EFFECT OF OSMOLARITY ON LDL BINDING AND INTERNALIZATION IN HEPATOCYTES

The present study has been performed to elucidate a possible role of c ell volume in low-density lipoprotein (LDL) binding and internalizatio n (LDLb+i). As shown previously, increase of extracellular osmolarity (OSMe) and K+ depletion, both known to shrink cells, interfere with th e formation of clathrin-coated pits and thus with LDLb+i. On the other hand, alterations of cell volume have been shown to modify lysosomal pH, which is a determinant of LDLb+i. LDLb+i have been estimated from heparin-releasable (binding) or heparin-insensitive (internalization) uptake of I-125-labeled LDL. OSMe was modified by alterations of extra cellular concentrations of ions, glucose, urea, or raffinose. When OSM e was altered by varying NaCl concentrations, LDLb+i decreased (by 0.5 +/- 0.1%/mM) with increasing OSMe and LDLb+i increased (by 1.2 +/- 0. 1%/mM) with decreasing OSMe, an effect mainly due to altered affinity; the estimated dissociation constant amounted to 20.6, 48.6, and 131.6 mu g/ml at 219, 293, and 435 mosM, respectively. A 25% increase of OS Me increased cytosolic (by 0.46 +/- 0.03) and decreased lysosomal (by 0.14 +/- 0.02) pH. Conversely, a 25% decrease of OSMe decreased cytoso lic (by 0.28 +/- 0.02) and increased lysosomal (by 0.17 +/- 0.02) pH. Partial replacement of extracellular Na+ with K+ had little effect on LDLb+i, although it swelled hepatocytes and increased lysosomal and cy tosolic pH. Hypertonic glucose, urea, or raffinose did not exert simil ar effects despite a shrinking effect of hypertonic raffinose. Monensi n, which completely dissipates lysosomal acidity, virtually abolished LDLb+i. In conclusion, the observations reveal a significant effect of ionic strength on LDLb+i. The effect is, however, not likely to be me diated by alterations of cell volume or alterations of lysosomal pH.