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Journal of Experimental Biology 150,425-442 (1990)
Published by Company of Biologists 1990

Ion Transport Across the Midgut of the Tobacco Hornworm (Manduca Sexta)

M. E. CHAMBERLIN ¹

¹ Department of Zoological and Biomedical Sciences and College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA; Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G2W1

1.The transport of K⁺, Na⁺ and Cl^- across the three morphologically distinct regions of the tobacco hornworm midgut was measured under open-circuit and short-circuit conditions. Using a saline which contained physiological levels of haemolymph ions, amino acids and sugars, it was shown that all three sections actively secrete K⁺ and Cl^- and absorb Na⁺.

2. The anterior section maintained the highest short-circuit current (I_sc), transepithelial potential difference (PD) and net K⁺ secretion. The middle section had the lowest I_sc, PD and K⁺ secretion, but absorbed Na⁺ at the greatest rate. The posterior section had the greatest rate of Cl^- secretion.

3. Omission of K⁺ depressed the I_sc. Subsequent addition of K⁺ stimulated the I_sc to control levels in the middle and posterior sections, but not in the anterior section. Omission of Cl^- or Na⁺ also inhibited the I_sc. Reintroduction of Cl^- had no stimulatory effect and, although reintroduction of Na⁺ stimulated the I_sc, control levels were not attained.

4. Unlike the results reported in previous studies, the net K⁺ transport exceeded the I_sc in all three midgut sections. The deficit in I_sc was not made up by the transport of Na⁺ and Cl^- . The results are discussed with respect to proposed models of ion transport across this epithelium.

Key words: tobacco hornworm, midgut, short-circuit current, transepithelial ion transport.

Accepted on January 23, 1990

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