Components of O2-uptake by excised frog nerve dependent upon externally supplied sodium ions.

Abstract

The steady rate of oxygen uptake of excised frog nerves equilibrated in a solution having a very low concentration of sodium ions increases to a new high steady rate when equilibrated with a solution containing a high concentration of this ion. The increase is suppressed by ouabain, indicating participation of the sodium pump. Part of this sodium-activated increase in oxygen uptake is inhibited by tetrodotoxin, indicating that passive influx of sodium ions into axons is part of the total process. Thus, two pathways for passive sodium influx into axons are suggested by these experiments. Procedures known to increase the passive permeability of axons for sodium ions also increase this sodium-activated oxygen uptake. A mechanism is proposed to explain that part of the sodium-activated oxygen uptake that is inhibited by tetrodotoxin.The regulation of the concentration of sodium ion (Na+) in the cytoplasm of nerve cells is of physiological interest because a low concentration is essential for the maintenance, in the resting state, of the capability for initiation and conduction of the action potential. The generally accepted model of this regulatory system has three interlocking component processes: net passive diffusion of external Na+ into the cytoplasm; activated extrusion of Na+ via a specific ATPase (EC 3.6.1.3; ATP phosphohydrolase) located in the plasma membrane; and rephosphorylation of the product adenosine diphosphate (ADP) by oxidative metabolism in mitochondria (1-4). The first process, dissipating the ionic gradient across the plasma membrane, is offset by the subsequent chemical reactions, and the Na+ gradient is maintained by utilizing oxygen and substrate to provide the necessary supply of adenosine triphosphate (ATP). This model suggests experiments on excised nerves to define further the relations among these processes when all components are interacting as an integrated regulatory system.Ouabain, a potent inhibitor of the membrane-bound adenosine triphosphatase that requires Na+ and K+ for full activity (5), (Na + K)ATPase, has been used to identify that part of Na+-efflux from axons that is transported by the sodium pump (6, 7). Secondarily, the oxygen uptake of excised peripheral nerves has been operationally divided into a ouabain-sensitive and a ouabain-insensitive component (3,(8)(9)(10). The former component is considered to be a measure of that part of the oxidative metabolism that is utilized for extrusion of Na+ from the cytoplasm. For example, the excised sciatic nerve of a frog, Rana pipiens, at rest (i.e., not conducting action potentials) in Ringer's solution uses about [5][6] ,gmol of oxygen per hr/g (dry weight) (11,12). Ouabain reduces this steady-state rate of oxygen uptake about 30% (9). In accordance with the model, these data have been interpreted to mean that 30% of the oxygen uptake of resting nerve is used to extrude Na+ at a rate equal to its influx (3, 9). This process maintains a constant low concentration of this ion in the cytoplasm, as measured in such excised n...

A continuous-flow perfusion system for the maintenance and NMR study of small tissue samples in vitro

“…Frog sciatic nerve has been reported to have an oxygen consumption rate of 6-12 × 10 −6 mol/h/g (dry weight) at 20 • C [13][14][15], corresponding to an estimated consumption rate of 40-80 × 10 −9 mol/h for a typical nerve sample (estimated at just under 7 mg dry weight) used in our studies. Allowing a buffer Fig.…”