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1. A study has been made of the effects of tetrodotoxin on the impulse activity, resting membrane potential, input resistance, and the generator potential and its after-hyperpolarization of the slowly adapting stretch receptor neurone of the lobster.2. Tetrodotoxin was able to abolish completely within about 2 min the impulse activity in most cells, when given in a dose of 2 x 10(-8) g/ml., but in all cells, when administered in a dose of 4 x 10(-8) g/ml. After blockage by the toxin in concentrations as high as 4 x 10(-6) g/ml. for periods of up to 30 min the action potential was restored by washing the preparation in physiological solution for 1 hr.3. In a concentration of 4 x 10(-8) g/ml. tetrodotoxin produced within 1-2 min an average increase of 4.8 mV of the resting membrane potential and a simultaneous 47% reduction of the resting input resistance. These effects were reversed by washing the preparation in physiological solution for 1 hr.4. Tetrodotoxin administered in doses as high as 4 x 10(-6) g/ml. for periods of up to 30 min had no effect on the amplitude of the steady phase of the generator potential.5. In a concentration of 4 x 10(-8) g/ml. tetrodotoxin produced within 5 min a 65% reduction of the amplitude of the hyperpolarization following the generator potential. This effect was reversed by washing the preparation in physiological solution for 1 hr.6. The simultaneous increase in resting membrane potential and decrease in membrane resistance is suggested to be due to an elevated potassium permeability besides a reduced sodium conductance. The constancy in height of the generator potential in the presence of a decreased membrane resistance makes necessary the assumption of an augmented generator current. The decrease in amplitude of the hyperpolarization following the generator potential may be the result of an increase in potassium conductance and/or a reduction in acceleration of an electrogenic pump in consequence of a diminished sodium influx during the generator potential.

Tetrodotoxin in concentrations of 0.8–1.0 × 10-7 g/ml completely blocked conducted activity of the afferent nerve of isolated frog muscle spindle without affecting the receptor potential. Development of block was characterized by an increase in threshold of the nerve, a gradual delay of the onset of the impulse response and a reduction and decomposition of the individual action potentials into smaller spikes. The spike decomposition appeared to be due to a de-synchronization of the conducted activity in the branched afferent terminals. After block by tetrodotoxin for up to 20 min the normal response could be restored by washing the preparation with Ringer's solution. The blocking action of tetrodotoxin was delayed and reduced by increasing calcium; removal of calcium had the opposite effect. Tetrodotoxin (1000 ng/ml) had no significant effect on the receptor potential. With low sodium plus tetrodotoxin the receptor potential was reduced by about 50 %; this reduction was reversed by addition of normal sodium. It is suggested that the sodium carrying system of the generator potential differs from that underlying the regenerative activity.

The effects of phospholipase C on the muscle membrane were studied by the recording of the resting membrane potential, the input resistance and the action potential generation in single fibres of the isolated extensor digitorum longus and soleus muscles of the rat. Incubation with phospholipase C abolished spike generation, depolarized the fibres by 15–20 mV and increased the ionic permeability of the membrane as shown by a fall of about 40 per cent of the “input” resistance. Removal of the enzyme by washing the preparation with Krebs-Ringer solution did not restore membrane exitability, the resting membrane potential nor the input resistance of the fibres. When the external calcium concentration was raised from the normal 2 mM to 15 mM the muscle fibres repolarized and action potentials with a reduced rate of rise and amplitude were recorded. No recovery of the input resistance occurred. When the calcium concentration was brought back to 2 mM the fibres again depolarized and became inexcitable. Potassium (10 or 15 mM), caesium (10 mM) or rubidium (6 mM) antagonized the aforementioned effects of 15 mM calcium. It was concluded that phospholipase C irreversibly affected the muscle membrane and that an increase in the external calcium ion concentration only functionally restored some of the membrane properties, i.e., spike generation and the resting membrane potential. The results support the view that the hydrophilic head of certain membrane phospholipids is connected with passive ion transport in the electrogenic membrane.

Albuquerque, E. X. and S. Thesleff. Effects of phospholipase A and lysolecithin on some electrical properties of the muscle membrane. Acta physiol. scand. 1968. 72. 248–252. The effects of incubation with phospholipase A and lysolecithin on the resting membrane potential, action potential, input resistance and acetyleholine sensitivity of isolated extensor digitorum longus and soleus muscles of the rat were compared with those previously obtained with phospholipase C. A common action of the agents studied were that they in sufficient doses depolarized the muscle fibres and reduced the input resistance. In contrast to phospholipase C which irreversibly blocked action potential generation, phospholipase A and lysolecithin had little effect on membrance excitability. The sensitivity of chronically dencrvated muscle fibres to locally applied acetylcholine was unaffected by the two phospholipascs and by Iysolccithin. The difference in action between phospholipasr C and phospholipase A on electrogenic membrane excitability is suggested to be explained by the respective mode of lipolytic action of the two enzymes. Phospholipasc C hydrolyses the diglyceride — phosphate, ester linkage in phosphatides and thereby removes the phosphate head from the molecult while phospliolipase A cleaves one fatty acid ester linking leaving the polar heads of phospholipids intact. Lysolecithin possibly rearranges membrane structures by its high surface activity.