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Mammalian urinary bladder permeability is altered by cationic proteins: modulation by divalent cations
C J Tzan1, J R Berg, S A Lewis
1Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77555.
The American Journal of Physiology
|October 1, 1994
Summary
Protamine sulfate (PS) and synthetic cationic polypeptides increase urinary bladder epithelial membrane conductance. Polyvalent cations modify this PS-induced conductance by blocking binding sites and increasing its loss rate.
Area of Science:
- Physiology
- Membrane Biophysics
- Urology
Background:
- Cationic polypeptides (CpP), including protamine sulfate (PS), increase mammalian urinary bladder epithelial apical membrane conductance.
- This PS- and CpP-induced conductance is voltage-dependent, mediated by saturable binding sites, and subject to self-inhibition by CpP.
Purpose of the Study:
- To investigate the mechanisms by which polyvalent cations modify protamine sulfate (PS)-induced membrane conductance in the mammalian urinary bladder epithelium.
- To identify potential binding sites for PS on the apical membrane.
Main Methods:
- Electrophysiological measurements of apical membrane conductance in mammalian urinary bladder epithelium.
- Assessment of the effects of various polyvalent cations (UO2(2+), La3+, Mn2+, Ba2+, Ca2+, Sr2+, Mg2+) on PS-induced conductance.
- Lipid bilayer experiments using negatively charged phospholipids to study PS interactions.
Main Results:
- Polyvalent cations interact with PS-induced conductance at three distinct sites: competitive inhibition of PS binding, reversible blockade of conductance, and acceleration of conductance loss.
- The relative binding affinity for blocking PS-induced conductance followed the order: UO2(2+) > La3+ > Mn2+ > Ba2+ >= Ca2+ > Sr2+.
- PS increased the conductance of lipid bilayers with negatively charged phospholipids, suggesting membrane lipids, potentially phosphate groups, as binding sites.
Conclusions:
- Polyvalent cations significantly modulate PS-induced membrane conductance in the urinary bladder epithelium through multiple mechanisms.
- The findings suggest that negatively charged membrane lipids are likely involved in the binding of PS to the apical membrane.