Influence of pH on the inhibitory action of local anaesthetics on smooth muscle contraction.

1. The guinea-pig isolated ileum kept in a calcium-free depolarizing solution contracts when calcium is added; the responses are proportional to the concentration of calcium and are reversed by washing.2. The contractions elicited by calcium are inhibited by three different classes of compounds: local anaesthetics, pentobarbitone and ethanol. The inhibitory action of the local anaesthetics increases with an increase in pH, the inhibitory action of pentobarbitone increases with a decrease in pH and the inhibitory action of ethanol is not affected by variations in Tyrode pH.3. It is suggested that the antagonists are active when unionized and that the inhibition is obtained when unionized compounds have dissolved in the lipid material of the cell membrane. The possible site of action of the antagonists is discussed.