Calcium and strontium ionic flux of lipid bilayers
Bilayer lipid membranes were formed from lecithin,. cerebroside, ceramide and tripalmitin. The electrical potential across the lecithin and cerebroside membranes were measured in the presence of Na+, Ca++ and Sr++, and from these membrane specific resistance, transference number and ionic flux were calculated for each membrane in the presence of each ion. The electrical properties of the lecithin membrane in the presence of Na+ were comparable with literature values. The measured potential of the lecithin membrane in presence of ·the various ions were in the order of Sr-H->ca:++>Na+, and for the cerebroside membrane the order was' Sr*>Ca++, Na+. No significant differences were found in the transference numbers of these ions in the two membrane systems. The ionic fluxes across the lecithin membrane·were in the order Na+>Ca++>Sr++ and those for the cerebroside membrane were Na+>Ca++, Sr++. The water permeabilities and ionic fluxes were measured for the several membranes. The water permeability coefficient of lecithin membrane was comparable with that of dog red blood cell. Ca++ fluxes across membranes showed the order of cerebroside-lecithin>lecithin> cerebroside>ceramide>tripalmitin, and sr++ fluxes followed the order: lecithin, cerebrosidelecithin> cerebroside>ceramide, tripalmitin. Sr++ flux across cerebroside membranes was higher than'the Cq.;+ flux across this membrane. Ca* fluxes across c.eramide and cerebrosidelecithin membranes were higher than Sr* fluxes across these membranes. Comparing the flux of each ion in either experiment set, indicated that cation permeselectivity of the membrane depends on the field strength of the membrane negative charges, and ion sieving. It is also dependent on cationic site interaction, site spacing, entropy effects, and hydration energy. The isotopically measured flux was about eight orders of magnitude larger than the flux calculated from the electrical potential. A two component mechanism of Ca++ and Sr-++ transport across the membrane was postulated: (1) linear flux through the lipid kinks and (2) a carrier mediated flux as a result of binding of these cations to lipid molecules accompanied by a "flip-flop" mechanism to transport the ions to the other side of the membrane. This hypothesis agreeswith postulated mechanism of Ca++ and Sr++ transport across intestinal membranes.