SURFACE-CHARGE, FLUIDITY, AND CALCIUM-UPTAKE BY RAT INTESTINAL BRUSH-BORDER VESICLES

Biological membrane outer surfaces are negatively charged and interact with positively charged calcium ion during calcium uptake. Positively charged polycations such as polyarginine bind to membranes with high affinity, displacing bound calcium from the membrane. We tested the ef fect of polyarginine on uptake of calcium by brush-border membrane ves icles and examined the responses in terms of membrane fluidity by elec tron paramagnetic resonance (EPR). Polyarginine inhibited the saturabl e component of calcium uptake by a mechanism combining inhibition char acteristics of strontium (competitive) and magnesium (non-competitive) . Unlike the inhibition of non-saturable calcium uptake by strontium a nd magnesium, polyarginine increased k(D), the rate constant for non-s aturable calcium uptake, by a concentration dependent mechanism. These effects of polyarginine on calcium uptake were associated with decrea sed membrane fluidity at the uptake temperature. These findings are co nsistent with a role for surface negative charge in determining both s aturable and non-saturable calcium uptake. Increased membrane fluidity is associated with decreased saturable and increased non-saturable ca lcium uptake. Although increased fluidity might be involved in the inc reased k(D) for non-saturable uptake, the concentration-specific stimu lating effect of polyarginine suggests a gating mechanism.