Abstract
The isolated midgut of a lepidopteran larva (Manduca sexta), 5th instar was investigated with voltage-clamp and fluctuation analysis techniques. With high K+ insect saline on both sides the outward-directed short-circuit current (Isc) was carried by K+ (IK) from serosal to mucosal compartment. IK could be blocked, in a dose-dependent manner by serosal Ba2+ ions. There was no current with serosal Na+. Noise analysis of IK revealed a Lorentzian component in the power spectrum when Ba2+ was present in the serosal solution. The Ba2+/receptor kinetics show pseudo-first order characteristics only at low [Ba2+]s. For [Ba2+]s greater than KBa, the apparent Ba2+ association rate decreases with a hyperbolic course as a function of serosal [Ba2+] which could indicate some "substrate-inhibition"-like interaction of Ba2+ at its receptor site. It is concluded that the serosal membranes of the K+-secreting intestinal cells contain the common type of Ba2+-blockable K+ channel which provides the serosal pathway for K+ during secretion which is ultimately driven by the mucosally-located electrogenic K+-ATPase.
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