Contractions induced by sodium withdrawal in crab (Callinectes danae) muscle fibres

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Anemonia sulcata toxin (ATX II) enhances spontaneous electrical activity and tension in chronically denervated rat diaphragm

In isolated strips of rat diaphragm denervated 9-21 days prior to experimentation, spontaneous action potentials were recorded extracellularly and twitch and resting tension were measured. The sea anemone toxin ATX II enhances the occurrence of spontaneous action potentials, increases resting tension and depresses twitch tension. These effects are essentially irreversible. In low sodium solution substituted with sucrose the effects of ATX II are attenuated, however, they fully develop upon return to normal sodium solution with a marked transient increase in the incidence of spontaneous action potentials and in resting tension. ATX II remains uneffective after pretreatment with tetrodotoxin. Reelevation of the extracellular sodium concentration after exposure to low sodium solution per se causes a marked increase in occurrence of fibrillation potentials, however the transient increase in resting tension was much smaller than in the presence of ATX II. Substitution of chloride with the impermeable anion methylsulphate enhances spontaneous activity and resting tension without an effect on twitch tension. Addition of ATX II elevates resting tension although the concomitant further increase in incidence of spontaneous action potentials is small. It is concluded that the increase in resting tension reflects a summation of the fibrillatory activity, but fibrillations become more effective when the preparations are exposed to ATX II. This finding points at the possible rôle of sodium ions in excitation contraction coupling of denervated skeletal muscle.

The effect of leakage on micro-electrode measurements of intracellular sodium activity in crab muscle fibres

The effect of lowering extracellular Na (Nao) on the intracellular Na activity has been measured in single muscle fibres from the crab Carcinus maenas using Na+-sensitive glass micro-electrodes. Measurements have been made with recessed-tip micro-electrodes inserted radially into intact fibres, and with axial electrodes in cannulated fibres. Reducing Nao to one-tenth normal caused local contractions in intact fibres. The apparent steady-state internal Na (Nai) and the fall in Nai when Nao was reduced were found to vary considerably not only between different fibres but also when measured with different electrodes in the same fibre. The steady-state Nai, and the extent and rate of its decrease when Nao was reduced, could be reduced by pushing the Na+-sensitive electrode deeper into the fibre. Cannulated fibres generally had higher internal Na activities than intact fibres, but at comparable levels of Nai the rate of fall recorded from cannulated fibres when Nao was reduced was much slower than with intact fibres. In both intact and cannulated fibres the decrease in Nai was reduced by ouabain. The level of Nai recorded in cannulated fibres was sensitive to depolarization. Depolarizations from -50 to -30 mV resulted in a rise in Nai while further depolarization to 0 mV resulted in a fall in Nai. We conclude that both our results and those of Vaughan-Jones (1977) on undissected fibres are contaminated by leakage into the fibre round the micro-electrode. The true internal Na activity is probably much lower than previously reported.