Rapid activation of calcium-sensitive [formula omitted] exchange induced by 20-hydroxyecdysone in salivary gland cells of Drosophila melanogaster

Abstract

Ecdysteroids play an important role in the larval moulting process of insects. 20-Hydroxyecdysone (20E) causes the induction of specific ‘puffs’ in polytene chromosomes of Drosophila melanogaster salivary gland cells. Although it is known that inorganic ions control pretranscriptional 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 salivary gland cells. Isolated glands of the third larval stage were superfused with a solution mimicking the haemolymph. The relative K + conductance 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 × 10 −6 M 20E increased t K+, within 2 min by 19.1 ± 4.2% ( n = 15). This rapid response to 20E was elicited only in the presence of calcium. Moreover, starting 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. Amiloride (10 −3 M), a blocker of plasma membrane Na + H + exchange, prevented the 20E-induced intracellular alkalinization. We conclude that 20E activates a calcium-sensitive plasma membrane Na + K + exchange leading to a rise of plasma membrane K + conductance and intracellular alkalinization both being prerequisites for steroid hormone induced gene activation.