Lidocaine and Barium Distinguish Separate Routes of Transbasal K⁺ Uptake in the Posterior Midgut of the Tobacco Hornworm (Manduca Sexta)

The isolated posterior midgut of the tobacco hornworm maintains a vigorous transepithelial K⁺ transport from the hemolymphal side to the lumen side at a rate accurately measured by its short-circuit current. Previous studies using the K⁺ channel blocker Ba²⁺ suggested that partial inhibition of the short-circuit current by hemolymphal Ba²⁺ was due to blockage of one of at least two parallel transbasal entry routes for K⁺ into the intracellular transport pool. The present studies show that the local anesthetic lidocaine, at a concentration of 5 mmol I^-1 on the hemolymphal side, partly inhibits net transepithelial K⁺ transport. The inhibition is accompanied by hyperpolarization of the basal membrane and an increase in transbasal resistance, suggestive of a block of transbasal K⁺ conductance. The effects of lidocaine and Ba²⁺ are additive, suggesting that the inhibitors distinguish separate, parallel K⁺ uptake processes.