PH BUFFERING IN MUSCA-DOMESTICA MIDGUTS

The M. domestica midgut displays three morphological regions with the following luminal pH values: anterior, 6.1; middle, 3.1; posterior, 6. 8. Looking for enzymes that might be related to the acidification of m iddle midguts or to the neutralization of luminal contents in the ante rior and posterior midguts, M. domestica larvae were placed on layers of 10% starch gels containing 0.1% Congo Red or 0.1% lacmoid and one o f the following compounds: acetazolamide, vanadate, KSCN, NaF, ouabain , calcium acetate, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, or amiloride. None of these compounds induced alteration in anterior or posterior midgut pH, whereas acetazolamide, vanadate, NaF, and ouab ain affected luminal pH in middle midgut. Ammonia and phosphate predom inate in anterior and posterior midguts, chloride concentrates in midd le midgut, and bicarbonate occurs in low concentration along the midgu t lumen. Ouabain and vanadate cause a relative decrease of chloride in the middle midgut and an increase of ammonia mainly in the posterior midgut. The results suggest that chloride may follow the protons trans located into middle midgut contents by a type-P ATPase, similar to tha t found in vertebrate stomachs, which is inhibited by high intracellul ar concentration of Na+ . Ammonia probably neutralizes the contents of the anterior and mainly posterior midguts and is secreted by mechanis m probably involving an, apical Na+/K+-ATPase.