Osteonectin is a minor component of mineralized connective tissues in rat

Osteonectin is a major glycoprotein of porcine and bovine bones and teeth that is found associated with hydroxylapatite crystal surfaces. From the ability of osteonectin to bind calcium ions, it has been proposed as a possible nucleator of hydroxylapatite crystal formation. Analysis of hydroxylapatite-bound proteins of rat bone and dentine, however, has revealed that osteonectin represents only 2.5 +/- 1.5% of the hydroxylapatite-bound protein in long bones, 0.9 +/- 0.5% in calvariae, and less than 0.1% in incisor dentine of animals of different ages. Further, in vivo pulse-chase studies carried out in young adult rats have shown osteonectin to be synthesized at low levels in these tissues. Similarly, low levels of osteonectin were synthesized by rat calvarial cells in vitro. In contrast, fibroblastic cells from periodontal ligament and gingiva synthesized significantly greater amounts of osteonectin. These studies indicate that the low quantities of osteonectin in rat mineralized tissues are a consequence of low rates of formation rather than being due to rapid turnover. The virtual absence of osteonectin in incisor dentine correlates with the lack of peritubular dentine in rat, whereas the low osteonectin content of rat bones may reflect differences in their structure and biophysical properties compared with bones of larger mammals.     

Germanium-68 as a possible marker for silicon transport in rat brain

Silicon (Si) is an essential trace element normally present in brain and cerebrospinal fluid, although the mechanism by which it enters and distributes in brain is largely unknown. Due to the short radioactive half-life of³¹Si (156 min) we have investigated the use of⁶⁸Germanium (68-Ge, half-life 282 days) as a possible marker for Si transport in rat brain over longer periods than are possible with³¹Si. Adult male anaesthetised rats were given a bolus of⁶⁸Ge I. V. and arterial blood samples taken during experiments that lasted between 5 min and 3 days. At termination, the brain was removed and analysed for radioactivity as were the plasma samples. Data were analyzed by Graphical Analysis which showed that the blood-brain barrier permeability to⁶⁸Ge (K_in10^–4 ml/min/g) is similar to that for many non protein-bound electrolytes in plasma and that⁶⁸Ge fluxes across cerebral capillaries are bidirectional. The autoradiographic distribution of⁶⁸Ge in brain was homogenous. Our results are in agreement with those of previous studies using³¹Si or⁶⁸Ge, which suggest⁶⁸Ge may be a useful marker for Si when investigating the role of this element in conditions such as neurodegenerative diseases.     

Insulin and glucagon stimulation of amino acid transport in isolated rat hepatocytes. Synthesis of a high affinity component of transport.

Insulin and glucagon stimulate amino acid transport in freshly prepared suspensions of isolated rat hepatocytes. The kinetic properties of alpha-amino[1-14C]isobutyric acid (AIB) transport were investigated in isolated hepatocytes following stimulation by either hormone in vitro. In nonhormonally treated cells (i.e. basal state), saturable transport occurred mainly through a low affinity (Km approximately equal to 40 mM) component. In insulin or glucagon-treated hepatocytes, saturable transport occurred through both a low affinity component (similar to that observed in the basal state) and a high affinity (Km approximately equal to 1 mM) component. At low AIB concentrations (less than 0.5 mM), insulin and glucagon at maximally stimulating doses increased AIB uptake about 2-fold and 5-fold, respectively. The high affinity component induced by either hormone exhibited the properties of the A (alanine preferring) mediation of amino acid transport. This component required 2 to 3 h for maximal expression, and its emergence was completely prevented by cycloheximide. Half-maximal stimulation was elicited by insulin at about 3 nM and by glucagon at about 1 nM. Dibutyryl cyclic AMP mimicked the glucagon effect and was not additive to it at maximal stimulation. Maximal effects of insulin and glucagon, or insulin and dibutyryl cyclic AMP, were additive. We conclude that insulin and glucagon can modulate amino acid entry in hepatocytes through the synthesis of a high affinity transport component.     

Localization of neurophysin in the rat supraoptic nucleus

Summary Localization of neurophysin in neurons of the supraoptic nucleus was accomplished using an unlabeled-antibody, post-embedding, immunoperoxidase technique. Neurophysin was exclusively associated with neurosecretory granules within cell bodies of supraoptic neurons and their processes.Supported by U.S.P.H.S. Grant HD-08867     

Immunochemical evidence for the transport of neurophysin in the hypothalamo-neurohypophysial system of the dog

Summary 1. An immunohistochemical study has been made of the hypothalamo-neurohypophyseal system of the dog, 20h after crushing the pituitary stalk.2. By use of a cross-species-reactive neurophysin antiserum it was shown that neurophysin is a component of the axons which originate in the supraoptic and paraventricular nuclei and terminate around blood vessels in the posterior pituitary.3. Neurophysin specific fluorescence accumulated in axons proximal to the constriction but was absent from the axons immediately distal to the site of injury.4. In dogs left for six days it was shown by radioimmunoassay that the amount of neurophysin in the hypothalamus and stalk proximal to the constriction increased twofold while that remaining in the posterior pituitary and stalk distal to the constriction decreased five-fold over the same period.5. The results are interpreted as evidence for a rapid axonal transport of neurophysin from its site of synthesis in the cell bodies of the hypothalamus to the posterior pituitary. Acknowledgements: This work was supported by a research grant to D. B. Hope from the Medical Research Council. L. O. Uttenthal was supported by a Medical Research Training Award and B. G. Livett by a Nuffield Dominions Trust Demonstratorship (Australia). We thank Mrs. Marion Martin for radioimmunoassay of neurophysin, Miss Wendy Jones for technical assistance and the U.C.L.A. Brain Information Service for help with the bibliography.     

A possible molecular basis for photoprotection in the minor antenna proteins of plants

The bioenergetics of light-harvesting by photosynthetic antenna proteins in higher plants is well understood. However, investigation into the regulatory non-photochemical quenching (NPQ) mechanism, which dissipates excess energy in high light, has led to several conflicting models. It is generally accepted that the major photosystem II antenna protein, LHCII, is the site of NPQ, although the minor antenna complexes (CP24/26/29) are also proposed as alternative/additional NPQ sites. LHCII crystals were shown to exhibit the short excitation lifetime and several spectral signatures of the quenched state. Subsequent structure-based models showed that this quenching could be explained by slow energy trapping by the carotenoids, in line with one of the proposed models. Using Fluorescence Lifetime Imaging Microscopy (FLIM) we show that the crystal structure of CP29 corresponds to a strongly quenched conformation. Using a structure-based theoretical model we show that this quenching may be explained by the same slow, carotenoid-mediated quenching mechanism present in LHCII crystals.     

Carrier-mediated uptake of lactate in rat hepatocytes. Effects of pH and possible mechanisms for L-lactate transport

The rate of uptake and the distribution ratio between intra- and extracellular compartments of L- and D-lactate were studied in hepatocyte preparations from fed rats. L- and D-lactate uptake apparently depended on both passive diffusion and carrier-mediated components. The apparent Km of the high-affinity carrier for L-lactate was in the range of 1.8 mM. The reciprocal competitive inhibitions between isomers of lactate suggest that L- and D-lactate might be transported by distinct carriers. Lactate transport was inhibited by various anions; pyruvate was the most potent anion, whereas only high concentrations of ketone bodies were effective. Acidic extracellular pH enhanced lactate uptake, this effect being more pronounced for L-lactate. At low pH, L-lactate was concentrated into hepatocytes, but its affinity for the carrier appeared unchanged, suggesting the existence of a process gaining energy from the pH gradient across the cell membrane. In the hypothesis of a lactate/H+ symport, the affinity for H+ was not dependent on lactate concentration and the apparent Km for H+ corresponded to a pH of 7.34. No trans-stimulation of lactate uptake after prior loading of the cells with pyruvate or lactate was observed. The present data suggest that, at physiological concentrations, lactate uptake by the liver might be largely carrier-mediated and the rate of transport across the liver cell membrane may be of a magnitude relatively comparable to the rate of metabolism.     

Permeability of rat neurohypophysial blood vessels for neurophysins and ferritin in acute hyperoxia

The time course of the release of vasopressin-binding (nicotine-stimulated) and oxytocin-binding (estrogen-stimulated) neurophysins (NPs) into the rat pituitary and blood serum has been studied during the convulsive phase of hyperbaric oxygenation (HBO) and the postconvulsive period (PCP). The ultrastructure of the posterior pituitary (neurohypophysis) and the state of the blood-pituitary barrier in the caudal region of the gland have been studied with the use of ferritin as an exogenous marker of vascular permeability.     

A new proposal concerning the botanical origin of Baltic amber

Baltic amber constitutes the largest known deposit of fossil plant resin and the richest repository of fossil insects of any age. Despite a remarkable legacy of archaeological, geochemical and palaeobiological investigation, the botanical origin of this exceptional resource remains controversial. Here, we use taxonomically explicit applications of solid-state Fourier-transform infrared (FTIR) microspectroscopy, coupled with multivariate clustering and palaeobotanical observations, to propose that conifers of the family Sciadopityaceae, closely allied to the sole extant representative, Sciadopitys verticillata, were involved in the genesis of Baltic amber. The fidelity of FTIR-based chemotaxonomic inferences is upheld by modern–fossil comparisons of resins from additional conifer families and genera (Cupressaceae: Metasequoia; Pinaceae: Pinus and Pseudolarix). Our conclusions challenge hypotheses advocating members of either of the families Araucariaceae or Pinaceae as the primary amber-producing trees and correlate favourably with the progressive demise of subtropical forest biomes from northern Europe as palaeotemperatures cooled following the Eocene climate optimum.     

A double-isotope method for the measurement of ketone-body turnover in the rat. Effect of L-alanine.

The synthesis of 4-3H-labelled ketone bodies, and their use along with 14C-labelled ketone-body precursors, is employed using an 'in vivo' rat infusion model to measure ketone-body turnover. The use of two isotopes is necessary to measure ketone-body turnover when ketogenesis may occur from more than one precursor such as glucose and fatty or amino acids. Requirements of isotopic equivalence in terms of metabolic similarity, valid stoichiometry and the lack of differences in the kinetics of relevant enzymes is demonstrated for the 4-3H- and 14C-labelled ketone bodies. The hypoketonaemic effect of L-alanine is shown by two distinct phases after the administration of L-alanine. During the first 12 min after alanine administration ther was a 50% decrease in acetoacetate and a 30% decrease in 3-hydroxybutyrate production, with no significant change in the utilization of either compound. The hypoketonaemic action of alanine during the following 16 min was primarily associated with an uptake of 3-hydroxybutyrate that was somewhat greater than the increase in its production. There were essentially equivalent decreases in production and utilization of acetoacetate, resulting in no significant net change in the level of this ketone body in the blood.