Changes in internal ionized Ca2+ and H+ in voltage clamped squid axons

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

The exchange in intact squid axons

The exchange in intact axons displays a number of features in common with other systems, but a number of interesting points remain to be examined. Both forward (Nao-Cai) and reverse (Cao-Nai) exchange are sensitive to changes in membrane potential, but potassium depolarization can also stimulate Cao-Nai exchange by chemical activation at a monovalent cation-binding site. By monitoring lithium uptakes into intact axons, activating cations do not appear to be transported on the exchange, but this deserves further examination under more stringent conditions. Cao-Nai exchange in intact axons appears activated by monovalent cations to a greater extent compared to dialyzed axons that exhibit little, if any, shift in the Km for Cao. The catalytic effect of Cai on Cao-Nai exchange seen in dialyzed axons proves elusive to study in intact axons, with or without introduction of Ca chelators. Experiments using ruthenium red suggest that free calcium can be dissociated from Cao-Nai exchange fluxes; this finding is also important to those studies monitoring exchange activity using Ca indicators. The possibility that Ca chelators may effect changes in the kinetics of Na-Ca exchange is a subject that needs further investigation.

Na(+)-Ca2+ exchanger mediates Ca2+ influx during anoxia in mammalian central nervous system white matter

White matter of the mammalian central nervous system suffers irreversible injury after prolonged anoxia, which can result in severe neurological impairment. This type of injury is critically dependent on Ca2+ influx into cells. We present evidence that the Na+,Ca2+ exchanger mediates the majority of the damaging Ca2+ influx into cells during anoxia in white matter. Anoxic injury was studied in the isolated rat optic nerve, and functional recovery was monitored using the compound action potential. Blockers of voltage-gated Na+ channels (tetrodotoxin and saxitoxin) significantly improved recovery, as did perfusion with zero-Na+ solution; both maneuvers would prevent intracellular [Na+] from rising and thus prevent Ca2+ influx by inhibiting reverse operation of the Na+,Ca2+ exchanger. Direct pharmacological blockade of the Na+,Ca2+ exchanger during anoxia with bepridil or benzamil also significantly improved recovery. These findings suggest that reverse operation of the Na+,Ca2+ exchanger during anoxia is a critical mechanism of Ca2+ influx and subsequent white matter injury.

Calcium entry in squid axons during voltage clamp pulses

Squid giant axons were injected with aequorin and tetraethylammonium and were impaled with sodium ion sensitive, current and voltage electrodes. The axons were usually bathed in a solution of varying Ca2+ concentration ([Ca2+]o) containing 150mM each of Na+, K+ and an inert cation such as Li+, Tris or N-methylglucamine and had ionic currents pharmacologically blocked. Voltage clamp pulses were repeatedly delivered to the extent necessary to induce a change in the aequorin light emission, a measure of axoplasmic Ca2+ level, [Ca2+]i. The effect of membrane voltage on [Ca2+]i was found to depend on the concentration of internal Na+ ([Na+]i). Voltage clamp hyperpolarizing pulses were found to cause a reduction of [Ca2+]i. For depolarizing pulses a relationship between [Ca2+]i gain and [Na+]i indicates that Ca2+ entry is sigmoid with a half maximal response at 22 mM Na+. This Ca2+ entry is a steep function of [Na+]i suggesting that 4 Na+ ions are required to promote the influx of 1 Ca2+. There was little change in Ca2+ entry with depolarizing pulses when [Ca2+]o is varied from 1 to 10mM, while at 50mM [Ca2+]o calcium entry clearly increases suggesting an alternate pathway from that of Na+/Ca2+ exchange. This entry of Ca2+ at high [Ca2+]o, however, was not blocked by Cs+o. The results obtained lend further support to the notion that Na+/Ca2+ exchange in squid giant axon is sensitive to membrane voltage no matter whether this is applied as a constant change in membrane potential or as an intermittent one.