Effects of Chemical Sympathectomy on the lncreases in Plasma Catecholamines and Dopamine-B-Hydroxylase Induced by Forced lmmobilization and lnsulin-lnduced Hypoglycemia: Origin and Fate of Plasma Dopamine-B-Hydroxylase'

Abstract

The effect of acute stresses on plasma norep¡nephr¡ne, epinephrine and dopam¡ne-B-hydroxylase (DBH) were evaluated ¡n control and 6-hydroxydopam¡ne-treated, awake cannulated guinea p¡ss. Forced ¡mmob¡lization fo|l hr caused a 3- añd 5- fold ¡ncrease in plasma DBH and norepinephrine, respect¡vely. Pretreatment w¡th 6-hydroxydopamine (23 mglkg b.wt. i.a., 72 and 48 hr before stress) reduced by 70% the ¡ncrease in plasma DBH and totally prevented the r¡se in plasma catecholam¡ nes evoked by the restraining tress. lnjection ol ¡nsulin (5U/kg b.wt. .a.) induced a 60% decrease ¡n blood glucose, a 1-told ¡ncrease ¡n plasma DBH and a select¡ve 4-fold increase in plasma epinephr¡ne; these ettects were not mod¡fied by chem¡cal ympathectomy. Our results ¡nd¡cate that forced immobilization and hypoglycem¡a roduce a preferential act¡vation of the sympathetic postganglion¡c nerves and of the adrenal medulla, respect¡vely, and that in gu¡nea pigs both stresses increase plasma DBH. The kinet¡cs of disappearance of plasma DBH were stud¡ed after subjecting the gu¡nea p¡gs for t hr to torced ¡mmobilization. Although 7 of 12 an¡mals showed a biphasic rate of fall of plasma DBH, in each case there was a rap¡d in¡t¡alfall possibly due to the distr¡but¡on" of the enzyme with a Trl2 of 1.65 hr. Similar t¡nd¡ngs were observed in 6- hydroxydopam¡ne-treated guinea pigs. These results suggest that the disiribut¡on of DBH ¡s the most ¡mportant process ¡n reduc¡ng the augmented plasma DBH levels el¡cited by a shortterm stress and that th¡s process is not dependent on the ¡ntegrity of the sympathetic neryes nor on the adrenal or sympathetic or¡gin of the enzyme. This study supports the view that the ralio, content of releasable DBH present ¡n sympathet¡c nerves and adrenal glands/total circulating pool of DBH, is the factor that determines whether an increase ¡n plasma DBH would occur in an¡mals exposed to an acule stress.