Cyclic Nucleotides, Calcium and Ion Transport

Abstract

Fluid transport across epithelia plays an important role not only in osmoregulation, but also in digestion. In many cases, the rate of fluid movement is carefully regulated by hormonal or nervous stimuli. Although the nature of the hormones and nerves responsible for this regulation have been characterized, not much is known about how these various stimuli bring about the very large changes in fluid flow which are seen during stimulation of such epithelia as the pancreas or mammalian salivary gland. One reason for our ignorance about how epithelia are controlled stems from the fact that we still know very little about how ions and water are transported across most epithelia. In order to unravel the control mechanisms within epithelial cells, it is first necessary to understand the basic features of the transport mechanisms. Such essential information has been difficult to obtain for most vertebrate epithelia because of their structural complexity. Such problems are greatly reduced in various insect epithelia such as the salivary glands of adult blowflies. These insect salivary glands not only have a relatively simple structure (1) but they are particularly amenable to in vitro studies which have provided insights into both the mechanism and the control of secretion.