Calcium is often used to stabilize membranes and enhance membrane fusion. We have used the fatty acid spin label, 5-nitroxy stearic acid to measure fluidity changes in the plasma membrane of carrot suspension culture cell protoplasts in response to divalent cations. Electron spin resonance spectra from spin-labeled protoplasts showed no membrane fluidity changes (as determined by the hyperfine splitting constant, 2A(max)) in the presence of Mg from 0 to 10 millimolar or Ca from 0 to 5 millimolar. Protoplasts in 10 millimolar Ca, however, showed a dramatic increase of 5 gauss in 2A(max) and evidence of exchange-broadening. The original (control) spectrum was regained by removing bound Ca with a Ca chelator. Polyethylene glycol, which enhances protoplast fusion, did not alter the membrane fluidity in the region of the 5-nitroxy stearic acid probe if added simultaneously with or following 10 millimolar Ca. Pretreatment with polyethylene glycol did, however, inhibit the Ca-induced phase separation. These data on a living system describe membrane structural changes under conditions similar to those used for protoplast fusion.
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Boss et al. (1980) studied this question.