Intersection of two signalling pathways: extracellular nucleotides regulate pollen germination and pollen tube growth via nitric oxide.

Plant and animal cells release or secrete ATP by various mechanisms, and this activity allows extracellular ATP to serve as a signalling molecule. Recent reports suggest that extracellular ATP induces plant responses ranging from increased cytosolic calcium to changes in auxin transport, xenobiotic resistance, pollen germination, and growth. Although calcium has been identified as a secondary messenger for the extracellular ATP signal, other parts of this signal transduction chain remain unknown. Increasing the extracellular concentration of ATPgammaS, a poorly-hydrolysable ATP analogue, inhibited both pollen germination and pollen tube elongation, while the addition of AMPS had no effect. Because pollen tube elongation is also sensitive to nitric oxide, this raised the possibility that a connection exists between the two pathways. Four approaches were used to test whether the germination and growth effects of extracellular ATPgammaS were transduced via nitric oxide. The results showed that increases in extracellular ATPgammaS induced increases in cellular nitric oxide, chemical agonists of the nitric oxide signalling pathway lowered the threshold of extracellular ATPgammaS that inhibits pollen germination, an antagonist of guanylate cyclase, which can inhibit some nitric oxide signalling pathways, blocked the ATPgammaS-induced inhibition of both pollen germination and pollen tube elongation, and the effects of applied ATPgammaS were blocked in nia1nia2 mutants, which have diminished NO production. The concurrence of these four data sets support the conclusion that the suppression of pollen germination and pollen tube elongation by extracellular nucleotides is mediated in part via the nitric oxide signalling pathway.

Fluconazole- and levofloxacin-induced torsades de pointes in an intensive care unit patient.

Patients initiated on fluconazole and levofloxacin should be closely monitored for QTc-interval prolongation. While there have been published reports of fluconazole and levofloxacin causing QTc-interval prolongation when given alone, coadministration of these two agents may further increase this risk. This case describes an episode of TdP in which levofloxacin and fluconazole were likely significant factors. QT prolongation was present at baseline prior to drug initiation (QTc = 454-505 ms) and levofloxacin resulted in further prolongation (QTc = 480-536 ms). After two days of therapy with fluconazole, overlapping with levofloxacin, the patient had an episode of PMVT with syncope, and progressive QT prolongation was evident (QTc = 554 ms). Only mild hypokalemia (potassium concentration = 3.6 meq/L) was present, and not additional etiologies for TdP were identified. Levofloxacin and fluconazole were discontinued and no further PMVT was observed, but the QT interval did not return to normal until after an additional 11 days (QTc = 436 ms). As in many cases of TdP, multiple factors were involved. Renal failure, drug dosing, mild hypokalemia, and a baseline abnormal QT interval potentiated the role of levofloxacin and fluconazole in the development of TdP. We recommend that neither drug be used alone or in combination when there is baseline QT prolongation. We also recommend that concomitant use of these agents be avoided when possible. If combination therapy is required, caution is warranted, particularly in patients with risk factors for QT prolongation. Specific attention should be given to drug dosing, interactions, electrolytes, and ECG monitoring.