Segmental differences in secretion by the Malpighian tubules of the fresh water dwelling corixid, Cenocorixa blaisdelli (Hung.) (Corixidae, Hemiptera)

Abstract

Fluid and ion secretion in the four segmented Malpighian tubules of the fresh water dwelling corixid, Cenocorixa blaisdelli (Hung.) was studied. Rate of secretion increased with increasing numbers of segments. Segment II (distal from midgut) contributed 40% of fluid secretion, and segment III and IV contributed 30% each. The length-specific rate of secretion for segments II–IV were 22.8, 3.8 and 27.4 nl mm −1 h −1, respectively. Ion concentrations did not change with increasing number of segments, but the highest rate of ion secretion per mm length of segment occurred in segment IV. An increase in pH of secreted fluid was found to occur in segment II from 7.00 to 8.07, and the alkalinity was correlated with an increase in total carbon dioxide, suggesting that bicarbonate is secreted, the first indication of bicarbonate secretion in Malpighian tubules. Acetazolamide (0.1 mmoll −1) in the bathing solution blocked both the increase in pH and total carbon dioxide. Mean transepithelial potential was 5.6 ± 2.2 mV (lumen positive) and indicated that anions could diffuse into the lumen passively. These measurements were compared with values of ion and fluid secretion of various segments of the Malpighian tubules of the alkaline-lake-dwelling C. bifida (Hung.) under similar in vitro conditions.