Coupling between oxidative metabolism and active transport in the midgut of tobacco hornworm.

Active K transport (Isc) in the midgut of tobacco hornworm Manduca sexta has been shown to be highly dependent on oxidative metabolism. However, the oxygen consumption rate (rO2) was not altered by conditions that drastically affect Isc. Respiration was normally maximal, inasmuch as uncouplers did not increase rO2. This rate could be maintained without any added substrate probably by oxidation of endogenous substrates. Additional succinate increased rO2 by 17%. Simultaneous monitoring of Isc and the redox level of the respiratory chain components demonstrated that 1) succinate (5 mM) reduced all the respiratory enzymes while increasing Isc by 17%; 2) sesamol (5 mM), a mitochondrial uncoupler, reoxidized all respiratory enzymes and inhibited Isc by about 50%; 3) cyanide (1 mM) fully reduced the cytochromes and completely inhibited Isc. These redox responses indicate that the mitochondria in this tissue are normally coupled, even if respiration is maximal and is not modulated by active transport. Mitochondria isolated from the midgut show coupling and respiratory control by ADP, appearing to behave like mitochondria from other tissues. Therefore, a cytoplasmic constraint must exist in this tissue that continually elicits an unmodulated maximal respiratory rate.

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.

Dilantin-calcium interaction and active Na transport in frog skin.

The effects of diphenylhydantoin (DPH) and calcium were simultaneously studied on solute transport in frog skin. In the presence of external Ca, DPH elicited a significant increase in active Na transport; in its absence, DPH had no significant effect on active transport. DPH increased passive permeability, but this action was independent of Ca. It appears that the increase in active transport elicited by DPH is due to a specific interaction with Ca at the transport site.