Extremely high pH in biological systems: a model for carbonate transport.

The pH profiles along the midgut of several lepidopteran species are described in detail. The peak pH recorded (12.0) is the highest observed in a living system and is too high to be achieved by conventional bicarbonate transport. By calculating the rate of net alkali transport in vivo, it is shown that the midgut transports equal amounts of potassium and alkali from blood to gut lumen. It is suggested that the specialized goblet cavities in lepidopteran midgut allow coupling of electrogenic transport of potassium with the electrostatic removal of protons from transported bicarbonate, producing a unique potassium carbonate secretion system.