Dynamic and static calcium gradients inside large snail (Helix aspersa) neurones detected with calcium-sensitive microelectrodes

Calcium-sensitive mini- and microelectrodes

It is widely agreed that the best method for measuring the ionized free calcium concentration ([Ca(2+)]) in large volumes of biological solutions is to use Ca(2+)-sensitive macroelectrodes. These are commercially available. To measure [Ca(2+)] in small volumes of solution, minielectrodes with 1-2-mm tips can easily be made and used, and may also be commercially available. Ca(2+)-sensitive microelectrodes (CaSMs, with 0.5-2-μm tips) can also be made and used extracellularly or intracellularly in robust cells, but interest in their use has recently been largely eclipsed. This is because of practical difficulties and the introduction of a large number of fluorescent and other optical calcium probes with calcium sensitivities varying from the nanomolar to the millimolar range, such as Fura-2, Indo-1, Fluo-4, and many others. In this article, we emphasize the utility of Ca(2+)-selective electrodes and show that their use is complementary to use of fluorescent and other optical methods. Each method has advantages and disadvantages. Because numerous reviews and books have been dedicated to the theoretical aspects of ion-selective electrode principles and technology, this article is mainly intended for investigators who have some degree of electrophysiological experience with ion-selective electrodes or microelectrodes.

The Ca(2+): H(+) coupling ratio of the plasma membrane calcium ATPase in neurones is little sensitive to changes in external or internal pH

To explore the effects of both external and internal pH (pH_o and pH_i) on the coupling between Ca²⁺ extrusion and H⁺ uptake by the PMCA activity in snail neurones H⁺ uptake was assessed by measuring surface pH changes (ΔpH_s) with pH-sensitive microelectrodes while Ba²⁺ or Ca²⁺ loads were extruded. Ru360 or ruthenium red injection showed that injected Ca²⁺ was partly taken up by mitochondria, but Ca²⁺ entering through channels was not. External pH was changed using a mixture of three buffers to minimise changes in buffering power. With depolarisation-induced Ca²⁺ or Ba²⁺ loads the ΔpH_s were not changed significantly over the pH range 6.5–8.5. With Ca²⁺ injections into cells with mitochondrial uptake blocked the ΔpH_s were significantly smaller at pH 8.5 than at 7.5, but this could be explained in part by the slower rate of activity of the PMCA. Low intracellular pH also changed the ΔpH_s responses to Ca²⁺ injection, but not significantly. Again this may have been due to reduced pump activity at low pH_i. I conclude that in snail neurones the PMCA coupling ratio is either insensitive or much less sensitive to pH than in red blood cells or barnacle muscle.

Modeling of the modulation by buffers of Ca2+ release through clusters of IP3 receptors

Intracellular Ca(2+) release is a versatile second messenger system. It is modeled here by reaction-diffusion equations for the free Ca(2+) and Ca(2+) buffers, with spatially discrete clusters of stochastic IP(3) receptor channels (IP(3)Rs) controlling the release of Ca(2+) from the endoplasmic reticulum. IP(3)Rs are activated by a small rise of the cytosolic Ca(2+) concentration and inhibited by large concentrations. Buffering of cytosolic Ca(2+) shapes global Ca(2+) transients. Here we use a model to investigate the effect of buffers with slow and fast reaction rates on single release spikes. We find that, depending on their diffusion coefficient, fast buffers can either decouple clusters or delay inhibition. Slow buffers have little effect on Ca(2+) release, but affect the time course of the signals from the fluorescent Ca(2+) indicator mainly by competing for Ca(2+). At low [IP(3)], fast buffers suppress fluorescence signals, slow buffers increase the contrast between bulk signals and signals at open clusters, and large concentrations of buffers, either fast or slow, decouple clusters.

The plasma membrane calcium ATPase (PMCA) of neurones is electroneutral and exchanges 2 H+ for each Ca2+ or Ba2+ ion extruded

The coupling between Ca2+ extrusion and H+ uptake by the ubiquitous plasma membrane calcium ATPase (PMCA) has not been measured in any neurone. I have investigated this with Ca2+- and pH-sensitive microelectrodes in large voltage-clamped snail neurones, which have no Na+-Ca2+ exchangers. The recovery of [Ca2+]i and surface pH after a brief depolarization or Ca2+ injection was not slowed by hyperpolarization to -90 mV from a holding potential of -50 mV, consistent with a 1 Ca2+ : 2 H+ coupling ratio. Since Ca2+ injections proved difficult to quantify, and Ca2+ currents through channels were obscured by K+ currents, Ba2+ was used as a substitute. When the cell was bathed in Ca2+-free Ba2+ Ringer solution, the K+ currents were blocked and large inward currents were revealed on depolarization. The Ca2+-sensitive microelectrodes were sensitive to intracellular Ba2+ as well as Ca2+. With equal depolarizations Ba2+ entry appeared larger than Ca2+ entry and generated similar but slower pH changes. Ba2+ extrusion was insensitive to hyperpolarization, blocked by eosin or high pH, and about 5 times slower than Ca2+ extrusion. The ratio of the pH change caused by the extrusion of unit charge of Ba2+ influx to that caused by unit charge of H+ injection was 0.85 +/- 0.08 (s.e.m., n = 8), corresponding to a Ba2+ : H+ ratio of 1 : 1.7. Both this ratio and the electroneutrality of the PMCA suggest that the Ca2+ : H+ ratio is 1 : 2, ensuring that after a Ca2+ influx [Ca2+]i recovery is not influenced by the membrane potential and maximizes the conversion of Ca2+ influxes into possible pH signals.

Calcium signal transmission in chick sensory neurones is diffusion based

In many cells, the cytosol is an excitable medium through which calcium waves propagate by calcium induced calcium release (CICR). Many labs. have reported CICR in neurones subsequent to calcium influx through voltage gated channels. However, these have used long depolarizations. We have imaged calcium within chick sensory neurones following 50 ms depolarizations. Calcium signals travelled rapidly throughout the cell, such that changes at the cell centre were delayed by 24 ms compared to regions 3 microm from the plasma membrane. The nuclear envelope imposed a delay of 9 ms. A simple diffusion model with few unknowns gave good fits to the measured data, indicating that passive diffusion is responsible for signal transmission in these neurones. Simulations run without indicator dye did not reveal markedly different spatiotemporal dynamics, although concentration changes were larger. Simulations of calcium changes during action potentials revealed that large calcium transients occurring in the cytosol close to the nucleus are significantly attenuated by the nuclear envelope. Our results indicate that for the brief depolarisations that neurones will experience during normal signal processing calcium signals are transmitted by passive diffusion only. Diffusion is perfectly capable of transmitting the calcium signal into the interior of nerve cell bodies, and into the nucleoplasm.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.