Suppression of rat tumor colonies in the lung by oxygen at high pressure is a local effect

The effect of hyperbaric oxygen (HBO) on the growth of anaplastic carcinoma colonies in rat lungs after intravenous tumor cell injection was studied. From the first day after tumor cell injection, the rats were exposed to HBO for 16-21 days, 90 min per day. Oxygen at a pressure of 300 kPa (3.0 ATA) significantly decreased the number of lung tumor colonies and increased the survival of tumor-bearing rats, whereas the application of oxygen at a pressure of 100 kPa had no effect. An oxygen-nitrogen normoxic mixture balanced with nitrogen to 300 kPa (3.0 ATA) did not affect the number of colonies, suggesting that the effect was specific for oxygen and not for the increased pressure itself. A 6-day application of oxygen at a 300 kPa pressure suppressed the growth of lung tumor colonies when applied on days 1-6 and 7-12 after intravenous tumor cell injection, but had no effect when applied on days 13-18. In contrast to dramatic effects of HBO on the development of artificial lung metastases, the oxygen at the same 300 kPa pressure had no effect on the growth of tumor cells injected in the hind foot. Thus it appears that the suppression of lung tumor colonies by HBO was due to local oxygen effects in the lungs.