Catecholamine metabolism and related functions in experimental hypertension

TIPS - lkcmhrr After

I980

retiring

-115

from the School of Phar-

macy, Professor Amarow

kindly allowed

antagonism to accty Icho!ine h> atrt)pmc 1111 the vascular \ide and b;, tuhtxtirarine

me to finish this research in his department

the neuromuscular

at the Royal

be specific.

Veterinary

College*. There-

after I went to Mexico City. then to Ethi-

on

side had been found to

the antagc;ntsts bcmg

more specific than the agomst

much

The work

opia and finally spent four years in Saudi

has been continued with the alpha and beta

Arabia.

blockers

After

returning

to this country in

1978. Dr Vane kindly suggested I should become

a Consultant

Laboratories.

where

at the

Wellcome

1 had started to work

of

adrenaline

and

later

uith

specific antihistamines. In 1939 there were 15011 hcbrsesat the Laboratories

If I tn to asses!, the rcsultc ofour Hc)rh III pharmacology

and antitoxic

serum produc-

1425, has been the mart produLti\c.

in 1925. and it was a great pleasure to see

tion was important.

how the Laboratory

duce active immunization have practIcalI) taken the place of all passibc antitoxm

chcmcnherapb

thcrap!,

huted 30 much tc) m_xJical hno\:Irtl~c

had developed.

The work at ihe Wellcome had altered since

Laboratories

I left in 1039. In phar-

macology there was a great development of specific

drugs.

Many

years

ago.

the

In l9HO agents to pro-

and thcrapcutics III gerwal.

the worh done at Bechcnham f:clm l(J_7:,to

and few horses remain

Tlte field of chemotherapy expand.

particularly

cclntinuec IO

with some

VINS

Jk-

Catecholamine metabolism and related functions in experimental hypertension Masayori Ozaki Hypertension

is the result of a breakdown

this strain was subdivided to produce SHR

in a variety of complex regulatory mechan-

stroke prone rats (SHR-SP)

isms. In most cases of elevated blood pres-

and his colleagues. and in these animals.

sure (BP) there is an increased sensitivit!

cerebral

to catecholamine.

occur in aremarkably

the metabolism of which

is cl~~sely related to the sympathetic new-

hemorrhages

b> Ohamoto

and/or

infarction4

high incidence. after

6 months of agea.

ous system. Thus, numerous investigations have evolved around the important role of this system in the pathogenesis and

inbred

Wistar-Kyoto

which were finally sporrtaneously

and Aoki selectively rats (WKY) produced

hvpertensive

from

a strain of rats (SHR)‘.

This strain is being used internationally

as

an excellent model of the essential hyperrenskon seen in humans. These rats develop

(CA) metabolism in the

central nervous system When

maintenance of high BP. In 1963. Okamoto

Catecholamine

norepinephrine

intraventriculnrly,

(NE)

is gibcn

there is a fall in s>strmlc

BP and a bradycardia

in animals of diffcr-

ent species including the rat. Prior treatment with a-blockers these effects.

markedly diminishes

Neurotransmitters

such as

dopamine (DA). NE and epinephrine (EN). as well as the enzymes involved in

at the rate of 100% of the

their biosynthesis,

are also found in the

population. with no treatment whatever. In

brain. Several CA

rich areas in the brain

I hese SHR. hemodynamic features such as

reputedly

rardiovascular

lation

hypertension

pathology

occur. in 1974

involved in cardiovascular

are localized

in the anterior

rrguand

It ‘ea\

a lurh> chance that kd u%to umkrtahe the treatment IO the efficienq

t)f bacterial of thcr

infxtltm\

dwawc

ot therapeutic\.

the end

has contrtand

TIPS - lkcembrr

after administration of 6-OHDA. A weak inverse a~rrclation was seen between the BP and NE contents in the medulla-ports. In addition, 2 weeks after administration of 60HDA. there puete degenerative changes in the noradrenergic cell bodies of the LC and an accuaoulatio~ of catecholaminergic fibe,rs surrounding the LC. as observrd with %rorescence histochemist@. These finclings suggest that 6OHDA-induced dencrvation may occur mainly in the dorsal bvndle originating in the LC. When assessiug the changes in serotonin I’5-HT) levels in the hypertension-induced rats, we found increased levels of 5-I-F in the spinal cord oilly. 5-HT fcvels in the spinal cord were detectable not only during the hyperteasive state but also wnen the BP had returned to the level seen before chemical lesioning. Thus, there may be an interaction between noladrenergic and serotonergic mechanisr9s on the central regulation of BP in the!e stages of hypertension. The balance between the activity of central noradrenergii: and serotonergic neurons may thus play a, de in maintaining, nonnotension. In folIowing studies, the levels of 5-HT and 5-h) droxyindole acetic acid (5HIAA) content in the cortex and spinalcord were determtid in 3-week-old SHR in which there was a low activity of DBHI in the LC. As ~:~mpared to agematched WKY. the young SHR showed an increaSe in 5-HIAA levels in the cortex,

these lmdings being similar to those in the ratsgiven 6-OHDA into the LC. However, there was no significant difference of 5-H’I and 5I-BAA levels in the spinal cord between young SHR and the age-matched WKY. The BP of young SHR rats remairled normotensive. t?4&t ob60HDA on nmoate SiiR and WILY’ To cl&date the nature of catecbolamine metabolism between normotensive WKY and SHR as related to the development of hypertension inherent to the SHR, 6-OHDA (S0 mg kg-‘, s.c.) was administered &ly to neonates of the WKY or SHR for 1 week. The growth rate, BP, and CA contents in the brain and other organs were enalysed until the rats reached several months ofage.In comparison of the effects of chemical sympathectomy between SHR and WKY, we found that: (1) the growth rate of the WKY was affected to a greater extent, (2) the BPof WKY was reduced for a longer duration (4 months) than that of the SHC ‘(2 months): and (3) the NE contents in the heart of the WKY were affected for a longer period (4 months) than that of the SHR (3 months). The effects of 6-OHDA on BP of SHR were similar to those seen in the case of inununological sympathectomy. Appar-

ently the sympatheticnervous system of the SHR is more resistant than that of WKY to the 6-OHDA treatment.

1980

Neurogenic factorsarc also importanl, in hct a close link has been found between neuwgenic and non-neurogenic factors in the pathogenesis of hypertension. In SHR, the importance of sympathetic hyperactivity in the development of hypertension, particularly in the area irnervated by splanchnic nerves, was also supported by the finding of histochemical and quantitative increases of NE and the increase of its synthesizing enzyme activities in the adrenal medulla. The ganglionic cells and terminal fibers of the sympathetic system are also hyperactive in SHR. Moreover, direct recording of the electrical activity of the left splanchnic sympathetic nerve of SHR revealed that the peripheral sympathetic tone is markedly increased compared with WKY. In 4-month-old SHR with established hypertension, NE contents per adrenal, per animal. and per gram of adrenal weight in SHR were almost twice as high as in controls. The activities of TH and DBH in the adrenals of SHR were also significantly than those of WKY’. ahigher Methyl-p-tyrosine, a TH inhibitor, has marked hypotensive effect in SHR. Enzyme activity in the adrenals of normotensive control rats increased gradually up to 16 weeksof age, while these activities in the SHR increased more rapidly up to 16 weeks and remained at a level about twice as high as in the rontrols. It is now generally considered that in SHR, NE levels and DBH activities are decreased in specific nuclei of the brain stem, in both young and adult rats. Decrease in the synthesis of NE can result under conditions where there is a reduced activation of central a-adrenergic receptors in depressor areas, and hypertension ensues. The activity of an EN-forming enzyme, phenyl-ethanolamine-N-methyltransferast! (PNMT) is increased in the brain stem of young SHR, and plasma NE and DBHactivity are also increased in these young rats. In addition plasma EN is increased in young SHR-!jP. These findings clearly central and peripheral implicate catecholaminergic neurons during the ouet and maintenance of experimental hyertension. fiypnteaslve rne&a&n ofa-metbyklopa u-Methyldopa (a-MD) is an effective decarboxylase inhibitor in virro, and this agent prevents the pharmacologic effects of dopa. The concept that a-MD effectively inhibits decarboxylation in humans and that it possesses useful antihypertensive and sedative components has been challenged since a-MD is not directly respons-

TIPS - December I980

447

TABLE I. Endogcnous catecht~lammeand a-MNE amtcnts in bin

l-iiue Hypothalamus Pon~medulla Spinal cord

Hean Adrenal Bland (Ug)

Control (vehicle)

NE (ngg-‘) Control (a-MD)

6-OHDP (a-MD)

1209r 92 537 f 1% 271 f 1.5 458 + 178 363 2 30

1604 2 163 420% 34 193% I8 378 _t 66 327 2 79

1426 + 127 5792 61 65~ 14. 496 2 137 3t’b ? 8.5

regonr and organs 4 h after a-MD adrnmetratwn

(‘ontnll (whick) 1172 c 3% 48% 9 41: 3 22r s 20.3 ? 2.8

a-MVF (/.lg g ‘)

DA(ngg”) Control (a-MD) 457 t 203 36’ h 175 5 IOr 2 12.7 z 0.8

f?-oHDP (a-MD) 16X r In* 3hr 4 hf 2’ 28: 3 171r4r

C-ontn,l

I 17 f 0 ‘ih ? 0 34. 11.26= 34 30 t

II’.? II.1x

hX)HDP 1 2h 5

Il.X

otn

I 19: II It* Il.115z IIIII

(I.02 2.10

1r.33 = (I ofi 41 711z 1Ol)o

l

lPCO.S in respect IO the controlvalue.Each value is the average c SEof four IO SIXdetermina:wm

ible for the depletion of NE and 5-HT from tissues’. In fact NE is depleted for several days even after its precursors DA and 5-HT have returned to normal levels. The product of decarboxylation of a-MD is a-methyldopamine (a-MDA) which is converted to a-methylnorepinephrine (a-MNE) by the enzyme, DBH. Other workers suggested that in patients ingesting a-MD, a-MNE is formed and stored in the sites that normally retain NE. a-MNE assumes the transmitter function of NE. although a-MNE was shown to possess a weaker transmitter activity. Thus followed the false transmitter hypothesis and it was suggested that a partial sympathetic nerve blockade occurs and that the sympathetic tone is impaired. a-MNE is considered to stimulate catecholaminergic receptors in the mediocaudai part of the nucleus tractus soiitarii (NTS). As there is little direct evidence that a-MD is converted to a-MNE in the CNS in viva, when given systemically, we investigated changes in the BP response to a-MD in SHR with damaged central

catecholaminergic neurons. Pregnant SHR were anesthetized Hith ether. and 6-hydroxydopa (h-OHDP). 50 r_Bg-‘, S.C.was given directly into the fetus through the uterine wall on the 19th and 2lst days of gestation’. The fetuses were delivered at full term and beginning at 4 weeks. the BP was measured in the conscious animals. The S-HT content was determined fiuorometrically and simultaneous estimation was made of endogenous catecholamine. a-MDA and a-MNE measured with a gas-liquid chromatographic methods and an electron capture detector. These rats treated with 6-OHDP were more labile ar the ore- or initial hypertensive state than controls. At the age of 12 weeks, the BP was as high in the 6OHDP-treated rats as in controls. The hypotensive effect of a-MD was examined in both groups a: 3(wO weeks. The 6-OHDP-treated rats had a much weaker response (40%) than that seen in the nontreated controls when 300 mg kg-‘. i.p. of a-MD was administered (Fig. 2).

8 0

. 1

2

J

3

4

hr5

In addition. MK-486 (25 mg kg-*. i.F 1 strongly inhibited the a-MD-cnduced decrea.se in the BPof the h-OHDP-treated rats. NE rc!ntents in 6-OHDP-treated rats in the pans-medulla (+7h%i and cerebellum (+ 146%) showed a remarkable increa.se of NE whereas there was a significant decrease of NE contents in the spinal cord (-87%) and the other parts of the brain (-30%). when compared with controls. Concentration of endogenous EA. \E and a-MNE in different regions c,f the brain and in various organs 4 h after a-MD adminisrration is shown in Table I. l-be endogecous DA contents in the hgpo’halamus, spinal cord and adrenal gland and ~he NE content of the spinal cord were smaller in 6-OHDP-treated rats than that in controls. when a-MI: was administered. The a-MDA content in the hOHDP-treated rats remamed unchanged in all regions. However. in this group the a-!blNE content considerably increased in the pons-meCdla and even more so in the spinal card. When 6-OHDA wa injected into the spinal cord at the Cd levels. rhe cwntenrs of both NE and a-MNE in the spinal cord caudal to the injection site were reduced ubenever the hypotensive effect of a-b¶D uas markedly reduced. These results indica.lrethat the hypot:nsive effect of a-blD involves mainly the SYS, including rhe spinal cord. Acute bypdendve effects of various &ocbPnoccptorblock&r8ents We gave &blockers to conGous 13KI’ in dIfferem doses via various routes .,rd found an acute h!;potrnsive effect vihcn optimal doses of t&e agents *err @en i p.‘@In SHR, a rather large dose ut Jr+ is required to effectiveI> induce a sfatc of phenomeno,l alw hgorension , This demonstrates that the activity of s>,mpathrtic nerves is greater in SHR than IIJ WKY. In wetbane-anesthetized rats, intravenous infusions of the blocking agents prLtiuced a rise in BP and an in~rrase in the levels of EN, NE and DA in adrenal venolis blood.

448 The evidence obtained from all these studies clearly shows the close link-up between catecholamines and hypertension and pha~a~~jo~i~al control of the Ieveis of these amines should make for a longer and healthier fife.

I thank M. Ohlara. Kyoto University for helping me to prepare this manuscript.

RerKRnghst I Okamotn. K. and Anki. 2 3 4

5 h

7

8 9 10

K. { l%3)&. Ctrr. f. 27. 282-293 Okamoto, K.. ‘f’amori. Y. and Nagaoka. A. ( 1974) ci:m-.Res. 34-35 (Suppi. I). 143-I53 Yam&. Y.. Lovenberg. W. and S~oetdsma. A. (lY?qS&nce 1xX544-546 Ogaw. M.. Fujiw, Y. and O&i. M. (1977) in sponlI??wwsHyfJerIenw. ILi Padlogmcs& and Compli~afbns. np. 15%160. U.S. Government Printing Office. Washington. D.C. Ozaki. hi.. Suzuk;~.Y.. Yamori, Y. and Okamoto. KE.(1%8)/q. C&c. 1. 32.1367-1372 Hes. A.. Connamacher, R. H.. Or&i. M. and lJde&-iend. S. (1961) J. Phmmcd. Erp. 7”n. 134.129-138 Nira. hf., Kawmo. T.. Fqita. Y.. Mzycmurs. S. and O&i. H. f 1’979) in Prophybic Approucfz lo Hpip~nerlnve 17ismse (Yamori Y. ed.). pp. 337-345. Rwen F’r&. Sew York Kawano. T.. Niwa, M., Fujita. Y.. O&i, hi. and Mori. K.(1978)Jup. 1. Phamacol. 28.168471 Sugawara. k;. ancl O&i, M. (1980) Gen. P&P mu&-.tI.ZtIi-ZI9 Sugsuara. IL. TaLami. N.. Mayemura. S., Niwa. M. and O&i. M (19&o) Eur. 1. Phamtaccd. 62, 287-295

A phiarmacological active on sm :ardiovascular Alphonse ,J. Ingenito Depwtrnen~ of Pharmacoiogy, EM Cutvlina University .‘MovI oj Medicine. Gmwilk, .-

The intent of the.following commentary is to present some viewpoints on the relationship between the known pha~acologi~ actions of tobacco smoke and its constituents, and some cardiovascular diseases that smoking iseithl~ said to be canral to or to increase the risk of developing. As such it represents the author’s opinions, some based on persona~l reseat&, and it is neither intended as an aut~~tative review nor as a mmprchmsive summary. The reader is referred tcl the U.S. Surgeon-Genend’s Reports particularly of 1964 and 1979. and the colnpendia published by Larson and his colileagues from 1961 to 1975 for a more t~~ro~h discussion and references. It is regirttable that the Larson volumes will not b: continued, sinee there is nothmg presentl:y available. under one cover, having their completeness for the tobacco and health researcher. The pubRc amtravarsy over smeking andhealth Considering the attention given the smoking and health issue in the scientific and lay presses in recent years, its contemporary importance needs no reemphasis here. It is curious that the smoking habit incurs such rancorous controversy and is #regarded as a major societal menace even though the habit is detrimental mainly to the individual and involves society to a much Iegrer degree than does, say, dependence on alcohol and other drugs of abuse. The hitter produce much more antisocial behavior than does smoking and they cause a greater number of auto accidents, crimes of violence and disruption of family life than does the cigarette. While opponents of the tobaazo habit may argue that tobaccogenic iIhtesses cause great ex~nd~t~es of public fttnds fearmedical care of these i& nesses, the same can be said tot aIo>holism and other drug abuse. In the writer’‘’ opinion it behooves each physician, who must advise his p&ems on their smoking habits, to assess the impact

NC2 7834. U.S.A.

of the habit, and its extent, in each individual disease state and patient, in the light of the known pha~a~o~ogy and toxicology of tobacco smoke and its constituents. The physician should know more about tobacco’s effects than what he reads in summaries of epiderniological data, which, though of obvious importance. do not in themselves constitute safe grounds for therapeutic advice. The decision as to the extent of the danger, should be left to the individual, with expert counsel from his physician. Driving an automobile can certainly be dangerous to one’s health, yet we see no admonitory signs affixed to cat doors. Just as the relative danger of driving depends on many factors, e.g. how and where one drives, the relative danger of tobacco smoking to each individual depends on many factors also. In view of the established role of dietary salt as a ‘risk factor’ in hypertension. it is a.wonder why warning iabets are not inch&~ t on sah packages and on the sahy convenience foods that are so popular. The disapprobation of society for tobacco smoking is as old as the custom itself, which established it&f among Westem cuhures in the 16th century. Historicaky, much of the opposition to smoking stemmed from cultural biases although medical reasons were also involved at an early stage. Concerted efforts to marshal scientific evidence against smoking began around the turn of the 20th century’@; the strongest evidence linking smoking with disease being with lung cancer, and less so with the cardiovascular diseases. In attempting to relate the smoking habit with diseases that it either causes or contributes to, the evidence should inch&, as a foundation, ex~~en~l studies in animaIs or humans, demonstrating phatmacologieaI actions of tobacco smoke or its constituents that either initiate or exacerbate the underlying disease process. Existing definitive studies of this bind are fewm, probably because they are difficult to do and even