Role of conjugation and red blood cells for inactivation of circulating normetanephrine.
Plasma and red blood cell (RBC) concentrations of normetanephrine (NMN), in both free and glucuronide-conjugated forms, were measured before, during, and after forced immobilization, an intense stressor of the sympathoadrenal system of rats. In this study NMN glucuronide was deconjugated by enzymatic hydrolysis; free and total NMN were assayed by radioenzymatic, thin-layer chromatographic procedures. In plasma, free NMN and NMN glucuronide are 777 +/- 99 and 792 +/- 74 pg/ml, respectively, when rats are at rest. Both free NMN and NMN glucuronide increased about 200% after 15 min of stress; in absolute amounts, increases were equivalent to that of the simultaneous increase in norepinephrine (NE). At 2 h of stress, NMN glucuronide, but not free NMN, increased further and significantly. The mean concentration of RBC-free NMN is about 50 times higher than that of plasma-free NMN, and it did not change significantly during stress; RBCs do not contain conjugated NMN. RBC NMN levels showed a strong correlation with RBC catechol methyltransferase activity. The latter seems to operate under conditions of substrate saturation; an acute release of NE leads to temporary storage of NE in RBCs but not conversion to NMN. The results indicate that conjugation of NMN with glucuronic acid is an important route for inactivation of plasma NMN formed during forced immobilization stress, whereas free NMN does not accumulate in RBCs during stress.
D000818:Animals; D002394:Catechol O-Methyltransferase / Q000378:metabolism; D002855:Chromatography, Thin Layer; D004912:Erythrocytes / Q000378:metabolism*; D005965:Glucuronates / Q000097:blood; D007103:Immobilization; D008297:Male; D009647:Normetanephrine / Q000097:blood*; D051381:Rats; D011911:Rats, Gunn; D011919:Rats, Inbred Strains; D013312:Stress, Physiological / Q000097:blood*
D005965:Glucuronates; D009647:Normetanephrine; D002394:Catechol O-Methyltransferase