Expression of purinergic P2X2 receptor-channels and their role in calcium signaling in pituitary cells

Pituitary cells express purinergic receptor-channels (P2XR), the activation of which by ATP is associated with the facilitation of Ca2+ influx. Pharmacological, RT-PCR, and nucleotide sequence analyses confirm the presence of a wild type P2X2aR and a spliced isoform P2X2bR, which lacks a portion of carboxyl terminal amino acids. Wild type and spliced isoform receptors have a similar EC50 for ATP and time-course for activation, but the spliced isoform exhibits rapid and complete desensitization, whereas the wild type channel desensitizes slowly and incompletely. Deletion and insertion studies have revealed that a 6 residue sequence located in carboxyl tail (Arg371-Pro376) is required for sustained Ca2+ influx through wild type receptors. When co-expressed, the wild type and spliced channels form functional heteropolymeric channels. The patterns of Ca2+ signaling in the majority of pituitary cells expressing ATP-gated receptor-channels are highly comparable to those observed in cells co-transfected with P2X2aR and P2X2bR. ATP-induced [Ca2+]i response in pituitary cells is partially inhibited by nifedipine, a blocker of voltage-gated L-type Ca2+ channels, suggesting that P2X2R not only drive Ca2+ into the cell, but also activate voltage-gated Ca2+ entry. Our results indicate that ATP represents a paracrine and (or) autocrine factor in the regulation of Ca2+ signaling, and that its actions are mediated in part by heteropolymeric P2X2R. Key words: ATP-gated channels, gonadotrophs, lactotrophs, somatotrophs.

Changes in ATP content, enzyme activity and inorganic nitrogen species during composting of organic wastes

Changes in several biochemical parameters, exchangeable [Formula: see text] and [Formula: see text] were evaluated during the composting of sewage sludge and city refuse. Except for casein-protease activity, all enzyme activities were higher in sewage sludge than in city refuse, at all times, denoting larger microbial populations; this was also confirmed by a high initial ATP level. In addition to differences in microbial biomass, the lower enzyme activity in city refuse might also depend on inhibition by a high content of heavy metals. Absolute urease and phosphatase activities decreased during composting, showing a minimum at 91 and 65 d, respectively; thereafter they increased slightly. ATP also decreased markedly during the first month, then it stabilized to a low level. The correlation found between urease-phosphatase-ATP and the lack of correlation between proteases, and between both proteases and ATP-urease-phosphatase, confirmed the different synthesis and behaviour of these enzymes. Proteases might partially exist outside living cells, thus playing an important role in composting when microbial activity is low. High levels of ATP and enzyme activity at the beginning of the composting, their sharp decrease and the increase in casein-protease followed by a decrease, characterized the "mineralization" phase, while low levels in ATP and casein-protease and a re-activation of the other hydrolases, characterized the "maturation" phase of the composting. The ammonium content decreased during composting while nitrate increased, indicating that the process evolved exhaustively and under fully aerobic conditions. Key words: ATP, city refuse, enzyme activity, hydrolases, sewage sludge

Effect of humic substances on photosynthetic sulphate assimilation in maize seedlings

Maize plants were grown in nutrient solution containing 0, 15, 30, 50 mg organic carbon L−1 of humic substances (HS) (M.W. < 12 KDa) extracted from a cambisol. After 14 d of growth the second and third leaves were collected and analyzed for chlorophyll and protein content and for the activities of ATP sulphurylase (ATP-s) and O-acetylserine sulphydrylase (OAS-s) to evaluate the effect of HS on maize sulphate assimilation pathway. Humic substances induced significant decreases in chlorophyll content, whereas the protein level slightly increased up to 30 mg L−1 of HS. ATP-s and OAS-s were positively affected by HS. The stimulations were different according to the enzyme and leaf investigated. Our results suggest that HS might play an important role in the efficiency of maize plants to assimilate sulphate. Key words: ATP-sulphurylase, chlorophyll, humic substances, maize, O-acetylserine sulphydrilase

ATP as a Measure of Living Phytoplankton Carbon in Estuaries

An attempt has been made to evaluate the accuracy of ATP as a measure of living phytoplankton carbon in estuaries. Phytoplankton carbon estimated from ATP was compared to estimates from cell counts. In high biomass samples the agreement between the two estimates was quite good. In the low biomass samples the cell count method underestimated phytoplankton carbon relative to the ATP estimate. This was interpreted to be due in part to low cell counts in these samples. Contamination by microzooplankton (essentially only tintinnids) was, in 14 out of the 18 samples, < 3% of the ATP estimated carbon. However, it was as high as 19% in one case. In the lowest biomass samples (< 0.5 μg chlorophyll a/L) bacterial populations may contribute as much as 50% of the total living carbon. It is concluded that, with caution, ATP is a useful measure of living phytoplankton carbon in estuaries during periods of moderate to high biomass (> 1 μg chlorophyll a/L for the St. Lawrence). Since carbon to chlorophyll ratios in the high biomass samples varied considerably, a constant ratio appears inapplicable for transforming chlorophyll to carbon. Key words: ATP, phytoplankton biomass, estuaries, carbon to chlorophyll ratio