RAPID ACTIVATION OF CALCIUM-SENSITIVE NA+ H+ EXCHANGE INDUCED BY 20-HYDROXYECDYSONE IN SALIVARY-GLAND CELLS OF DROSOPHILA-MELANOGASTER/

Ecdysteroids play an important role in the larval moulting process of insects. 20-Hydroxyecdysone (20E) causes the -induction of specific 'p uffs' in polytene chromosomes of Drosophila melanogaster salivary glan d cells. Although it is known that inorganic ions control pretransorip tional processes in the cell nucleus, the intracellular mechanisms of gene activation are still unclear. Therefore, we examined the effects of 20E on plasma membrane ion transport of Drosophila melanogaster sal ivary gland cells. Isolated glands of the third larval stage were supe rfused with a solution mimicking the haemolymph. The relative K+ condu ctance of the cell membrane (t(K+)) was measured with microelectrodes by performing ion substitution experiments. Under control conditions t (K+) averaged to 0.16 +/- 0.02 (n = 15). Addition of 5 x 10(-6) M 20E increased t(K+) within 2 min by 19.1 +/- 4.2% (n = 15). This rapid res ponse to 20E was elicited only in the presence of calcium. Moreover, s tarting from a steady-state intracellular pH of 7.20-7.60, 20E induced a rise in cytoplasmic pH by 0.27 +/- 0.06 (n = 6) within minutes. Ami loride (10(-3) M), a blocker of plasma membrane Na+/H+ exchange, preve nted the 20E-induced intracellular alkalinization. We conclude that 20 E activates a calcium-sensitive plasma membrane Na+/H+ exchange leadin g to a rise of plasma membrane K+ conductance and intracellular alkali nization both being prerequisites for steroid hormone induced gene act ivation.