enhanced T(1)-weighted MRI of a diffusable tracer: standardized quantities and symbols.

The biochemistry and medical significance of the flavonoids.

Non-insulin-dependent diabetes mellitus (NIDDM) results from an imbalance between insulin sensitivity and insulin secretion. Both longitudinal and cross-sectional studies have demonstrated that the earliest detectable abnormality in NIDDM is an impairment in the body's ability to respond to insulin. Because the pancreas is able to appropriately augment its secretion of insulin to offset the insulin resistance, glucose tolerance remains normal. With time, however, the beta-cell fails to maintain its high rate of insulin secretion and the relative insulinopenia (i.e., relative to the degree of insulin resistance) leads to the development of impaired glucose tolerance and eventually overt diabetes mellitus. The cause of pancreatic "exhaustion" remains unknown but may be related to the effect of glucose toxicity in a genetically predisposed beta-cell. Information concerning the loss of first-phase insulin secretion, altered pulsatility of insulin release, and enhanced proinsulin-insulin secretory ratio is discussed as it pertains to altered beta-cell function in NIDDM. Insulin resistance in NIDDM involves both hepatic and peripheral, muscle, tissues. In the postabsorptive state hepatic glucose output is normal or increased, despite the presence of fasting hyperinsulinemia, whereas the efficiency of tissue glucose uptake is reduced. In response to both endogenously secreted or exogenously administered insulin, hepatic glucose production fails to suppress normally and muscle glucose uptake is diminished. The accelerated rate of hepatic glucose output is due entirely to augmented gluconeogenesis. In muscle many cellular defects in insulin action have been described including impaired insulin-receptor tyrosine kinase activity, diminished glucose transport, and reduced glycogen synthase and pyruvate dehydrogenase. The abnormalities account for disturbances in the two major intracellular pathways of glucose disposal, glycogen synthesis, and glucose oxidation. In the earliest stages of NIDDM, the major defect involves the inability of insulin to promote glucose uptake and storage as glycogen. Other potential mechanisms that have been put forward to explain the insulin resistance, include increased lipid oxidation, altered skeletal muscle capillary density/fiber type/blood flow, impaired insulin transport across the vascular endothelium, increased amylin, calcitonin gene-related peptide levels, and glucose toxicity.

Pathogenesis of NIDDM: A balanced overview

The nuclease hypersensitivity element III1 upstream of the P1 promoter of c-MYC controls 85–90% of the transcriptional activation of this gene. We have demonstrated that the purine-rich strand of the DNA in this region can form two different intramolecular G-quadruplex structures, only one of which seems to be biologically relevant. This biologically relevant structure is the kinetically favored chair-form G-quadruplex, which is destabilized when mutated with a single G → A transition, resulting in a 3-fold increase in basal transcriptional activity of the c-MYC promoter. The cationic porphyrin TMPyP4, which has been shown to stabilize this G-quadruplex structure, is able to suppress further c-MYC transcriptional activation. These results provide compelling evidence that a specific G-quadruplex structure formed in the c-MYC promoter region functions as a transcriptional repressor element. Furthermore, we establish the principle that c-MYC transcription can be controlled by ligand-mediated G-quadruplex stabilization.

https://www.pnas.org/content/pnas/99/18/11593.full.pdf

Direct evidence for a G-quadruplex in a promoter region and its targeting with a small molecule to repress c-MYC transcription.

Crone, C. The permeability of capillaries in various organs as deter-mined by use of the ‘Indicator Diffusion’ method. Acta physiol. scand. 1963. 58. 292—305. — The theory of a single injection technique, the ‘Indicator Diffusion’ method, for quantitative studies of capillary permeability is developed. It is shown that the permeability of a capillary area can be expressed by three parameters: the initial extraction (E) of test substances added in a single injection to the blood flowing to an organ, the blood flow (Q) and the surface area (A) of the capillaries. The equation relating these figures is: P = (=/A) × loge1/(1—E). The permeability coefficients of capillaries in kidney, liver, lung, brain and hind limb to inulin and sucrose are reported. It is found that the permeability of capillaries varies considerably from organ to organ. It is questioned whether the pore model adequately describes the functional characteristics of the capillaries in the muscles. The existence of pores should result in a pronounced deviation of the ratio between the permeability coefficients for sucrose and inulin from the ratio between the free diffusion coefficients. This was not found to be the case.

The Permeability of Capillaries in Various Organs as Determined by Use of the ‘Indicator Diffusion’ Method

Diabetic patients with increased plasma glucose concentrations may develop cerebral symptoms of hypoglycemia when their plasma glucose is rapidly lowered to normal concentrations. The symptoms may indicate insufficient transport of glucose from blood to brain. In rats with chronic hyperglycemia the maximum glucose transport capacity of the blood-brain barrier decreased from 400 to 290 micromoles per 100 grams per minute. When plasma glucose was lowered to normal values, the glucose transport rate into brain was 20 percent below normal. This suggests that repressive changes of the glucose transport mechanism occur in brain endothelial cells in response to increased plasma glucose.

https://science.sciencemag.org/content/sci/214/4519/456.full.pdf

Blood-brain glucose transfer: repression in chronic hyperglycemia

Crone, C. The permeability of brain capillaries to non-electrolytes. Acta physiol. scand. 1965. 64. 407–417. – By means of the“Indicator diffusion” method blood-brain barrier permeability characteristics were studied on dogs. The permeability coefficient of fructose, glycerol, propylene glycol, urea, thiourea, antipyrine, ethanol, propanol and butanol was calculated. If these substances are ordered according to their rate of passage from blood into brain tissue a hierarchy is found which corresponds to that which is typical of the permeability of cell membranes to non-electrolytes. The experiments lend support to the view advanced by the late August Krogh that the permeability characteristics of the blood-brain barrier are analogous to those of cell membranes in general. This conclusion is discussed against the background of recent findings of morphologically-demonstrated tightness of cerebral capillaries which imply that material which diffuses from blood into brain tissue must pass the plasma membranes of the endothelial cells.

The permeability of brain capillaries to non-electrolytes.

Fed and starved rats were studied on successive days during a 5-day starvation period. The ability of ketone bodies to pass the blood-brain barrier was estimated by single common carotid injections of labeled ketone bodies and water, and results were expressed as the ratio between the normalized activities of tracers in tissue and blood, the brain uptake index (BUI). BUI of D-3-hydroxybutyrate and acetoacetate decreased as their total concentrations increased in the injectate bolus: BUI of D-3-hydroxybutyrate decreased significantly from 8% at 0.2 mM to 3--4% at 20.2 mM in fed rats and from 11.5% at 0.2 mM to 6% at 20.2 mM in starved rats, indicating saturation of the uptake mechanism. The BUI of both ketone bodies increased significantly with increasing duration of starvation, indicating adaptation to ketonemia. Enzymatic kinetics explained the uptake behavior of D-3-hydroxybutyrate in both fed and starved rats and involved a rise of Km and Vmax during starvation consistent with a doubling of the transport rate at the degree of ketonemia found in starved rats. The uptake of glucose was not influenced by starvation or ketonemia.

Induction processes in blood-brain transfer of ketone bodies during starvation

Classical techniques for studying modulations of microvascular permeability have a time resolution of minutes. A newly developed method allows continuous measurement of the electrical resistance of the microvascular membrane in vivo (Olesen & Crone 1983). The technique exploits microelectrodes impaled into the vascular lumen and is based on cable analysis of the vessel. It was applied to venules on the surface of the frog brain to test the effect on microvascular permeability of a wide variety of substances. The following agents increased ionic permeability reversibly within seconds: 5-hydroxytryptamine, bradykinin, ATP, ADP, AMP, phospholipase A2, arachidonic acid, leukotriene C4, oxygen-derived free radicals, ionophore A23187, and unbound Evans blue dye. An irreversible permeability increase was induced by protamine sulphate, neuraminidase, trypsin, melittin, and snake venoms from Crotalus durissus terrificus and Bothrops atrox. The following substances were without effect within an administration period of 5 min: histamine, epinephrine, putrescine, angiotensin II, vasoactive intestinal polypeptide (VIP), substance P, neurotensin, vasopressin, adenosine, PGE2, PGF2 alpha, prostacyclin (PGI2), leukotriene B4, albumin, heparin, plant cytokinins, hyaluronidase, thrombin, wasp venom. Variations in pH between 5.1 and 8.6 did not change permeability. Three conclusions are drawn from the observations: (1) the permeability of cerebral microvessels can be modulated by specific agents, (2) the agents induced changes in the endothelium within a few seconds, and (3) the rapid permeability increase induced by inflammatory mediators was less than two-fold and reversible within minutes.

Christian Crone

Papers

The Permeability of Capillaries in Various Organs as Determined by Use of the ‘Indicator Diffusion’ Method

Blood-brain glucose transfer: repression in chronic hyperglycemia

The permeability of brain capillaries to non-electrolytes.

Induction processes in blood-brain transfer of ketone bodies during starvation

Substances that rapidly augment ionic conductance of endothelium in cerebral venules