Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance

Globally, over one-third of irrigated land is affected by salinity, including much of the land under lowland rice cultivation in the tropics, seriously compromising yields of this most important of crop species. However, there remains an insufficient understanding of the cellular basis of salt tolerance in rice. Here, three methods of 24Na+ tracer analysis were used to investigate primary Na+ transport at the root plasma membrane in a salt-tolerant rice cultivar (Pokkali) and a salt-sensitive cultivar (IR29). Futile cycling of Na+ at the plasma membrane of intact roots occurred at both low and elevated levels of steady-state Na+ supply ([Na+]ext=1 mM and 25 mM) in both cultivars. At 25 mM [Na+]ext, a toxic condition for IR29, unidirectional influx and efflux of Na+ in this cultivar, but not in Pokkali, became very high [>100 μmol g (root FW)−1 h−1], demonstrating an inability to restrict sodium fluxes. Current models of sodium transport energetics across the plasma membrane in root cells predict that, if the sodium efflux were mediated by Na+/H+ antiport, this toxic scenario would impose a substantial respiratory cost in IR29. This cost is calculated here, and compared with root respiration, which, however, comprised only ∼50% of what would be required to sustain efflux by the antiporter. This suggests that either the conventional ‘leak-pump’ model of Na+ transport or the energetic model of proton-linked Na+ transport may require some revision. In addition, the lack of suppression of Na+ influx by both K+ and Ca2+, and by the application of the channel inhibitors Cs+, TEA+, and Ba2+, questions the participation of potassium channels and non-selective cation channels in the observed Na+ fluxes

BK Channels Regulate Spontaneous Action Potential Rhythmicity in the Suprachiasmatic Nucleus

Background: Circadian (,24 hr) rhythms are generated by the central pacemaker localized to the suprachiasmatic nucleus (SCN) of the hypothalamus. Although the basis for intrinsic rhythmicity is generally understood to rely on transcription factors encoded by ‘‘clock genes’’, less is known about the daily regulation of SCN neuronal activity patterns that communicate a circadian time signal to downstream behaviors and physiological systems. Action potentials in the SCN are necessary for the circadian timing of behavior, and individual SCN neurons modulate their spontaneous firing rate (SFR) over the daily cycle, suggesting that the circadian patterning of neuronal activity is necessary for normal behavioral rhythm expression. The BK K + channel plays an important role in suppressing spontaneous firing at night in SCN neurons. Deletion of the Kcnma1 gene, encoding the BK channel, causes degradation of circadian behavioral and physiological rhythms. Methodology/Principal Findings: To test the hypothesis that loss of robust behavioral rhythmicity in Kcnma1 2/2 mice is due to the disruption of SFR rhythms in the SCN, we used multi-electrode arrays to record extracellular action potentials from acute wild-type (WT) and Kcnma1 2/2 slices. Patterns of activity in the SCN were tracked simultaneously for up to 3 days, and the phase, period, and synchronization of SFR rhythms were examined. Loss of BK channels increased arrhythmicity but also altered the amplitude and period of rhythmic activity. Unexpectedly, Kcnma1 2/2 SCNs showed increased variability in the timing of the daily SFR peak

Activation kinetics of single P2X receptors

After the primary structure of P2X receptors had been identified, their function had to be characterized on the molecular level. Since these ligand-gated ion channels become activated very quickly after binding of ATP, methods with adequate time resolution have to be applied to investigate the early events induced by the agonist. Single-channel recordings were performed to describe conformational changes on P2X2, P2X4, and P2X7 receptors induced by ATP and also by allosteric receptor modifiers. The main results of these studies and the models of P2X receptor kinetics derived from these observations are reviewed here. The investigation of purinoceptors by means of the patch clamp technique following site-directed mutagenesis will probably reveal more details of P2X receptor function at the molecular level

Atp-activated channels in rat superior cervical ganglion neurones and their modulation by extracellular zinc

It has recently been shown that adenosine 5'-triphosphate (ATP) can act as a fast excitatory transmitter at neuro-neuronal synapses in both the central and the peripheral nervous systems. I have examined the ATP-activated inward current (IATP) cultured rat superior cervical ganglion (SCG) neurones and its modulation by extracellular zinc ions (Zn2+). ATP activated a non-specific cation conductance and caused a transient rise in intracellular Ca2+ which was dependent on extracellular Ca2+. The current was activated specifically by ATP and was reversibly blocked by the P2-purinoceptor antagonist, suramin. Low concentrations of extracellular Zn2+ rapidly and reversibly potentiated both IATP and the intracellular Ca2+ rise. Higher concentrations of Zn2+ reduced and prolonged the current. Other divalent cations mimicked the effect of Zn2+ with an order of potency of: Cu2+ [greater-than] Zn2+ [greater-than] Ni2+ [greater-than] Cd2+ [greater-than] Co2+ [greater-than] Mn2+. The potentiation by Zn2+ was dependent on the concentration of agonist; Zn2+ increased the apparent affinity of the receptor for ATP without potentiating the maximum response. Single channels activated by ATP, and reversibly blocked by suramin, were recorded using excised outside-out patches. The channels were small (conductance at -80 mV = 11 pS). Both single channel conductance and opening probability increased with hyperpolarization. Low concentrations of Zn2+ significantly increased the frequency of ATP-evoked channel opening and burst duration without altering the unitary conductance. Higher concentrations of Zn2+ further increased channel burst duration but also decreased unitary current amplitude. These results are consistent with two sites of action for Zn2+: a positively acting allosteric site which enhances macroscopic current amplitude and a site, possibly within the pore, which blocks conductance through the channel. In conclusion, the P2x-purinoceptor in rat SCG neurones is allosterically regulated by changes in concentration of extracellular Zn2+, increasing both IATP amplitude and ATP-evoked changes in intracellular Ca2+

Centennial Convocation

Mrs. Ether Goddard greets President Storke and Milton P. Higgin

Rifle Club

Members of the Rifle Clu

Alberti and Higgins

J. Norman Alberti '24 and Milton P. Higgin