AJP - Regulatory, Integrative and Comparative Physiology, Vol 247, Issue 1 208-R211, Copyright © 1984 by American Physiological Society
Role of conjugation and red blood cells for inactivation of circulating normetanephrine
S. Yoneda, N. Alexander, N. D. Vlachakis and R. F. Maronde
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.
