A new method for the simultaneous estimation of phosphate efflux and influx during glucose stimulation of isolated pancreatic islets

The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation

Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic beta-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 beta-cells, we revealed that both Ca(2+)-phosphate- and oxidant-induced MPT is remarkably different from other tissues. Ca(2+)-phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in beta-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in beta-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic beta-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca(2+), suggesting a novel type of nonclassical MPT in beta-cells. It results in the conversion of electrical transmembrane potential into DeltapH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca(2+)- and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in beta-cell glucose-sensing and secretory activity.

Theoretical interpretation of isotope labelling experiments in cells in which the label is chemically incorporated: the example of orthophosphate

Studies of transport across the plasma membrane in intact cells frequently involve measuring the incorporation of a labelled extracellular species into the cells. Unfortunately, if the labelled species is metabolized in the cell, the kinetics of labelling are made more complicated. Using the example of the incorporation of 32P-labelled orthophosphate into cells, we describe a mathematical model which allows for this complication, and show how this may alter the interpretation of experiments. The analysis is widely applicable to cellular labelling studies with any species that undergoes chemical exchange with a large cellular pool.

Net fluxes of orthophosphate across the plasma membrane in human red cells following alteration of pH and extracellular Pi concentration

Even though net fluxes of Pi (orthophosphate) across the cell membrane may be important in clinical disorders involving the abnormal extracellular Pi concentration, in acid-base disturbances, and in the responses of some cells to hormones, relatively few studies have been made of these fluxes, owing to the complexities of interpretation. Here we have studied net fluxes in response to changes in extracellular pH and Pi concentration in the simple case of the human red cell. The permeability of the cell membrane to net Pi fluxes was described in terms of a first-order rate constant, epsilon. By means of a mathematical model, it was possible to discriminate between transmembrane Pi movement, net intracellular generation or consumption of Pi by organic phosphates, and extracellular generation of Pi from the cells lysing during the experiment. We show that net Pi influx into the cell during experimental alkalosis was probably driven by net consumption of Pi by organic phosphates, and that this was reversed during acidosis. Inhibition of net Pi influx by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate (SITS) suggests that, like Pi self-exchange, net influx is at least partly mediated by the band 3 transport protein. Unexpectedly, epsilon increased from 2 h-1 at extracellular pH 7.4 to approx. 7 h-1 at pH 7.8. From the value of epsilon at pH 7.4, we conclude that the apparent buffering or regulation of steady-state Pi concentrations, previously reported in red cells in vitro, was not an artifact of intracellular generation of Pi from organic phosphates.