Alkalinization by Manduca sexta anterior midgut in vitro:: requirements and characteristics

Manduca sexta anterior midgut alkalinizes its luminal contents in vivo, achieving pH values up to 12 with rates of up to 81.4 mu Eq OH- h(-1). The alkalinization rate dropped to 3.0 mu Eq h(-1) within minutes of mounting the tissue in an Ussing chamber, even though K+ secretion(as measured by short-circuit transepithelial current and open circuit voltage) continued at a high rate for several hours. In contrast, cannulation of the anterior midgut in situ resulted in a preparation that alkalinized at rates up to 127 mu Eq h(-1) for several hours. Removal of the head or ventral nerve cord did not interrupt alkalinization. However, alkalinization 'could be blocked by increasing the volume of the saline in which the basal side was bathed. The buffer capacity of the saline had no effect on the maximum pH attained or the rate of base secretion. Doubling of the drip rate from 0.5 to 1.0 ml min(-1) had no effect on the maximum rate of OH- secretion but resulted in a reduction of the maximum pH attained from I1 to 8.5. These data suggest release of an alkalinization factor from the basal aspect of the gut that may act by altering the passive H+ permeability of the tissue. (C) 1998 Published by Elsevier Science Inc. All rights reserved.