708 BILE SALTS FUNCTION AS CALCIUM IONOPHORES

Abstract

It has been previously demonstrated that bile salts (BS) induce calcium (Ca) dependent hemolysis which is directly proportional to Ca influx into red blood cells. To examine how BS's might facilitate Ca movement across membranes, we investigated Ca uptake by unilamellar phospholipid vesicles (PV)prepared from soy phosphatidates. PV's were incubated with 5.0 mM ⁴⁵Ca either alone (control) or with added 750 μM BS - taurodeoxycholate (TDC) or tauroeholate (TC) (Hepes buffer, pH 7.0, 37°C). After 10 min, Ca uptake was 1.88 ± .08 nmoles by the control PV's, 3.28 ± .04 nmoles by the PV's with TC, and 5.06 ± .03 nmoles by the PV's with TDC. ⁴⁵Ca uptake was also compared to sodium-22 (²²Na) or rubidium-86 (⁸⁶Rb) uptake in the presence of TDC. After 5 min of incubation, TDC-associated Ca uptake was nearly 2 times greater than those of either Na or Rb. Finally, PV's were exposed either to 25 nM valinomycin (V) which induced an electrochemical gradient across PV membranes, or to 25 nM nigericin (N) which induced a pH gradient. TDC without V or N increased Ca uptake 1.67 ± .08 times over controls. Exposure to V increased TDC-associated Ca uptake 1.45 ± .05 times, and exposure to N increased it 1.90 ± .03 times.

Conclusions: (1) BS's induce cation uptake by PV's that is more specific for Ca than for Na or Rb; (2) as was true of red blood cells, TDC induces greater Ca uptake than TC; (3) pH gradients promote greater BS-indueed Ca uptake than electrochemical gradients. Hypothesis: The mechanism of BS-induced Ca uptake may be similar to that of Ca ionophore A23187 whose activity is similarly promoted by pH gradients.

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