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.