The effects of sodium, potassium and ATP on a developmental puff sequence in Drosophila salivary glands in vitro

Salivary glands of late third instar larvae of Drosophila melanogaster were isolated at a developmental stage when the release of ecdysone had already taken place. They were then incubated in a chemically defined medium. An ecdysone-dependent developmental puff sequence was measured in vitro and influenced by adding various substances or by changing iso-osmotically the sodium or potassium content of the medium. Trinactin, valinomycin, N-ethylmaleimide and KCN blocked the puff sequence, i.e. the regression of the early and the induction of the late ecdysone-dependent puffs probably by increasing the Na+ influx and depleting the ATP content of the cell. A medium that contained Na+ as the only monovalent cation decreased the size of the late ecdysone-dependent puffs and increased the size of other puffs, such as 50 CD. Addition of tetrodotoxin to the normal medium had the opposite effect, i.e., it increased 63 E and inhibited 50 CD. Na+ free medium, inhibition of K+ flux by tetraethylammonium chloride, and application of ouabain did not considerably influence the size of the puffs measured. It is concluded from these results that Na+ in particular has an inhibitory effect on the induction of late ecdysone-dependent puffs. Na+ (and perhaps also K+) may act by modulating the effect of proteins that are involved in gene control mechanisms.