Benzyl alcohol is known to have dual effect on the red blood cell shape change. At low concentration up to 50 mM benzyl alcohol transformed the shape from discocyte to stomatocyte by preferent binding to the inner hemileaflet, however, at higher concentratransformed the shape from discocyte to stomatocyte by preferential binding to the inner monolayer, however, at higher concentration above 50 mM benzyl alcohol transformed to echinocyte by affecting both monolayers. These results suggest that the effect of benzyl alcohol on the red blood cell shape and Ca<sup>++</sup> transport across cardiac cell membranes to assess the effects of the drug on the structures and functions of the biological cell membranes. The results are as follows: 1) Benzyl alcohol up to 40 mM caused progressive stomatocytic shap change of the red blood cell but above 50 mM benzyl alcohol caused echinocytic shape change. 2) Benzyl alcohol up to 40 mM inhibited both osmotic hemolysis and osmotic volume change of the red blood cell in hypotonic and hypertonic NaCl solutions, respectively. 3) Benzyl alcohol inhibited both Bowditch Staircase and Wood-worth Staircase phenomena at rat left auricle. 4) Benzyl alcohol at concentration of 5 mM increased Ca<sup>++</sup>-ATPase activity of red blood cell ghosts slightly but above S mM benzyl alcohol inhibited the Ca<sup>++</sup>-ATPase activity. 5) Benzyl alcohol at concentrations of 5 mM and 10 mM increased Ca<sup>++</sup>-ATPase activity slightly at rat gastrocnemius muscle S.R. but above 10 mM benzyl alcohol inhibited the Ca<sup>++</sup>-ATPase activity. Above results indicate that benzyl alcohol inhibit water permeability and Ca<sup>++</sup> transport across cell membranes in part via effects on the fluidity and transition temperatures of the bulk lipid by preferential intercalation into cytoplasmic monolayer and in part via other effect on the conformational change of active sites of the Ca<sup>++</sup>-ATPase molecule extended in cytoplasmic face.
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