Effect of 5-hydroxytryptamine on tissue blood flow and microwave heating of rat tumors

We have investigated the effect of 5-hydroxytryptamine (5-HT) on SMT-2A mammary adenocarcinoma blood flow during localized microwave hyperthermia treatment. Tissue blood flow in isogeneic female W/Fu rats was estimated using 25-micron-diameter 113Sn-labeled microspheres. An intraarterial injection of 5-HT (1 mg/kg) into either conscious or anesthetized (Nembutal, 20 mg/kg) animals resulted in a 53% reduction in tumor blood flow, while that of the surrounding skeletal muscle remained unchanged. Because of the selective reduction in tumor perfusion, the blood flow of the normal and malignant tissue was equal after 5-HT injection. This blood flow equivalence remained unaltered after 45 min of heating at 42 degrees C. Consequently, the temperature in the tumor was not significantly different from that in the surrounding normal tissue. In contrast, when the tissues were heated at 42 degrees C without 5-HT, the tumor blood flow was significantly greater than that in the surrounding musculature, resulting in the tumor being 1 degree C lower than the muscle temperature. An intratumoral injection of 5-HT (0.25 mg) reduced the tumor blood flow by 92%, and the blood flow of the surrounding muscle was reduced by 57%. These tissue blood flows were not significantly altered by heating at 44 degrees C for 45 min, and the tumor temperature was 0.7 degrees C greater than that in the muscle. When heating at 44 degrees C was performed without 5-HT injection, the tissue temperatures were equal. Thus, both an intraarterial and an intratumoral injection of 5-HT prior to hyperthermia treatment significantly improved the temperature differential between the neoplastic and surrounding normal tissue. Of additional interest was the observation that an intratumoral injection of 0.15 M NaCl also resulted in a preferential increase in the tumor temperature.