Slow heat rate increases yeast thermotolerance by maintaining plasma membrane integrity.

Thermal resistance of Saccharomyces cerevisiae was found to be drastically dependent on the kinetics of heat perturbation. Yeasts were found to be more resistant to a plateau of 1 h at 50 degrees C after a slope of temperature increase (slow and linear temperature increments) than after a shock (sudden temperature change). Thermotolerance was mainly acquired between 40-50 degrees C during a heat slope, i.e., above the maximal temperature of growth. The death of the yeasts subjected to a heat shock might be related to the loss of membrane integrity: intracellular contents extrusion, i.e., membrane permeabilization, was found to precede cell death. However, the permeabilization did not precede cell death during a heat slope and, therefore, membrane permeabilization was a consequence rather than a cause of cell death. During a slow temperature increase, yeasts which remain viable may have time to adapt their plasma membrane and thus maintain membrane integrity.     

Action potentials of the rabbit, guinea pig, dog and albino rat working ventricular myocardium under interpolated extrasystole conditions during postnatal ontogenesis

Experiments were carried out on the working myocardium of the right heart ventricle of newborn and adult rabbits, guinea-pigs, dogs and albino rats. In the dog, the guinea-pig and the rabbit, after ten action potentials (AP) elicited with 1 Hz frequency we always interpolated an extrasystole at an interval (TE) of 100-900 ms. In albino rats we used a basic frequency of 2 Hz and a TE of 30-370 ms from the last regular AP. Using glass microelectrodes, we recorded the extrasystolic AP (EAP) and the next subsequent AP (2AP). The results were evaluated by constructing graphs of the correlations of the duration of the plateau phase (D0) to TE and of the duration of repolarization to -60 mV level (D60) to the TE. In the myocardium of newborn rabbits, guinea-pigs and dogs, with short TE both D0 and D60 of the EAP are shorter than in the steady state (SS), while for the 2AP the same parameters are influenced only a little. As the TE lengthens, the EAP gradually acquire a length corresponding more to the SS. With TE longer than half the duration of the cycle in the steady state the EAP return to normal, while the 2AP become shorter. The effect of extrasystole on the rat EAP and 2AP diminished with advancing age. In the myocardium of adult rabbits and adult guinea-pigs, and slightly in the myocardium of adult dogs and newborn rats, we observed that the duration of the EAP, with certain TE, was greater than in the steady state.(ABSTRACT TRUNCATED AT 250 WORDS)