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Exposure of clone MTC rat 13672NF mammary adenocarcinoma cells to continuous heating at 42 degrees C or to heating for 20 min at 45 degrees C followed by incubation at 37 degrees C induced or enhanced synthesis of several heat-stress proteins (hsp) detectable by [3H]leucine pulse labeling, gel electrophoresis, and fluorography. Hsp synthesis occurred during development and expression of thermal resistance. For example, continuous heating of MTC cells at 42 degrees C after a rapid temperature transient (about 4 min) produced thermal resistance within 4 to 5 hr of initiation of heating. Hsp synthesis was observed within 2 hr of the cells reaching 42 degrees C and continued throughout 8 hr of heating; four major hsp appeared at nominal molecular weights of 112, 90, 70, and 22 kDa. A slow temperature transient from 37 degrees to 42 degrees C over a 3-hr period increased thermal resistance by a factor of about 2 relative to a rapid transient or "shock" to 42 degrees C. However, hsp synthesis was not significant during the slow heat transient and was either reduced (at 70 and 22 kDa) or not increased (at 112 and 90 kDa) compared with the rates of hsp synthesis at the same time intervals after the rapid transient to 42 degrees C. In these mammary tumor cells, no differences in the number of hsp were detected during heating at 42 degrees or after 45 degrees C heating. Other proteins did not appear to change their relative rates of synthesis, with the exception of a clear decrease in proteins with nominal molecular weights of histones H2A, H2B, H3, and H4. Hsp synthesis was not triggered by cold shock to 23 degrees or 0 degree C, or by radiation of from 2.5 to 10 Gy. Thus hypothermic stress did not enhance hsp synthesis nor were hsp nonspecifically associated with eventual cell lethality. In these experiments, the synthesis of hsp was generally correlated with the development of thermal resistance. Determining the quantity, function, and location of stress proteins may identify targets for thermal killing and resistance. However, one paradoxical observation remains to be resolved. Under the slow temperature transient conditions, the tumor cells attained more thermal resistance with generally reduced rates of hsp synthesis. These results have two possible implications: (a) one or more hsp were not directly related to thermal resistance, or (b) hsp were involved in thermal resistance but their relative rate of synthesis depended on the severity of the heat transient. The latter would imply that the cells made other regulatory or metabolic adjustments and either utilized the hsp more effectively or required less additional hsp.