Polyamines and Ca²⁺ Mediate Hyperosmolal Opening of the Blood‐Brain Barrier: In Vitro Studies in Isolated Rat Cerebral Capillaries

Abstract

We recently presented evidence that the reversible opening of the blood-brain barrier (BBB) by the infusion of 1.6 M mannitol into the rat internal carotid artery is mediated by a rapid stimulation of ornithine decarboxylase (ODC) activity and putrescine synthesis in cerebral capillaries. We have now investigated this hypothesis further, using isolated rat cerebral capillaries as an in vitro model of the BBB. The ODC activity of cerebral capillary preparations was enriched up to 15-fold over that of the cerebral homogenate. Hyperosmolal mannitol in physiological buffer evoked a rapid (less than 15 s), concentration- and time-dependent increase in capillary ODC activity and an accumulation of putrescine and spermidine which was blocked by the specific ODC inhibitor, alpha-difluoromethylornithine (DFMO, 10 mM). Mannitol (1 M), as well as 2 M urea, evoked a two- to fivefold increase in the temperature-sensitive influx of 45Ca2+ and uptake of horseradish peroxidase (HRP) and 2-deoxy-D-[1-3H]glucose (DG), but not alpha-[1-14C]aminoisobutyrate, during a 2-min incubation. DFMO (10 mM) abolished 1 M mannitol-mediated stimulation of 45Ca2+ influx and uptake of HRP and DG, whereas 1 mM putrescine replenished capillary polyamines and reversed the DFMO effects. Mannitol (1 M)-induced stimulation of ODC activity and membrane transport processes was Ca2+-dependent and verapamil- and nisoldipine-sensitive. Phorbol myristate acetate (PMA, 10 nM), a protein kinase C activator, also evoked a two- to threefold stimulation of 45Ca2+ transport and HRP and DG uptake. This PMA effect was abolished by DFMO, suggesting involvement of rapid, ODC-controlled polyamine synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Polyamine metabolism in different pathological states of the brain

“…In addition, in isolated capillaries, the mannitol-induced influx of 45 Ca2 + and uptake of horseradish peroxidase or deoxyglucose were inhibited by DFMO -whereas putrescine abolished this inhibition (Koenig et al, 1989c). Interestingly, the protein kinase C activator phorbol myristate acetate supplemented the effect of mannitol and this was also DFMO sensitive (Koenig et al, 1989c).…”

Polyamines Induce Blood-Brain Barrier Disruption and Edema Formation in the Rat

“…The effect of DFMO was reversed by the administration of exogenous putrescine. Koenig et al 125,26/ also showed that hyperosmolar mannitol injected into the carotid artery induces ODC-regulated synthesis of microvascular PA allowing BBB breakdown. These studies indicate that PA synthesis is obligatory for BBB breakdown; they also suggest that the mechanism of PA action involves a ΡΑ-mediated stimulation of calcium fluxes into microvessel endothelium, leading to an increase in free cyctosolic Ca 2+ .…”

Blood‐brain barrier permeability in rats is altered by exposure to magnetic fields associated with magnetic resonance imaging at 1.5 T

“…This suggests that the same mechanisms may be activated across a wide bandwidth of the non-ionising portion of the electromagnetic spectrum. Further, there is evidence that reversible opening of the BBB is mediated by stimulation of ornithine decarboxylase activity (Koenig et al, 1989) which may be dependent on calcium ion transport across the cell membrane and/or intracellular free calcium concentration. Since it has been shown that ornithine decarboxylase activity (Litovitz et al, 1991), intracellular free calcium concentration (Carson et al, 1990), and calcium ion transport across the cell membrane (Walleczek and Budinger, 1992) can be altered by ELF magnetic field exposure, the modulation in permeability by the total imaging procedure over the changes produced by the static exposure alone may be initiated by the stimulation of the calcium second messenger system produced by exposure to the MRI gradients which produces an ELF magnetic field.…”