The effect of ooplasmic pH regulation on the formation of yolk spheres in the telotrophic ovariole of Dysdercus intermedius

Abstract

Vitellogenic oocytes of Dysdercus intermedius (Heteroptera: Pyrrhocoridae) were treated with the proton ionophore monensin in order to load the ooplasm with protons along the electrochemical gradient. Additionally, changes in the ooplasmic pH ( ΔpH OOC ) were recorded during exposure the oocytes to potassium-free medium ( K + MED =0 mM; choline for K +) or sodium-free medium ( Na + MED =0 mM; 40 mM of choline for 40 mM of Na +). The following observations were made: 1) The average ooplasmic pH ( pH OOC ) recorded during immersion in physiological saline solution (PSS) was pH OOC(PSS) =7.40. 2) K + MED had no effect on pH OOC ( pH OOC(K-FREE)≅pH OOC(PSS) ). 3) In sodium-free medium the pH OOC decreased by H + influx in the magnitude of ΔpH OOC(Na-FREE)=pH OOC(Na-FREE)−pH OOC(PSS) =−0.05 pH units. 4) The decreased pH OOC observed in sodium-free medium returned to initial values (7.40) by pumping out H + when 40 mM of choline were replaced by 40 mM of Na +. 5) Addition of monensin (10 μM; under the condition of Na + MED =0 mM) reduced pH OOC in the magnitude of ΔpH OOC(MON)=pH OOC(MON)−pH OOC(PSS) =−0.14. 6) Monensin induced ooplasmic proton loading was reversible when 40 mM choline were replaced by 40 mM Na +. Vitellogenesis was demonstrated by the accumulation of fluorescence labelled hemolymph proteins in yolk spheres in the cortex of the oocyte: 1) Yolk formation continued in potassium-free medium. 2) The formation of yolk spheres came to a halt in sodium-free medium and, additionally, in the presence of monensin (10 μM; Na + MED =0 mM). 3) Breaks in yolk formation under the condition of Na + MED =0 mM or during monensin treatment were stopped by replacing 40mM of choline with 40 mM of Na +. The results obtained using proton-specific microelectrodes and the in vitro assay to detect the formation of yolk spheres indicate that both the ooplasmic pH regulation and the acidification of vesicles during vitellogenesis are under control of a H +/Na + antiporter.