Yolk formation in the telotrophic-meroistic ovariole of Dysdercus intermedius depends on extracellular sodium

Abstract

With the present study we attempted to integrate electrophysiological results with effects of ion substitution in the vitellogenic oocyte of Dysdercus intermedius. The permeability of the oocyte membrane to ions in the in vitro medium was determined by equimolar substitutions of K + or Na + by choline; Ca ++ by Mg ++ Mg ++ by Sr ++, and by sulfate for Cl −. The impermeability of the oocyte membrane to Na +, Ca ++, and Mg ++, as opposed to the permeability of K + and Cl −, renders the latter two as the major ions that contribute to the membrane potential ( E m ) of −34 mV. Intracellular activities of K +, Na +, Ca ++, Mg ++ and Cl − were measured with ion selective microelectrodes. Liquid Ion Exchange (LIX) pH microelectrode determination revealed an ooplasmic pH of 7.4 which is 0.6 pH units higher than the pH (6.8) of the in vitro medium. Variations of pH med was of no influence on pH ooc. We visualised the effects of ion substitution on yolk formation by in vitro uptake of rhodamine-labeled hemolymph proteins derived from vitellogenic females which results in the formation of large fluorescent yolk spheres. Cl − replacement did not affect the protein uptake. However, in the absence of Ca ++ yolk formation was restrained. Replacement of external K + by choline did not influence protein uptake when [Na +] med was present, but resulted in a marked inhibition of yolk formation when in vitro medium was Na +-free. Thus this process appears to depend on extracellular Na + rather than intracellular or extracellular K +.