DEVELOPMENTAL ASPECTS OF CALCIUM DISTRIBUTION IN THE TROPHOCYTE-OOCYTE SYNCYTIUM OF THE TELOTROPHIC-MEROISTIC OVARIOLE (DYSDERCUS-INTERMEDIUS)

We describe the intracellular activities of Ca++, K+, Na+, Mg++, and C l- in two compartments of the trophocyte-oocyte syncytium of Dysdercus intermedius (rod cotton bug; Heteroptera: Pyrrhocoridae) using voltag e and ion-selective microelectrodes. Three days after transition to th e adult stage, a difference in the resting potentials (Delta E(m)=E(m( TROPH))-E(m(OOC))=-5.4 mV) has developed within the syncytium between the trophocytes and previtellogenic growing oocytes: The peripheral cy toplasm (''pockets'') of the lateral trophocytes (E(m(TROPH))=-33.3 mV ) being more negative than the central core (E(m(CORE))=-29.7 mV). E(m ) of the previtellogenic oocytes, each attached by a trophic cord to t he base of the central core, averages E(m(OOC))=-27.9 mV. K+, Na+, Mg+, and Cl- are distributed in the syncytium according to their charge and the potential difference between trophocyte and oocyte (Delta E(m) ): K+, Na+, and Mg++ activities are higher in the more negatively char ged pockets of trophocytes than in the ooplasm, whereas the activity o f Cl- is greater in the oocyte than in the trophocyte as expected for a negatively charged ion. Ca++ differs strikingly from these passively distributed ions: in the trophocytes the Ca++ activity (aCa(TROPH)(+)=124 nM) is lower than in the more positively charged ooplasm (aCa(OO C)(++)=275 nM). Such a distribution opposite to the potential differen ce elucidates Ca++ as a candidate generating the potential difference between trophocyte and oocyte, and this implies a calcium current movi ng from the previtellogenic oocytes to the trophocytes, which in turn suggests more active calcium pumping out across the trophocyte membran e and an influx of calcium into the ooplasm. In the complete telotroph ic-meroistic ovariole trophocytes are enveloped by a layer of inner sh eath cells (ISC). During removal of the ISC layer (''skinning''; under the condition of [Ca++](med) =0 mM), E(m(TROPH)) hyperpolarizes in th e magnitude of -7.7 mV during the first 3 min and to E(m(skinned TROPH ))=-42.6 mV after 5 min. This hyperpolarization in the magnitude of De lta E(m(skinned TROPH))approximate to -10 mV is accompanied with a dep letion of intracellular Ca++ down to aCa((skinned TROPH)(++))= 63.4 nM . No permeability for Ca++ across the trophocyte membrane was found wh en conditions were arranged to favor Ca++ influx from the regular in v itro medium ([Ca++](med)=1 mM) into skinned trophocytes (E(m(skinned T ROPH))=-42.6 mV): Hyperpolarization and intracellular Ca++ activity ha rdly changed. Thus, hyperpolarization of the trophocyte membrane under [Ca++](med)= 0 mM conditions reflects an electrogenic Ca++ extrusion rather than diffusion of Ca++. The Ca++-based potential difference acr oss the trophic processes in the syncytium is steep enough to maintain different concentrations of charged solutes contributing to the abili ty of trophocytes to diverge from oocytes in development while sharing a common cytoplasm.