The influence of barbiturate drugs on tetraethylammonium induced contractures of the chick biventer cervicis muscle in vitro.

Methohexitone sodium (METHO) 8.8 X 10(-5) M markedly potentiates tetraethylammonium (TEA)-induced contractures of the chick biventer cervicis muscle in vitro, (Elliott, 1979). A group of barbiturates were compared with METHO for their ability to potentiate TEA induced contractures. The concentrations of TEA which produced a 50% maximum contracture (EC 50) in the presence of METHO (8.8 X 10(-5) M) and in the optimum potentiating concentrations of the other barbiturates were determined. The EC 50 ratios (= EC 50 METHO/EC 50 test drug) were: barbitone sodium 0.25, phenobarbitone sodium, 0.20, pentobarbitone sodium 0.90, thiopentone sodium, 2.43. The concentrations of the barbiturates used were: barbitone 3.1 X 10(-2) M, phenobarbitone 3.15 X 10(-3) M, pentobarbitone 6.04 X 10(-4) M, thiopentone 1.89 X 10(-4) M. The EC 50 ratios were adjusted by dividing by the ratio [test barbiturate]/[METHO]. The adjusted ratios gave the following potencies (METHO = 1), barbitone 7.13 X 10(-4), n = 6, phenobarbitone 5.58 X 10(-3), n = 8, pentobarbitone 1.33 X 10(-1), n = 6, thiopentone 1.13, n = 6. The potencies correlate well with the partition coefficients of the barbiturates between cell membranes and buffer (P m/b). The regression y = 1.49x -3.12, where y = log (adjusted ratio) and x = log (P m/b) fits data with a correlation coefficient of 0.99.