Action of thyroxine on active transport in isolated membranes of Bufo bufo

Thyroxine in a near-physiological concentration causes a 50% increase in active sodium transport across the isolated skin and bladder of the toad within one hour of application of the hormone. This has been measured both electrically and by using isotopic techniques. The application of thyroxine also results in an increase in oxygen consumption, both oxygen consumption and sodium transport increasing to their maxima after the same time interval. It is shown that this rise in respiration greatly exceeds that required by the increased, energy-dependent, active transport mechanism. This implies that thyroxine not only stimulates the phenomena involved in the transport process, but also accelerates those enzymatic processes concerned directly or indirectly with general respiratory metabolism of the cell. A relationship is suggested between the results of the present in vitro work and in vivo studies on the physiology of moulting, where it is known that thyroxine is related to moulting and may be responsible for higher sodium influx at this time. It is suggested from the data that thyroxine causes alterations in the cell membranes and that this may be the fundamental action of the hormone.