Effects of metabolic and transport inhibitors on sodium transport and CO2 production of toad bladders: use of an uncoupling agent to distinguish between effects on transport and metabolism.

Agents that directly inhibit sodium transport secondary reduce the rat of metabolism. Conversely, agents that directly inhibit energy metabolism cause a secondary inhibition of active transport. The aim of the present investigation was to distinguish between these two effects with an uncoupling agent, 2,4-dinitrophenol (DNP). The rates of sodium transport (TNa) and CO2 production (QCO2) by paired urinary hemibladders from Dominican toads were simultaneously measured. DNP alone inhibited TNa, but stimulated QCO2 of the hemibladders, consistent with its action as an uncoupling agent. Ouabain, which directly inhibits sodium transport, inhibited TNa and QCO2 of toad hemibladders; the subsequent addition of DNP stimulated QCO2 to a rate similar to that of the control hemibladders. The metabolic inhibitors antimycin A and rotenone also inhibited TNa and QCO2; in contrast to the results with ouabain, the subsequent addition of DNP did not stimulate QCO2 in the presence of antimycin A or rotenone. Therefore, the respiratory response to DNP clearly distinguishes between direct inhibition of metabolism and direct inhibition of active transport. This approach may be useful for the investigation of the mechanism of action of drugs, hormones and other biological phenomena.