Acute in vivo exposure of rat (Sprague-Dawley SPF, male; body weight, 320±10g) to 100 and 200 ppm NO was performed in a small exposure chamber, and the time course of nitrosylhemoglobin (Hb-NO) and methemoglobin (Met Hb) formation in blood was simulated by the computer.
(1) The exposure chamber (volume, 1400 ml) for single rat was constructed so as to minimize NO₂ formation by the reaction of NO with O₂. The rat was anesthetized under NO exposure in the chamber by ether.
(2) The content of Hb-NO and Met Hb in the blood approached to a steady state at 30 to 60 min after exposure to 100 and 200 ppm NO. The concentration of Met Hb was 7 to 15 times of those of Hb-NO. Both hemoglobins disappeared within a half time of 20 min by the successive exposure to fresh air.
(3) The computer simulation for Hb-NO and Met Hb formation under NO exposure was carried out on the simplified reaction model by varying the following velocity constants: the association of 0₂ with deoxyhemoglobin, the dissociation of 0₂ from oxyhemoglobin, the association of NO with deoxyhemoglobin, the conversion of Hb-NO to Met Hb, and the reduction of Met Hb. The processes with unknown mechanisms were included in some velocity constants. The time courses of Hb-NO and Met Hb was semiquantitatively discussed on the basis of the computed kinetic parameters.
(4) In conclusion, (i) Hb-NO content under a steady inhalation of NO is maintained at fairly low level (0.4% for 100 ppm NO), (ii) Met Hb content is also kept at low level (less than 10% for 100 ppm NO) due to the enzymatic reduction of Met Hb in red cells. However (iii) the activity of Met Hb reductase seems to be inhibited by exposure to higher concentrations of NO.