Unequal depolarization of the membrane of the rat diaphragm muscle fibres caused by veratrine.

The effect produced by veratrine on transmembrane potential was investigated in five distinct regions of the isolated rat diaphragm blocked with either d-tubocurarine or alpha-bungarotoxin. It was found that small (0.4 micrograms/ml) and large (2.0 micrograms/ml) doses of veratrine depolarize only two or three of these regions. With the use of a very large (10.0 micrograms/ml) dose depolarization occurred in all five regions of the diaphragm but the effect was much larger in those areas of the muscle fibre membrane which were selectively depolarized by the smaller ones. These results clearly indicate either an unequal distribution of sodium channels activated by veratrine or differences in sodium channel density in distinct areas of the muscle fibre membrane. Supersensitivity of veratrinized muscles to potassium was confirmed. However, the sensitiveness to the depolarizing action of potassium was only increased in the regions of the diaphragm partially depolarized by veratrine. This suggests that partial depolarization of the muscle fibre membrane was actually the cause of the supersensitivity. The possible involvement of potassium and of the unequal depolarization of the muscle fibre membrane in the veratrine response is considered.