Involvement of calcium in phosphoinositide metabolism in the blood-brain barrier

Mechanism of arachidonic acid-induced Ca2+ mobilization in liver nuclei

Arachidonic acid treatment in isolated liver nuclei resulted in a rapid and transient increase of Ca2+ concentration in the nucleoplasm which was monitored with the Ca(2+)-sensitive dye fura-2 dextran. This effect was associated with a decrease of Ca2+ concentration in the nuclear envelope as measured with fura-2 AM. Our results indicate that arachidonic acid causes a Ca2+ release from the nuclear envelope to the nucleoplasm similar to that evoked by inositol trisphosphate (IP3). The arachidonic acid-induced Ca2+ mobilization in the nucleus was not due to the metabolites of arachidonic acid. Experiments performed in the presence of ATP and Ca2+ indicate that arachidonic acid-induced Ca2+ mobilization in the nucleus takes place in a non ATP-dependent way. Taken together, these results suggest that arachidonic acid may contribute to the regulation of nuclear Ca2+ mobilization.

Regulation of phosphoinositide cycle by intracellular sodium in the blood-brain barrier

In the present study of cerebral microvessels, we report that monensin, a Na+ ionophore, elicits a decrease in 32P radioactivity incorporation into phosphoinositides in cerebral microvessels. In addition, monensin evokes enhanced production of inositol-1-monophosphate (IP) and inositol-1,4-bisphosphate (IP2), together with an increase in the diacylglycerol (DAG) mass. These results indicate that monensin evokes a phosphoinositide hydrolysis by phospholipase C (PLC). The absence of inositol-1,4,5-trisphosphate (IP3) production leads us to think that although phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis occurs in this process, there is a very rapid disappearance of IP3. The net decrease in 32P radioactivity incorporated into phosphoinositides suggests that a partial inhibition of their re-synthesis is also evoked. Experimental evidence with pharmacological tools suggests that: (1) these effects are secondary to an increase in Ca2+ through the Na+/Ca2+ exchanger; and (2) the intracellular Ca2+ release is not involved in these effects of monensin. Since some neuropeptide receptors in cerebral microvessels have been reported to be coupled to either the Na+/H+ exchanger or to PLC, we discuss the possibility that cross-talk exists between these intracellular signalling pathways (phosphoinositide metabolism and Na+ transport) in the blood-brain barrier (BBB).

Identification of nitric oxide synthases in isolated bovine brain vessels

We have studied the presence of neuronal nitric oxide synthase (nNOS) and endothelial nitric oxide synthase (eNOS) in parenchymal and pial bovine cerebral vessels by using western blot analysis. The different vessel structures were analysed by microscopic observation and their biochemical features determined by using gamma-glutamiltranspeptidase (gamma-GTP) as an endothelial marker and alpha-smooth muscle actin (alpha-SMA) as a smooth muscle cell marker. nNOS could not be found in parenchymal vessels, being present only in pial vessels; eNOS was present in all vessel-fractions studied, albeit a different distribution was found. Thus, the eNOS is more abundant in the parenchymal vessels which are associated with smooth muscle cells whereas both pial vessels and brain microvessels, almost devoid of actin, show extremely low levels. Two main conclusions can be obtained: first, nNOS is exclusively restricted to pial vessels and, second, eNOS is present in endothelial cells which are in association with smooth muscle cells.