EXTRACELLULAR AND INTRACELLULAR HYDROGEN ION CONCENTRATION IN RELATION TO ANESTHETIC EFFECTS OF BARBITURIC ACID DERIVATIVES

¹ From the Lilly Research Laboratories, Indianapolis, Indiana, and The Marine Biological Laboratory, Woods Hole, Massachusetts

1. Certain theoretical considerations have been presented in an attempt to interpret the gradations in anesthetic effect which are exerted by substituted barbituric acids on fertilized eggs and larvae of Arbacia punctulata at varying levels of extracellular and intracellular pH.

2. The apparent dissociation constants at an ionic strength equivalent to that of sea water are presented for 30 substituted barbituric acids.

3. For each of the substituted barbituric acids investigated it is found that the extracellular concentrations of undissociated molecules required to produce a 50 per cent reduction in the rate of cell division of fertilized Arbacia eggs is independent of the extracellular pH. A similar result is obtained for the action of two barbituric acids on the movement of Arbacia larvae. These observations indicate that the barbiturates penetrate the fertilized eggs and larvae of Arbacia only in the form of undissociated molecules.

4. For each of two representative barbituric acids investigated it is found that the extracellular concentration of undissociated molecules required to produce a 50 per cent reduction in the rate of cell division of fertilized Arbacia eggs is not affected by a decrease in the intracellular pH. For each of three representative barbituric acids investigated it is found that the extracellular concentration of undissociated molecules required to produce a 50 per cent reduction in the rate of cell division is also not affected by an increase in the intracellular pH. These observations indicate that it is the barbituric acid molecule and not the barbiturate anion which is active inside the cell in producing the anesthetic effect; thus, the barbiturates are to be classified with non-ionizable anesthetics like the ethers and alcohols in respect to their mechanism of action.

5. The extracellular concentrations of undissociated molecules (which are nearly equivalent to the respective intracellular concentrations of undissociated molecules) required to produce a 50 per cent reduction in the rate of cell division of fertilized Arbacia eggs have been measured for 30 barbituric acids containing various substituents in the 1 and 5 positions of the molecule. These required molecule concentrations vary from 0.72 x l0^–4 M for 5-ethyl-5-(-ethylhexyl) barbituric acid up to 125 x 10^–4 M for 5,5-diethyl barbituric acid.

6. It is observed that there is a partial, but incomplete, parallel between the order of decreasing anesthetic effectiveness, the order of decreasing oil-water distribution coefficients, and the order of increasing solubility in water.