Evaluation of capillary supercritical fluid chromatography with mass spectrometric detection for the analysis of a drug (mebeverine) in a dog plasma matrix.

Supercritical fluid chromatography with mass spectrometric detection was evaluated as a technique for the analysis of drugs in biological fluids. Dog plasma was spiked with a model drug, mebeverine, and with a deuterium-labeled analog of mebeverine. The spiked plasma was prepared for analysis by solid-phase extraction on octadecylsilane cartridges. Mebeverine levels in the spiked dog plasma samples were determined by interpolation from a standard curve. Accuracy and precision of the analysis were determined within and between days. In general, accuracy was found to be 100 +/- 15% for plasma samples spiked with 6 to 60 ng mebeverine/ml. The relative standard deviation for replicate sample analysis over this concentration range was between 5 and 12.5%.

Active chloride transport in the leaf epidermis of Commelina communis in relation to stomatal activity.

A chloride selective micro-electrode has been used to determine vacuolar chloride concentrations in individual cells of the leaf epidermis of Commelina communis. When the stomata were open a gradient in chloride concentration across the stomatal complex was observed with the highest concentration in the guard cells. On stomatal closure the chloride gradient was reversed. Calculation of the driving forces on chloride indicated that active transport of chloride was occurring during both stomatal opening and closure. This transport appeared to be energetically independent of the transport of potassium. These results are discussed in relation to the behaviour of other anions during stomatal movements.

Direct Determination of pH in the stomatal complex of Commelina.

The vacuolar pH of individual cells of the epidermis of intact leaves of Commelina communis L. has been determined using pH sensitive microelectrodes. The results show that the pH of the guard cells increases by approximately 0.5 units during stomatal opening. The changes in vacuolar pH mirror the changes in vacuolar potassium concentration that occur during stomatal movements.

A study of potassium gradients in the epidermis of intact leaves of Commelina communis L. in relation to stomatal opening.

Potassium sensitive microelectrodes have been used to determine the potassium content of individual epidermal cells in intact leaves of C. communis L. Large gradients of potassium across the stomatal complex were observed. When the stomata were open a stepwise decrease in vacuolar potassium content from the guard cells outwards was found. With the stomata closed a gradient in the opposite direction was observed, with the potassium content of the guard cells much lower than that of the surrounding cells. Electrical potential differences between the cells were determined enabling driving forces on potassium to be calculated. The results indicate that the potassium gradients are maintained against the electrochemical gradient and that potassium transport both into and out of the guard cells is an active process. Potassium transport between the subsidiary cells also appears to be active.Potassium fluxes into the guard cell during stomatal opening were calculated and found to be relatively high. The implications of this are discussed.

Measurement of a gradient of oxygen partial pressure across the intact root.

Oxygen partial pressures in the vacuolar sap of root cells have been measured using oxygen sensitive polarographic microelectrodes. A significant gradient in oxygen partial pressure was observed across intact sunflower roots growing in air saturated culture solution. The relevance of this finding to the mechanism of salt transfer across the root and in particular to the Crafts-Broyer theory is discussed.

Measurement of profiles of potassium activity and electrical potential in the intact root.

Profiles of potassium activity and electrical potential in the vacuoles of cells across the intact root of Helianthus annuus have been measured. No gradient in either potassium activity or potential difference was detected from epidermis to pericycle. The trans-root electrical potential was found to be made up of two components, a large P.D. between the outside solution and the outer cells and a small P.D. at the pericycle-xylem interface. The results indicated that all the living cells of the root have the same capacity to actively accumulate potassium.

Evidence for a sodium influx pump in sunflower roots.

Transmembrane electrical potential differences in the cortical cells of the root of the sunflower (Helianthus annuus) have been measured using microelectrodes. The plants were grown in culture solution with a range of sodium concentrations. It was found that increasing the external sodium concentration had virtually no effect on the transmembrane potential. The vacuolar content of sodium did not change significantly with the age of the tissue indicating that sodium was in flux equilibrium in our experiments. This allowed the Nernst equation to be used to calculate the electrochemical potential gradient for sodium between the vacuole and the external solution. It was concluded that sodium was being transported into the vacuole against the electrochemical potential gradient. The location and role of the inwardly directed sodium pump implied by these results is discussed in relation to the efflux pumps for sodium reported for roots of other species. Potassium was also accumulated against the electrochemical potential gradient by these cells.Sodium was found to stimulate the growth of H. annuus when present in the culture solution at very low concentrations.

Active and passive ion transport in relation to transpiration in Helianthus annuus.

The effect of transpiration on the uptake of K, Ca, NO3 and SO4 from two strengths of culture solution by Helianthus plants was investigated. In the dilute solution transpiration had no effect on the uptake of all the ions studied. In the more concentrated solution the uptake of K, NO3 and SO4 were found to be sensitive to changes in water flux across the root. Calcium uptake was found to be completely independent of water uptake in all the experiments.The results are interpreted in the light of the results from an earlier study on excised Helianthus roots. It is suggested that transpiration only affects the nonmetabolic transport of the ions across the root.

Active transport of ions across sunflower roots.

The electrical potential difference across exuding roots of Helianthus annuus in two strengths of complete culture solution was measured. The determination of the concentration of the major nutrient ions in the outside solution and the xylem sap enabled the Nernst potential for each ion to be calculated. A comparison of the measured and calculated potentials indicated that the anions NO3, SO4, H2PO4 and HPO4 were actively transported into the sap against the electrochemical potential gradient. The cations Ca and Mg, on the other hand, appeared to move passively into the sap. The behaviour of potassium depended on its concentration in the medium. With a relatively low external concentration (0.75 mM) it appeared to be actively tansported into the sap, whilst at higher outside concentrations (7.5 mM) it was apparently moving passively into the xylem down the electrochemical potential gradient. The possibility of potassium being pumped out of the sap with relatively high external concentrations is discussed.