Effects of DNP on active transport processes and cell membranes in the Malpighian tubule of Formica

Abstract

In Formica Malpighian tubules KCl secretion is driven by a V-type H+ ATPase in the luminal membrane in parallel with a H+/K+ antiporter. The effect of the protonophore dinitrophenol (DNP) was investigated on the isolated, symmetrically perfused tubule. DNP was applied in two different concentrations: 0.2 mmol/l and 1 mmol/l. The effects were fast and rapidly reversible. The equivalent short-circuit current (I sc) was reduced significantly to respectively 25±3% Cn=4) and –3±7% (n=11) of the control value when 0.2 mmol/ l or 1 mmol/l was added to the bath. When 1 mmol/l DNP was applied the transepithelial resistance (R te) decreased significantly to 74±11% of the control value (n=11), and the luminal over basolateral voltage divider ratio (VDR), providing an estimate of luminal over basolateral membrane resistance, decreased to 37±12% of the control (n=6). A concentration of 1 mmol/l DNP was also applied from the lumen. The decrease in I sc was significant, but much less pronounced (74±5% of control; n=6) and no significant changes in R te and VDR were observed. It is argued that, when the concentration in the bath is high enough, DNP may cross the cell and have a protonophoric effect not only on the mitochondria but also across the luminal cell membrane explaining the drop in transepithelial and in relative luminal membrane resistance. The diminished effectiveness of DNP, when applied from the luminal side, suggests that the luminal membrane is somehow less permeable to toxic substances, but that DNP very rapidly enters the cell via the basolateral membrane and may bring about an initial protonophoric effect across this membrane.