Double-barrelled nitrate-selective microelectrodes have been used to measure the time course of the remobilisation of vacuolar stored nitrate in barley (Hordeum vulgare L. cv. Klaxon) root cells during 24 h of nitrate deprivation. These measurements showed that there are dierent time courses for this process in epidermal and cortical cells of the same root. The remobilisation was much slower from cortical cell vacuoles and had a time course which was similar to that obtained for tissue digests of the roots. The microelectrodes were also used to measure the nitrate concentration in sap exuding from detopped seedlings. These measurements showed that there was a gradual decrease in the delivery of nitrate to the shoot during this time. Root nitrate reductase activity of neither shoots nor roots changed signi®cantly during the ®rst 24 h. Direct measurement of the cytosolic nitrate in a root epidermal cell showed that during short-term changes, such as a 20-min exposure to zero external nitrate supply, cytosolic nitrate was maintained relatively unchanged. Net nitrate eux from the roots was measurable during the initial 5 h of the zero-nitrate incubation period; after this time no further nitrate eux was detectable. These measurements are discussed in relation to the nitrate budget of a root cell and we conclude that during the ®rst 24 h of nitrate withdrawal vacuolar nitrate can be readily mobilised to supply the nitrogen demands of the seedling and to maintain the cytosolic nitrate concentration.

The transcription factor NF-B plays an important role in the regulated expression of cytokines in human monocytes. A p100 subunit of NF-B has IB-like properties by sequestering the p65 transactivating subunit in the cytosol of cells. In transient transfection assays we demonstrated that p100 has an inhibitory effect on the NF-B-dependent IL-6 promoter activity. In view of this finding, we studied the regulation of the p100 subunit in human monocytes in response to LPS, the inflammatory cytokines IL-1␤ and TNF-␣ and lymphokines. The results demonstrate that LPS, IL-1␤, and TNF-␣ induce p100 expression at mRNA and protein level while IFN-␥, IL-3 and IL-4/IL-10 have no effect. The induction of p100 expression was shown to be mediated by a two-fold increase in the p100 transcription rate and a two-fold increase in p100 mRNA stability. Furthermore the p100 mediated upregulation was dependent on a tyrosine kinase dependent pathway rather than the protein kinase C pathway. NF-B is a complex of either p50 homodimers or a p50/p65 heterodimer. The latter is known to strongly autoregulate p100 transcription. We therefore examined the composition of NF-B induced by LPS vs the different lymphokines. LPSinduced NF-B showed a distinct p65 supershift whereas the composition of NF-B induced by different lymphokines did not show a change in p65. We conclude that the p100 subunit of the transcription factor NF-B is induced by different inflammatory mediators while lymphokines fail to induce p100 expression which may be caused by the induction of NF-B predominantly consisting of p50 homodimers.

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