Influence of methoxamine, 5-hydroxytryptamine and physically induced low pH on thermal sensitivity and thermotolerance.

: The influence of pH on split-dose hyperthermic response of non-resistant and thermotolerant Ehrlich tumors was studied. Vasoactive agents, methoxamine and 5-hydroxytryptamine (5-HT) and, mechanical interruption of the tumor blood flow by wire technique were applied in combination with hyperthermia at 72-hour intervals, where an increased thermal response was observed in Ehrlich tumor. A potential influence in suppressing the induction of thermotolerance (hyperthermia at 24-hour intervals) was evident in the case of 5-HT and methoxamine. These vasoactive agents and the wire technique showed a selective effect in reducing the tumor microcirculation as well as lowering its pH. However, unlike the wire technique, the pH was rather low in the postheating period in the case of 5-HT and methoxamine. Poor tumor vascularization induced by vasoactive agents or physical means would demonstrate a poor nutritional condition with a low pH, whereby an increase in thermal response would be obvious and may be expected to have maximal influence either in the decay or in delaying the induction of thermotolerance. A low tumor pH particularly during the postheating period may have some role against the development of thermotolerance.