Origin of the short circuit decay profile and maintenance of the cation transport capacity of the larval lepidopteran midgut in vitro and in vivo

Abstract

The larval midguts of Hyalophora cecropia and Manduca sexta contain two primary cell types: a columnar cell and a goblet cell. Employing scanning electron microscopy, goblet cells were found to contain within their cavities semi-viscous matrix plugs. Massive release or removal of goblet matrix plugs is noted following a variety of physiological insults to the tissues, including stretching, gaseous carbon dioxide anesthesia, gut evacuation, gut excision in the absence of cold anesthesia, and mounting the tissue in a chamber designed for the study of cation transport. A reduction in the capacity to actively transport cations in vitro or in vivo follows each of these treatments. When a current (short-circuit current = I SC ) is imposed across the isolated larval midgut that is equal but opposite in direction to the natural electromotive force generated by the tissue, a characteristic irreversible I SC (and potential = P.D.) decay profile is obtained. This decay profile normally consists of three phases : a transient increase in I SC , a rapid decay in I SC and a slower but continuous decay in I SC . The transient increase in I SC is an artifact associated with anoxia. The duration of this transient increase in I SC is related to elapsed time between mounting the midgut in a chamber designed for measuring I SC and the time at which the hemolymph side of the tissue is bathed in oxygenated saline. The rapid decay is associated with massive release of matrix plugs, an increase in membrane K + conductance and a reduced capacity to transport K +. The slower decay is associated with further loss of plugs coupled with cell death. Cell death is caused by inadequacies in the saline normally employed to bathe the midgut epithelium in vitro. These inadequacies promote tissue histolysis and disruption of the normal epithelial topology.