Potassium transport in the isolated pharate pupal integument of the tobacco hornworm,Manduca sexta

Abstract

Molting fluid accumulates in the exuvial space beneath the old cuticle between 24 and 12 h before the larval-pupal ecdysis in pharate pupae ofManduca sexta. Just before the molt (9 to 3 h), however, molting fluid disappears from the exuvial space. The integument separates the molting fluid (K+=140 mM) from the hemolymph (K+=35 mM). We have studied potassium transport across isolated, short-circuited preparations of the integument. In 32 mM potassium saline, potassium is actively transported in the hemolymph to exuvial direction at 0.27 and 0.34 μEq cm−2h−1 in preparations isolated at 15 and 6 h before the molt, respectively. The open-circuit potential difference (exuvial side positive) is 5.6 and 14.4 mV at 15 and 6 h before the molt, respectively. The short-circuit current, which can be accounted for by net potassium transport at 15 h before the molt, can not be completely accounted for by net potassium transport at 6 h before the molt (short-circuit current is 7.5 and 13 μA cm−2 at 15 and 6 h before the molt, respectively). At 15 h before the molt the values forKm andJmax are 15.2 mM and 0.4 μEq cm−2 h−1 and at 6 h before the molt these values are 32 mM and 0.7 μEq cm−2 h−1. The short-circuit current is not inhibited by ouabain, thallous ions, and acetazolamide, but is inhibited by 100% N2. During the period between 15 and 6 h before the molt, the integument undergoes a number of developmental changes. These are reflected in the tanning of the cuticle, the changes in the electrical properties, such as the increase in resistance, and the changes in the kinetic properties of potassium transport. The integument appears to play an active role in regulating the composition of the molting fluid, but its role in fluid transport remains to be determined.