Role of ATP in respiratory control and active transport in tobacco hornworm midgut.

The intracellular ATP, ADP, AMP, and orthophosphate (Pi) levels were measured in the midgut of Manduca sexta. The nucleotide levels were identical in tissues either "fresh" frozen or equilibrate in regular (32 mM) K or low (8 mM) K solutions. The calculated [ATP]/[ADP][Pi]ratio was approximately 300 M-1, which is low compared to other tissues. Given the ability of this ratio to control the respiratory rate, it is speculated that this low value may cause the maximal uncontrolled respiration normally observed in the midgut. The kinetics to anoxia of active transport (Isc) and the redox level of the mitochondrial cytochromes were measured simultaneously in the midgut. The cytochromes became reduced with a time constant of 0.75 +/- 0.15 min, whereas that for Isc inhibition was 2.1 +/- 0.15 min after a delay of 0.25 min. The difference between these two kinetic rates indicates that an intermediate form of energy exists in this tissue to energize active K transport. Measurements of ATP levels during the transition to anoxia indicate that its decay kinetics are sufficiently slow for ATP to be the immediate energy source for active transport in this tissue.