Effect of pH on development and decay of thermotolerance in CHO cells using fractionated heating at 43 degrees C

The development and decay of thermotolerance at pH 6.7, 7.1 and 7.7 was studied after fractionated hyperthermia at 43 degrees C using exponentially growing CHO cells. The maximum of thermotolerance and the time interval to reach this maximum were found to correlate with the survival decrement after the priming heat treatment. Both parameters were only affected by pH in so far as the pH altered survival after the priming treatment. Decay of thermotolerance was exponential. For a given priming heat treatment for the time t1, the half-time of decay, tau 1/2, increased linearly with increasing cell doubling time, tau d, measured for non-heated cells growing at different pH. On the other hand, for a given cell doubling time, tau d, the half-time, tau 1/2, increased exponentially with increasing duration of the priming heat treatment, t1. For all measured data the half-time of thermotolerance decay could be described by the equation tau 1/2 = alpha. tau d.exp(k.t1), with k = 2.2 +/- 0.2 h-1 and alpha = 0.094 +/- 0.009 for all pretreatments applied and all pH conditions tested. This relationship might indicate that the decay of thermotolerance is governed by a single mechanism.