Second messenger modulation of steady-state calcium efflux in Aplysia bag cells

Self-referencing, non-invasive, ion selective electrode for single cell detection of trans-plasma membrane calcium flux

Biological systems have very different internal ion compositions in comparison with their surrounding media. The difference is maintained by transport mechanisms across the plasma membrane and by internal stores. On the plasma membrane, we can classify these mechanisms into three types, pumps, porters, and channels. Channels have been extensively studied, particularly since the advent of the patch clamp technique, which opened new windows into ion channel selectivity and dynamics. Pumps, particularly the plasma membrane Ca(2+)-ATPase, and porters are more illusive. The technique described in this paper, the self-referencing, ion-selective (or Seris) probe, has the ability to monitor the behavior of membrane transport mechanisms, such as the pumps and porters, in near to real-time by non-invasively measuring local extracellular ion gradients with high sensitivity and square micron spatial resolution. The principles behind the self-referencing technique are described with an overview of systems utilizing ion, electrochemical and voltage sensors. Each of these sensors employs the simple expedient of increasing the system resolution by self-referencing and, thereby, removing the drift component inherent to all electrodes. The approach is described in detail, as is the manner in which differential voltage measurements can be converted into a flux value. For the calcium selective probes, we can resolve flux values in the low to sub pmol.cm(-2)s(-1) range. Complications in the use of the liquid ion exchange cocktail are discussed. Applications of the calcium selective probe are given, drawing on examples from the plant sciences, developmental biology, muscle physiology, and the neurosciences.

Plasma membrane Ca2+-ATPase extrudes Ca2+ from hair cell stereocilia

Mechanically sensitive hair cells of the auditory and vestibular systems use Ca2+ to control adaptation of mechanical transduction, to effect frequency tuning, to trigger neurotransmitter release, and to mediate efferent synaptic signaling. To determine the role that pumps play in regulation of Ca2+ in the hair bundle, the organelle responsible for mechanoelectrical transduction, we localized and quantified the plasma membrane Ca2+-ATPase (PMCA) of the bundle. We found that each hair bundle contains approximately 10(6) PMCA molecules or approximately 2000 per square micrometer of bundle membrane and that PMCA is the principal calmodulin binding protein of the bundle. Consistent with biochemical estimates of PMCA density, we measured with extracellular Ca2+-selective electrodes a substantial Ca2+ efflux from bundles. The number of bundle Ca2+ pumps and magnitude of resting Ca2+ efflux suggested that PMCA should generate a substantial membrane current as bundles expel Ca2+. Measurement of whole-cell currents revealed a transduction-dependent outward current that was consistent with the activity of PMCA. Finally, dialysis of hair cells with PMCA inhibitors led to a large increase in the concentration of Ca2+ in bundles, which suggests that PMCA plays a major role in regulating bundle Ca2+ concentration. Our data further indicate that PMCA could elevate the extracellular Ca2+ concentration close to hair bundles above the low level found in bulk endolymph.

Non-invasive ion probes--tools for measuring transmembrane ion flux

Ionophores, used in non-invasive vibrating electrodes, can directly monitor steady-state ion flux from single cells, even when the source is non-electrogenic.