YOLK FORMATION IN THE TELOTROPHIC-MEROISTIC OVARIOLE OF DYSDERCUS-INTERMEDIUS DEPENDS ON EXTRACELLULAR-SODIUM

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 ion s in the in vitro medium was determined by equimoIar substitutions of K+ or Na+ by choline; Ca++ by Mg++, Mg++ by Sr++, and by sulfate for C l-. The impermeability of the oocyte membrane to Na+, Ca++, and Mg++, as opposed to the permeability of K+ and Cl-, renders the latter two a s the major ions that contribute to the membrane potential (E(m)) of - 34 mV. Intracellular activities of K+, Na+, Ca++, Mg++, and Cl- were m easured 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, Varia tions of pH(med) was of no influence on pH(ooc). We visualised the eff ects of ion substitution on yolk formation by in vitro uptake of rhoda mine-labeled hemolymph proteins derived from vitellogenic females whic h results in the formation of large fluorescent yolk spheres, C1- repl acement did not affect the protein uptake, However, in the absence of Ca++ yolk formation was restrained. Replacement of external K+ by chol ine 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 f rather than intracellular or extracellular K+. Copyright (C) 1996 El sevier Science Ltd