Changes in muscle pH following hyperthermia

Considerable attention has been paid to the modification of intratumor pH in response to hyperthermia. It has been hypothesized that observed reductions in intralesional pH are involved in the ultimate response of tissue to hyperthermia treatment. Further, it has been shown that significant differences exist in hyperthermia-induced changes in blood flow between tumor and normal tissue in many systems. Changes in blood flow are hypothesized to be related to observed changes in pH. Since reduced blood flow is not observed in normal tissue under normal treatment conditions, changes in pH in normal tissue have not been considered significant in their response to hyperthermia treatment. However, this conclusion has not been verified or documented experimentally. The purpose of this study was to examine the distribution of pH in normal tissue (muscle) as a function of time following hyperthermia treatments which in the same animal system resulted in subcurative (TCD 10/30) or curative (TCD 90/30) tumor (mammary adenocarcinoma) responses. The observed distribution of pH in normal tissue was compared with that obtained in tumors under identical conditions. The results indicate that some post-treatment changes in muscle pH do occur following hyperthermia, but that these changes are small compared to those observed in tumors. More importantly, unlike the response observed in tumors, no hyperthermia dose dependency is observed in the muscle response. From these studies it can be concluded that changes in normal muscle pH are probably not associated with normal tissue response to hyperthermia.     

Thermal sensitivity of endothelial cells

Experimental work indicates that one of the mechanisms of tumor control by hyperthermia may be damage to blood vessels, resulting in decreased blood flow to the neoplasms. Among the various elements of the microvasculature, endothelial cells are the most important possible targets of thermal injury. Furthermore, neoplasms have a significantly higher proportion of proliferating endothelial cells than do normal tissues. Thus it is necessary to establish the thermal sensitivity of endothelial cells and to explore possible differences in response between resting and proliferating endothelium. We studied the in vitro thermal sensitivity of murine and human capillary endothelial cells compared to human fibroblasts by following cell survival and growth recovery. Nonstimulated endothelial cells are more sensitive than fibroblasts. Their sensitivity is dose dependent within the range of 42 to 45 degrees C/30 min. Stimulation to proliferate by endothelial cell growth factor (ECGF) renders these cells even more sensitive. Morphologic studies confirm these thermal effects in endothelial cells and fibroblasts. These findings support a direct effect of hyperthermia on endothelial cells, which appears to be more severe in proliferating cells. This may explain the reduced blood flow in heated tumors and may indicate a valuable therapeutic gain for hyperthermia.     

Local heating of human skin by millimeter waves: effect of blood flow

We investigated the influence of blood perfusion on local heating of the forearm and middle finger skin following 42.25 GHz exposure with an open ended waveguide (WG) and with a YAV mm wave therapeutic device. Both sources had bell-shaped distributions of the incident power density (IPD) with peak intensities of 208 and 55 mW/cm(2), respectively. Blood perfusion was changed in two ways: by blood flow occlusion and by externally applied vasodilator (nonivamide/nicoboxil) cream to the skin. For thermal modeling, we used the bioheat transfer equation (BHTE) and the hybrid bioheat equation (HBHE) which combines the BHTE and the scalar effective thermal conductivity equation (ETCE). Under normal conditions with the 208 mW/cm(2) exposure, the cutaneous temperature elevation (DeltaT) in the finger (2.5 +/- 0.3 degrees C) having higher blood flow was notably smaller than the cutaneous DeltaT in the forearm (4.7 +/- 0.4 degrees C). However, heating of the forearm and finger skin with blood flow occluded was the same, indicating that the thermal conductivity of tissue in the absence of blood flow at both locations was also the same. The BHTE accurately predicted local hyperthermia in the forearm only at low blood flow. The HBHE made accurate predictions at both low and high perfusion rates. The relationship between blood flow and the effective thermal conductivity (k(eff)) was found to be linear. The heat dissipating effect of higher perfusion was mostly due to an apparent increase in k(eff). It was shown that mm wave exposure could result in steady state heating of tissue layers located much deeper than the penetration depth (0.56 mm). The surface DeltaT and heat penetration into tissue increased with enlarging the irradiating beam area and with increasing exposure duration. Thus, mm waves at sufficient intensities could thermally affect thermo-sensitive structures located in the skin and underlying tissue.     

Simultaneous measurements of local tissue temperature and blood perfusion rate in the canine prostate during radio frequency thermal therapy

Local tissue temperature and blood perfusion rate were measured simultaneously to study thermoregulation in the canine prostate during transurethral radio-frequency (RF) thermal therapy. Thermistor bead microprobes measured interstitial temperatures and a thermal clearance method measured the prostatic blood perfusion rate under both normal and hyperthermic conditions. Increase in local tissue temperature induced by the RF heating increased blood perfusion throughout the entirety of most prostates. The onset of the initial increase in blood perfusion was sometimes triggered by a temporal temperature gradient at low tissue temperatures. When tissue temperature was higher than 41°C, however, the magnitude and the spatial gradient of temperature may play significant roles. It was found that the temperature elevation in response to the RF heating was closely coupled with local blood flow. The resulting decrease in or stabilization of tissue temperature suggested that blood flow might act as a negative feedback of tissue temperature in a closed control system. Results from this experiment provide insights into the regulation of local perfusion under hyperthermia. The information is important for accurate predictions of temperature during transurethral RF thermal therapy.     

Cancer hyperthermia using magnetic nanoparticles

Magnetic-nanoparticle-mediated intracellular hyperthermia has the potential to achieve localized tumor heating without any side effects. The technique consists of targeting magnetic nanoparticles to tumor tissue followed by application of an external alternating magnetic field that induces heat through Néel relaxation loss of the magnetic nanoparticles. The temperature in tumor tissue is increased to above 43°C, which causes necrosis of cancer cells, but does not damage surrounding normal tissue. Among magnetic nanoparticles available, magnetite has been extensively studied. Recent years have seen remarkable advances in magnetite-nanoparticle-mediated hyperthermia; both functional magnetite nanoparticles and alternating-magnetic-field generators have been developed. In addition to the expected tumor cell death, hyperthermia treatment has also induced unexpected biological responses, such as tumor-specific immune responses as a result of heat-shock protein expression. These results suggest that hyperthermia is able to kill not only local tumors exposed to heat treatment, but also tumors at distant sites, including metastatic cancer cells. Currently, several research centers have begun clinical trials with promising results, suggesting that the time may have come for clinical applications. This review describes recent advances in magnetite nanoparticle-mediated hyperthermia.     

Changes in erythropoiesis due to radiation or chemotherapy as studied by flow cytometric determination of peripheral blood reticulocytes

Flow cytometric determination of time dependent changes of numbers of reticulocytes in peripheral blood were investigated as a parameter for changes in erythropoiesis induced by radiation- or chemotherapy. Rats irradiated or treated with drugs (such as e.g. cyclophosphamide 100 mg/kg, vincristin 0.2 mg/kg, or mitomycin C 1.0 mg/kg) showed clear changes in erythropoietic activity. Reticulocyte numbers decreased rapidly until day 3-4 after treatment; this period was followed by a gradual increase and normal control values were seen at day 8-11. Radiation effects of doses as low 0.5 Gy could be detected in such a way. Similar studies were performed with patients with ovarian tumors treated with cis-platinum, a drug that may cause non-immune haemolysis. During prolonged treatment some patients showed increasing numbers of reticulocytes, measured at the first day of each hospitalization period, whereas leucocyte and platelet counts stayed more or less constant. Increasing numbers of reticulocytes generally indicates stimulation of erythropoietic activity of the bone marrow (due to increased blood loss); in this study increasing numbers often preceeded a decrease in hemoglobin values later on. Flow cytometric analysis of reticulocytes is therefore a potentially useful tool to detect changes in erythropoiesis, and considered more sensitive for the early recognition of patients that develop anemia, than hemoglobin measurements only.     

Effects of hydralazine on the blood flow in RIF-1 tumors and normal tissues of mice

Hydralazine is a peripheral vasodilator used as an antihypertensive agent. Hydralazine has been reported to potentiate tumor damage by hyperthermia as well as by hypoxic-cell-specific drugs through the reduction of tumor blood flow and pO2. In the present study, we investigated the changes in blood perfusion caused by hydralazine in S.C. RIF-1 tumors and normal tissues in C3H mice using the 86Rb uptake technique and laser Doppler flowmetry. The tumor blood flow was decreased significantly by an intravenous administration of 0.5-10.0 mg/kg hydralazine, as determined by both uptake of 86Rb and laser Doppler flowmetry. The tumor pO2 was also decreased significantly by the injection of hydralazine. On the other hand, the uptake of 86Rb was increased significantly in the skin and muscle by hydralazine. The changes seen in the skin and muscle after injection of hydralazine as assessed by laser Doppler flowmetry were similar to those assessed by uptake of 86Rb, indicating a significant increase in blood circulation in these tissues. Uptake of 86Rb remained unchanged in the kidney and decreased in the liver and spleen in the presence of hydralazine in a dose-dependent manner at 0.5-10.0 mg/kg. The decline in uptake of 86Rb in normal tissues strongly suggests that hydralazine decreases the blood flow in these normal tissues. Thus the recent proposal to use hydralazine to increase the antitumor activity of heat or certain drugs needs to be reexamined.     

Predicting effects of blood flow rate and size of vessels in a vasculature on hyperthermia treatments using computer simulation

Background

Pennes Bio Heat Transfer Equation (PBHTE) has been widely used to approximate the overall temperature distribution in tissue using a perfusion parameter term in the equation during hyperthermia treatment. In the similar modeling, effective thermal conductivity (K_eff) model uses thermal conductivity as a parameter to predict temperatures. However the equations do not describe the thermal contribution of blood vessels. A countercurrent vascular network model which represents a more fundamental approach to modeling temperatures in tissue than do the generally used approximate equations such as the Pennes BHTE or effective thermal conductivity equations was presented in 1996. This type of model is capable of calculating the blood temperature in vessels and describing a vasculature in the tissue regions.

Methods

In this paper, a countercurrent blood vessel network (CBVN) model for calculating tissue temperatures has been developed for studying hyperthermia cancer treatment. We use a systematic approach to reveal the impact of a vasculature of blood vessels against a single vessel which most studies have presented. A vasculature illustrates branching vessels at the periphery of the tumor volume. The general trends present in this vascular model are similar to those shown for physiological systems in Green and Whitmore. The 3-D temperature distributions are obtained by solving the conduction equation in the tissue and the convective energy equation with specified Nusselt number in the vessels.

Results

This paper investigates effects of size of blood vessels in the CBVN model on total absorbed power in the treated region and blood flow rates (or perfusion rate) in the CBVN on temperature distributions during hyperthermia cancer treatment. Also, the same optimized power distribution during hyperthermia treatment is used to illustrate the differences between PBHTE and CBVN models. K_eff (effective thermal conductivity model) delivers the same difference as compared to the CBVN model. The optimization used here is adjusting power based on the local temperature in the treated region in an attempt to reach the ideal therapeutic temperature of 43°C. The scheme can be used (or adapted) in a non-invasive power supply application such as high-intensity focused ultrasound (HIFU). Results show that, for low perfusion rates in CBVN model vessels, impacts on tissue temperature becomes insignificant. Uniform temperature in the treated region is obtained.

Conclusion

Therefore, any method that could decrease or prevent blood flow rates into the tumorous region is recommended as a pre-process to hyperthermia cancer treatment. Second, the size of vessels in vasculatures does not significantly affect on total power consumption during hyperthermia therapy when the total blood flow rate is constant. It is about 0.8% decreasing in total optimized absorbed power in the heated region as γ (the ratio of diameters of successive vessel generations) increases from 0.6 to 0.7, or from 0.7 to 0.8, or from 0.8 to 0.9. Last, in hyperthermia treatments, when the heated region consists of thermally significant vessels, much of absorbed power is required to heat the region and (provided that finer spatial power deposition exists) to heat vessels which could lead to higher blood temperatures than tissue temperatures when modeled them using PBHTE.     

Response of mice to continuous 5-day passive hyperthermia resembles human heat acclimation

Chronic repeated exposure to hyperthermia in humans results in heat acclimation (HA), an adaptive process that is attained in humans by repeated exposure to hyperthermia and is characterized by improved heat elimination and increased exercise capacity, and acquired thermal tolerance (ATT), a cellular response characterized by increased baseline heat shock protein (HSP) expression and blunting of the acute increase in HSP expression stimulated by re-exposure to thermal stress. Epidemiologic studies in military personnel operating in hot environments and elite athletes suggest that repeated exposure to hyperthermia may also exert long-term health effects. Animal models demonstrate that coincident exposure to mild hyperthermia or prior exposure to severe hyperthermia can profoundly affect the course of experimental infection and injury, but these models do not represent HA. In this study, we demonstrate that CD-1 mice continuously exposed to mild hyperthermia (ambient temperature ~37°C causing ~2°C increase in core temperature) for 5 days and then exposed to a thermal stress (42°C ambient temperature for 40 min) exhibited some of the salient features of human HA, including (1) slower warming during thermal stress and more rapid cooling during recovery and (2) increased activity during thermal stress, as well as some of the features of ATT, including (1) increased baseline expression of HSP72 and HSP90 in lung, heart, spleen, liver, and brain; and (2) blunted incremental increase in HSP72 expression following acute thermal stress. This study suggests that continuous 5-day exposure of CD-1 mice to mild hyperthermia induces a state that resembles the physiologic and cellular responses of human HA. This model may be useful for analyzing the molecular mechanisms of HA and its consequences on host responsiveness to subsequent stresses.     

The effects of some physical factors on the production of hyperthermia by ultrasound in neoplastic tissues

Summary A one-dimensional and a three-dimensional computer model have been built in order to study the importance of blood flow and ultrasonic absorption in tissues during local hyperthermia. The decreased blood flow in the interior of certain tumours and possibly the increased ultrasonic absorption of the malignant tissue in some cases may cause selectively higher temperatures inside the tumours though the heat input is the same as in the surrounding tissues. Also, the vasodilation of blood vessels in normal tissues as a response to heat causes a therapeutically useful temperature difference. These blood flow differences can lead to enhanced effects during sonication to produce hyperthermia in the tumour. The inhomogenity of blood flow in the tumour causes a non-uniform temperature distribution leaving the well-perfused cells in the advancing front at a much lower temperature than the cells in the necrotic centre. Thus, the combination of local hyperthermia with radio-and chemotherapy seems to offer the most attractive means of destroying malignant tissue.     

Lymph flow from murine footpad tumors before and after sublethal hyperthermia.

The effect of local hyperthermia (43.5 degrees C for 1 h) on lymph flow from B16-F10 tumor-bearing foot pads of C57BL/6 mice was measured by monitoring the clearance of 99mTc-labeled human serum albumin. The foot was represented by a single-compartment model enabling a quantitative computation of lymphatic flow from the tumor to regional lymph nodes. Lymphatic flow from untreated tumors was 0.0059 +/- 0.0011 ml/min cm3 compared to 0.0118 +/- 0.0027 ml/min cm3 lymphatic flow from tumors immediately following heating. Morphological alterations in tumor blood vessels result in their high vascular permeability. The increase in lymphatic clearance from tumors after sublethal hyperthermia is compatible with the increase in interstitial fluid formation in tumors based on Starling's Law.     

Molecular mechanisms underlying thermal adaptation of xeric animals

For many years, we and our collaborators have investigated the adaptive role of heat shock proteins in different animals, including the representatives of homothermic and poikilothermic species that inhabit regions with contrasting thermal conditions. Adaptive evolution of the response to hyperthermia has led to different results depending upon the species. The thermal threshold of induction of heat shock proteins in desert thermophylic species is, as a rule, higher than in the species from less extreme climates. In addition, thermoresistant poikilothermic species often exhibit a certain level of heat shock proteins in cells even at a physiologically normal temperature. Furthermore, there is often a positive correlation between the characteristic temperature of the ecological niche of a given species and the amount of Hsp70-like proteins in the cells at normal temperature. Although in most cases adaptation to hyperthermia occurs without changes in the number of heat shock genes, these genes can be amplified in some xeric species. It was shown that mobile genetic elements may play an important role in the evolution and fine-tuning of the heat shock response system, and can be used for direct introduction of mutations in the promoter regions of these genes.     

Effects of hyperthermia and X-irradiation on mouse stromal tissue

The sensitivity of normal stroma to heat, irradiation and heat combined with irradiation has been studied using the tumour bed effect (TBE) assay. Irradiation before implantation led to a TBE. This TBE was dose dependent below 15 Gy, the TBE remaining relatively constant above 15 Gy. The interval (0-90 days) between irradiation and tumour implantation did not influence the magnitude of the TBE. Hyperthermia with large heat doses (45-60 min at 44 degrees C) before implantation may lead to a TBE. The interval between hyperthermia and tumour implantation proved to be very important. Our results show that the recovery from heat-induced stromal damage is very rapid. When the interval between hyperthermia and tumour implantation is 10 days or longer, no TBE could be observed. Irradiation combined with large heat doses (30-60 min at 44 degrees C) decreased the radiation-induced TBE. However, the combination of irradiation with mild heat treatments (15 min at 44 degrees C) could lead to a larger TBE than after irradiation alone. When hyperthermia was given prior to irradiation, the interval between heat and irradiation proved to be very important. With large intervals (21 days or longer) the TBE values were about the same as with irradiation alone. When heat was given after irradiation it always reduced the irradiation-induced TBE.     

The influence of pre-treatment temperature on the thermal sensitivity of a mouse tumour

The response of tumours to hyperthermia was tested by giving graded heat treatments and assessing local control at 90 days. Mice were divided into three groups which were pre-treated for 3 days in ambient temperatures of 4, 21 or 35 degrees C. This enabled the mean tumour resting temperature to be varied by up to 11 degrees C, before subsequent heat treatment. For the heat treatments, the tumours were clamped in order to eliminate blood flow, resulting in uniform temperature distributions and hence more uniform thermal sensitivity. TCD50 values were used to construct Arrhenius plots. For all three pre-treatment temperatures, these plots demonstrated a factor of 1.6 increase in heating time per degree Celsius reduction in heating temperature. However, tumours kept in a 4 degrees C environment before treatment were more thermally sensitive than those kept in 21 degrees C conditions, while those in a 35 degrees C environment were more resistant. Pretreatment at 4 degrees C was equivalent to an increase of either 0.5 degree C in heating temperature or 28 per cent in heating time, compared with pre-treatment at 21 degrees C. Pre-treatment at 35 degrees C was equivalent to a reduction of either 0.6 degree C in heating temperature or 25 per cent in heating time. These data indicate that the pre-treatment tumour temperature is an important parameter, but the effect of heat treatment is more closely related to absolute heating temperature rather than to the increase in temperature above the normal resting level.     

Treatment of murine SCC VII tumors with localized hyperthermia and temperature-sensitive liposomes containing cisplatin

The release of cisplatin (CDDP) encapsulated in temperature-sensitive unilamellar liposomes to murine SCC VII carcinoma by localized hyperthermia and the effects of the treatment on tumor growth were studied. A transition temperature of the temperature-sensitive liposomes containing cisplatin (LIP-CDDP) was 41 degrees C. Twenty-four hours after injection of LIP-CDDP, the heated tumors (42 degrees C, 60 min) contained 3.3 times more CDDP than the unheated tumors receiving free CDDP. Although the uptake of liposome-associated CDDP by liver was approximately threefold greater at 1.5 h after injection than uptake of free CDDP, it decreased about 50% over a 24-h period. No difference in uptake of the two forms of CDDP by kidney was observed. The combination of LIP-CDDP and localized heating at 42 or 43 degrees C was more effective relative to the amount of CDDP in delaying tumor growth than that of free CDDP and hyperthermia. Treatment with LIP-CDDP plus local heating resulted in a dose-modifying factor of 5.3 when compared with free CDDP and no hyperthermia. The dose-modifying factor was 2.8 when treatment with LIP-CDDP and heat was compared with treatment with free CDDP and heat. Thus CDDP could be released selectively from the temperature-sensitive liposomes by heat and resulted in both a greater uptake of the drug and a delay in tumor growth.     

Heat-induced thermal resistance and its relationship to lysosomal response.

Two separate effects of hyperthermia on mouse splenic lysosomes have been reported, dependent on the severity of the treatment. Heating to temperatures below 42.5 degrees C causes a transient increase in lysosomal acid phosphatase activity which can be correlated with the ability of moderate hyperthermia to potentiate X-ray damage. Heating to temperatures above 42.5 degrees C results in an immediate increase in lysosomal membrane permeability which may be involved in tissue necrosis. By giving a priming heat treatment at 41.8 degrees C, induced thermal resistance was demonstrated for the lysosomal membrane effect, but not for the enzyme activation. The degree of induced thermal resistance observed is similar to that reported for the cell-killing effect of heat on tissues in vivo and cells in vitro and occurs over a similar time course. The relevance of these results to the understanding of fractionated hyperthermia in cancer therapy is discussed.     

Improvement of tumor oxygenation by mild hyperthermia

There is now abundant evidence that oxygenation in rodent, canine and human tumors is improved during and for up to 1-2 days after heating at mild temperatures. An increase in tumor blood perfusion along with a decline in the oxygen consumption rate appears to account for the improvement of tumor oxygenation by mild hyperthermia. The magnitude of the increase in tumor pO(2), determined with oxygen-sensitive microelectrodes, caused by mild hyperthermia is less than that caused by carbogen breathing. However, mild hyperthermia is far more effective than carbogen breathing in increasing the radiation response of experimental tumors, probably because mild hyperthermia oxygenates both (diffusion-limited) chronically hypoxic and (perfusion-limited) acutely hypoxic cells, whereas carbogen breathing oxygenates only the chronically hypoxic cells. Mild hyperthermia is also more effective than nicotinamide, which is known to oxygenate acutely hypoxic cells, in enhancing the radiation response of experimental tumors. The combination of mild hyperthermia with carbogen or nicotinamide is highly effective in reducing the hypoxic cell fraction in tumors and increasing the radiation response of experimental tumors. A primary rationale for the use of hyperthermia in combination with radiotherapy has been that hyperthermia is equally cytotoxic toward fully oxygenated and hypoxic cells and that it directly sensitizes both fully oxygenated and hypoxic cells to radiation. Such cytotoxicity and such a radiosensitizing effect may be expected to be significant when the tumor temperature is elevated to at least 42-43 degrees C. Unfortunately, it is often impossible to uniformly raise the temperature of human tumors to this level using the hyperthermia devices currently available. However, it is relatively easy to raise the temperature of human tumors into the range of 39-42 degrees C, which is a temperature that can improve tumor oxygenation for up to 1-2 days. The potential usefulness of mild hyperthermia to enhance the response of human tumors to radiotherapy by improving tumor oxygenation merits continued investigation.     

Changes in erythropoiesis due to radiation or chemotherapy as studied by flow cytometric determination of peripheral blood reticulocytes

Summary Flow cytometric determination of time dependent changes of numbers of reticulocytes in peripheral blood were investigated as a parameter for changes in erythropoiesis induced by radiation- or chemotherapy. Rats irradiated or treated with drugs (such as e.g. cyclo-phosphamide 100 mg/kg, vincristin 0.2 mg/kg, or mitomycin C 1.0 mg/kg) showed clear changes in erythropoietic activity. Reticulocyte numbers decreased rapidly until day 3–4 after treatment; this period was followed by a gradual increase and normal control values were seen at day 8–11. Radiation effects of doses as low 0.5 Gy could be detected in such a way.Similar studies were performed with patients with ovarian tumors treated with cis-platinum, a drug that may cause non-immune haemolysis. During prolonged treatment some patients showed increasing numbers of reticulocytes, measured at the first day of each hospitalization period, whereas leucocyte and platelet counts stayed more or less constant. Increasing numbers of reticulocytes generally indicates stimulation of erythropoietic activity of the bone marrow (due to increased blood loss); in this study increasing numbers often preceeded a decrease in hemoglobin values later on.Flow cytometric analysis of reticulocytes is therefore a potentially useful tool to detect changes in erythropoiesis, and considered more sensitive for the early recognition of patients that develop anemia, than hemoglobin measurements only.In honous of Prof. P. van Duijn.     

Influence of hyperthermia on gamma-ray-induced mutation in V79 cells

Asynchronously growing V79 cells were assayed for mutation induction following exposure to hyperthermia either immediately before or after being irradiated with 60Co gamma rays. Hyperthermia exposures consisted of either 43.5 degrees C for 30 min or 45 degrees C for 10 min. Each of these heat treatments resulted in a survival level of 42%. For all sequences of combined treatment with hyperthermia and radiation, cell killing by gamma rays was enhanced. Mutation induction by gamma rays was enhanced when heat preceded gamma irradiation, but no increase was observed when heat was given after gamma exposures. Treatment at 45 degrees C for 10 min gave a higher yield in mutants at all gamma doses studied compared to treatment at 43.5 degrees C for 30 min. When heat-treated cells were incubated for different periods before being exposed to gamma rays, thermal enhancement of radiation killing was lost after 24 h. In contrast, only 5-6 h incubation was needed for loss of mutation induction enhancement.     

Histamine and histamine receptors: behavioral thermoregulation in the salamander Necturus maculosus

Low doses (0.01, 0.1 mg/kg, i.p.) of histamine (HA) caused selection of significantly lower temperatures, and higher doses (0.5, 1.0 mg/kg) increased temperatures by mudpuppies in linear thermal gradients. Injection of the HA precursor, L-histidine (500 mg/kg) produced an increase in the temperatures selected. Results from injections of HA H1-receptor agonist (2-pyridylethylamine) and antagonist (pyrilamine), and H2-receptor agonist (dimaprit) and antagonist (cimetidine) had significant effects on thermoregulation; H1-receptors may mediate behavioral hyperthermia and H2-receptors behavioral hypothermia. Responses to these histaminic compounds are significantly influenced by the time of day at which the responses are measured and by season and acclimation temperature. The equivalent behavioral responses in both endotherms and ectotherms to agents which produce physiological hyperthermia and hypothermia are probably behavioral hypothermia ("cold seeking") and behavioral hyperthermia ("heat seeking"), respectively.