Physiological and biochemical changes in frog sciatic nerve during anoxia and recovery.

TLDR: Frog (Rana pipiens) sciatic nerve was incubated, with and without stimulation, in an oil bath to study the correlation between changes in the magnitude of the compound action potential (α and β) and changes in metabolites, particularly energy reserves, during anoxia and recovery fromAnoxia.

Abstract: — Frog (Rana pipiens) sciatic nerve was incubated, with and without stimulation, in an oil bath. The correlation between changes in the magnitude of the compound action potential (α and β) and changes in metabolites, particularly energy reserves, during anoxia and recovery from anoxia was studied. The time to extinction of the action potential in anoxia was frequency-dependent. The action potential could not be restored, nor its extinction delayed, by washing the nerve in O2-free Ringer's solution. Therefore, in this system extracellular K+ accumulation was not a significant factor in blocking impulse conduction. At the time of complete nerve block resulting from anoxia (90 min at rest), ATP, P-creatine and glucose were 30, 10 and 10 per cent, respectively, of initial levels. Glycogen did not fall below 42 per cent of control levels even after 5 h of anoxia. Changes in the levels of energy reserves during anoxia were used to calculate the metabolic rate of nerves at rest and during stimulation. In one series of experiments, the resting metabolic rate was 0·12 mequiv. of ‘high-energy phosphate’ (∼P)/kg/min. Stimulation increased the metabolic rate to 0·22 mequiv. of ∼P/kg/min at 30 Hz and to 0·29 mequiv. of ∼P/kg/min at 100 Hz. The change in metabolic rate when the nerve passed from the resting to the stimulated state was quite abrupt, an observation suggesting that the slow transition observed with methods monitoring O2, consumption was largely instrumental. In nerve stimulated to exhaustion in the absence of O2, neither ATP nor P-creatine had fully recovered within 60 min after O2, was readmitted, although the action potential reached supranormal levels 15 min after return to O2. The ratio of lactate: pyruvate, which increased as expected during anoxia, paradoxically increased even further after O2, was readmitted. The rate of energy utilization during recovery was 0·30 mequiv. of ∼P/kg/min. Nerves stimulated at 100–200 Hz in O2, exhibited no changes in levels of P-creatine, ATP or lactate, an observation implying that the nerve could not be made to use ∼P faster than oxidation of glucose could provide it. This meant that the maximal metabolic rate was not limited by the rate of supply of chemical energy. Instead, the limitation may have arisen as a result of a limited rate at which ionic imbalance can result from stimulation or a limited pump capacity of the axonal membrane. Nerves stimulated at 200 Hz in O2 for 20 min and then transferred to an O2-free environment without further stimulation exhibited an increase in the rate of energy utilization (nearly two-fold) over the resting rate, a finding that suggested a metabolic (ionic?) debt as a result of activity which could not be met even though the energy supply was adequate. Therefore, restriction of energy expenditure by a limiting pumping rate seemed to be the most likely explanation. The resting metabolic rate of frog sciatic nerve was only one-quarter to one-third of the rate for rat sciatic nerve, when compared at the same temperature (25°C).