Permeability changes elicited by influenza and Sendai viruses: separation of fusion and leakage by pH-jump experiments.

Permeability changes elicited in Lettre cells by influenza virus at pH 5.3 were maintained when the pH was shifted to 7.4. Permeability changes elicited by Sendai virus at pH 7.4 were maintained when the pH was shifted to 5.3. In each case permeability changes at the new pH were sensitive to inhibition by extracellular Ca2+. The time at which the pH was shifted was critical: if the shift was made prior to the onset of permeability changes, no subsequent changes occurred. It is concluded that the pH-sensitive event, namely virus-cell fusion, is related to the induction of permeability changes through attainment of some type of 'threshold' level of membrane damage.