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518 CIRCULATION RESEARCH dog. Circ Res 19: 6-10 Martins JB, Zipes DP (1980) Effects of sympathetic and vagal nerve interventions on recovery properties of the endocardium and epkardium of the canine left ventricle. Circ Res 46: 100110 Mirro MJ, Watanabe AM, Bailey JC (1980) Electrophysiological effects of disopyraxnide and quinidine on guinea pig atria and canine cardiac Purkinje fibers. Dependence on underlying cholinergic tone. Circ Res 46: 660-688 Rabinowitz SH, Verrier RL, Lown B (1976) Muscarinic effects of vagosympathetic trunk stimulation on the repetitive extrasystole (RE) threshold. Circulation 63: 622-627 Rinkenberger RL, Prystowsky EN, Heger JJ, Troup PJ, Jackman WM, Zipes DP (1980) Effects of intravenous and chronic oral verapamil administration in patients with supraventricular tachyarrhythmias. Circulation 62: 997-1010 Roskoski R, Mayer HE, Schmid PG (1974) Choline acetyltransferase activity in guinea pig heart in vitro. J Neurochem 23: 1197-1200 Schmid PT, Greif BJ, Lund DD, Roskoski R (1978) Regional choline acetyltransferase activity in guinea pig heart. Circ Res 42: 657-660 VOL. 49, No. 2, AUGUST 1981 Smith RB (1971) Observations on nerve cells in human, mammalian and avian cardiac ventricles. Anat Anz 129: 436—444 Spear JF, Moore EN (1973) Influence of brief vagal and stellate nerve stimulation on pacemaker activity and conduction within the atrioventncular conduction system of the dog. Circ Res 32: 27^11 Tcheng KT (1951) Innervation of the dog's heart Am Heart J 41: 512-524 Watanabe AM, Besch HR (1975) Interaction between cyclic adenosine monophosphate and cyclic guanoside monophosphate in guinea pig ventricular myocardium. Circ Res 37: 309317 Waxman MB, Wald RW (1977) Termination of ventricular tachycardia by an increase in cardiac vagal drive. Circulation 56: 385-391 Wellens HJJ, Bar FWHM, Lie KL, Duren DR, Dohmen HJ (1977) Effect of procawamide, propranolol and verapamil on mechanism of tachycardia in patients with chronic recurrent ventricular tachycardia. Am J Cardiol 40: 579-685 Wildenthal K, Mierzwiak DS, Wyatt HL, Mitchell JH (1969) Influence of efferent vagal stimulation on left ventricular function in dogs. Am J Phyaiol 216: 577-581 Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 Blood Pressure Response to Central and/or Peripheral Inhibition of Phenylethanolamine iV-Methyltransferase in Normotensive and Hypertensive Rats JESSIE BLACK, BERNARD WAEBER, MARGARET R. BRESNAHAN, IRENE GAVRAS, AND HARALAMBOS GAVRAS SUMMARY We studied the effects on blood pressure and heart rate of two different phenylethanolamine JV-methyltransferase (PNMT) inhibitors in normotensive, in two-kidney renal hypertensive, and in deoxycorticosterone-salt (DOC-salt) hypertensive rats. One compound (SK&F 64139) blocks the conversion of norepinephrine to epinephrine in both the central and the peripheral nervous system, whereas the other (SK&F 26861) does not cross the blood-brain barrier and therefore is active mostly in the adrenal glands. In the rats given SK&F 29661, practically no acute blood pressure changes were observed. Central PNMT inhibition with SK&F 64139 induced only a minor blood pressure and heart rate response in normotensive and two-kidney renal hypertensive rats. However, in DOC-salt hypertensive rats, it reduced arterial pressure to approximately normal levels and concomitantly glowed pulse rate. There was a close correlation between the magnitude of the blood pressure response observed in all SK&F 64139-treated animals and the control plasma norepinephrine (r = —0.795, P < 0.001) and epinephrine (r — -0.789, P< 0.001) levels. These results suggest an important role for central epinephrine in regulating the peripheral sympathoadrenomedullary and the baroreceptor reflex activity, particularly when the maintenance of the high blood pressure is not renin-dependent. Circ Res 49: 618-524, 1981 CENTRAL catecholaminergic neurons are thought to participate in the regulation of normal blood pressure and in the development and maintenance of high blood pressure in several types of experiFrom the Department of Medicine and the Thomdlke Memorial Research Laboratories, Boston University Medical Center, Boston, M u sachuselLe. Thra work was completed during the tenure of Dr. H. Gavras as an Established Investigator of the American Heart Association. Supported in pan by U.S. Public Health Service Grant HL-183ia Dr. Waeber is supported by the Swiss National FoundationAddress for reprints: Dr H Gavraa, 80 E. Concord St., Boston, Massachusetts 02118. Received November 5. I960; accepted for publication April 7, 1981 mental hypertension (Chalmers, 1975). At least some of the brain areas involved in cardiovascular homeostasis are located in the brain stem (Doba and Reis, 1974; Chalmers, 1975). Such areas not only are innervated richly with catecholarninergic cell bodies and terminals (Fuxe, 1965; Bolme et al., 1972), but also have the highest activity of phenylethanolamine 7V-methyltransferase (PNMT) (Saavedra et al., 1974), the enzyme which catalyzes the last step in epinephrine formation (Axelrod, 1962). A neurogenic component has been proposed as a pathogenic factor in the hypertension induced by mineralocorticoids and salt (Nakamura et al., 1971; PNMT INHIBITION IN EXPERIMENTAL HYPERTENSION/Btoc* et al. Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 deChamplain and van Ameringen, 1972; Chalmers, 1975; Reid et al., 1975). In this model, rats with established high blood pressure have increased levels of PNMT activity in at least one region of the brain stem, and demonstrate a decrease in blood pressure to normotensive values following administration of a PNMT inhibitor (SK&F 7698) (Saavedra et al., 1976). However, the precise antihypertensive mechanism of that inhibitor remains unclear, particularly in view of the potent a-adrenergic-inhibiting properties of the compound (Pendleton et al., 1974). The purpose of this study was to evaluate the acute effect on blood pressure of two recently synthesized PNMT inhibitors in rats with experimental hypertension induced by renal artery stenosis or mineralocorticoid and salt excess, i.e., two forms of hypertension considered to be, respectively, the prototypes of renin-dependent (during the early phases of hypertension) (Brunner et al., 1971; Leenen et al., 1973; Carretero and Gulati, 1978) and non-renin-dependent hypertension (Gavras et al., 1975). One of these compounds (SK&F 64139) inhibits PNMT activity in both the central nervous system and the adrenal gland, and, in vitro, is a weak a-adrenergic antagonist (Pendleton et al., 1976; Pendleton et al., 1977); the other (SK&F 29661) does not cross the blood-brain barrier and thus does not block the conversion of norepinephrine to epinephrine in the brain (Pendleton et al., 1979). Methods Animals Male Wistar rats from Charles River Breeding Laboratories were used in all experiments. They were housed in a temperature- and humidity-controlled room with automatic lighting in 12-hour cycles. Normotensive Rats (Group A) These animals were kept on a usual diet of Purina Rat Chow and tap water ad libitum. At the time of study, the rats weighed 260 ± 6 g (mean ± SE) . Two-Kidney Renal Hypertensive Rats (Group B) These rats underwent a left lateral flank incision under ether anesthesia when they weighed 140-160 g, and a silver clip was applied, as previously described, to the left renal artery (Byrom, 1969). The right kidney was left intact. The rats then were allowed unrestricted access to laboratory chow and tap water. On the day of the experiment 3-4 weeks later, their weight averaged 284 ± 12 g. Deoxycorticosterone (DOC)-Salt Hypertensive Rats (Group C) A left nephrectomy was performed under ether anesthesia in rats weighing 140-160 g. One week later, the DOC-salt regimen was started. The rats were given subcutaneous injections of DOC (Percorten Pivalate, CIBA), 30 mg/kg body weight per 519 week in divided doses of 15 mg/kg body weight at 3- to 4-day intervals, and had free access to 1% NaCl as drinking water and usual laboratory chow. At the time of the study, 4 weeks following the initiation of the DOC-salt regimen, the animals weighed 293 ± 6 g. Procedures and Analytical Methods On the day of the experiment, under light ether anesthesia, the right femoral vein was cannulated with a PE-10 and the right external iliac artery with a PE-50 catheter. Both catheters contained a heparinized 5% dextrose solution. Arterial pressure then was monitored continuously with a HewlettPackard transducer (model P23 Db) and recorder (model 7702). Pulse rates were derived from the arterial pressure tracings. Upon awakening, the animals were maintained in a semirestrained position on a light mesh screen for 60-90 minutes until blood pressure rose to a steady baseline. At that time, 0.3 ml blood was withdrawn via the arterial catheter for determination of plasma catecholamines. That blood volume was replaced by an equivalent amount of 5% dextrose, and 20 minutes later, the first dose of the test drug was administered. Plasma norepinephrine and epinephrine levels were assayed, as described elsewhere (Bresnahan et al., 1980), with a modification of the method of Peuler and Johnson (1977). SK&F 64139 (7,8-dichloro-l,2,3,4-tetrahydroisoquinoline hydrochloride) and SK&F 29661 (1,2,3,4tetrahydroisoquinoline-7-sulfonamide) were supplied by Smith, Kline & French Laboratories. Both drugs were diluted in 5% dextrose to achieve a final concentration of 25 mg/ml. Each rat received two 5-mg doses of one inhibitor given 1 hour apart as a 0.2-ml solution injected over a 5-minute period. The effect on blood pressure of the different compounds was observed for 3 hours in all the animals. Seven rats in group A, six in group B, and eight in group C received SK&F 29661; whereas six in group A, eight in group B, and seven in group C were given SK&F 64139. Data Analysis Data are reported as means ± SE. Statistical evaluation of the results was made using analysis of variance for one group having repeated measures, one-way analysis of variance followed by Scheffe multiple comparison procedure, and Student's t-test for nonpaired data as appropriate. Correlation coefficients of regression lines were calculated by the method of least squares. A probability level of less than 0.05 was considered significant. Results The control mean blood pressures and pulse rates of these different groups of rats are summarized in Table 1. Figure 1 depicts the time courses of the effect on blood pressure of SK&F 29661 and SK&F 64139 administered to normotensive rats, two-kidney renal hypertensive rats, and DOC-salt hypertensive rats. CIRCULATION RESEARCH 520 VOL. 49, No. 2, AUGUST 1981 TABLE 1 Control Blood Pressures and Heart Rates in Normotenswe, Two-Kidney Hypertensive, and DOC-Salt Hypertensive Rats Two-kjdney renal hyperteruiive rata Normotensive rats Control mean blood pressure <mm Hg) Control pulse rate (beats/min) DOC-saJt hypertensive rau> SK&F 29661 in - 7) SK&F 64139 (n - 6) SK&F 29661 in - 6) SKfcF 64139 (n - 8) SK&F 29661 in - 8) SK&F 64139 in - 7) 120 ± 1 . 5 120 ± 1 3 L89 ± 7 179 ± 6.9 176 ± 6.7 194 ± 6.8 480 ± 8 460 ± 13 470 ± 18 473 ± 8 443 ± 11 446 ± 18 Baseline hlood pressures and heart rates in the three groups (mean ± SE) are not flignificantly diiTerent between animala that received SK&F 29G61 and those that received SK&F 64139. Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 In normotensive rats, SK&F 29661 did not decrease the blood pressure, whereas SK&F 64139 caused a significant reduction in blood pressure (P < 0.01) with a maximal drop of 12 mm Hg. In two-kidney renal hypertensive rats, SK&F 29661 had only a minor antihypertensive effect. SK&F 64139 produced a more pronounced blood pressure response, which was still nonsignificant. At the end of the experiment, rats which had received either agent were still hypertensive with mean blood pressures of 181 ± 7.5 and 165 ± 6.9 mm Hg, respectively. Although DOC-salt rats also responded poorly to the administration of SK&F 29661, those given the peripherally and centrally active PNMT inhibitor SK&F 64139 showed a progressive decrease of blood pressure toward approximately normal levels (P < 0.001). Three hours after initiation of the experiment, the mean arterial pressure was still low at 128 ±6.1 mm Hg in these animals, whereas it remained high at 165 ± 5.5 mm Hg in those treated with SK&F 29661. Figure 2 illustrates the changes in pulse rate I 5mq IV , 5mg IV + 2 0 - NORMOTENSIVE induced by SK&F 29661 and SK&F 64139 in the different groups of rats. With SK&F 29661, a trend toward a slower pulse rate was observed only in the DOC-salt hypertensive rats. The administration of SK&F 64139 to normotensive and two-kidney renal hypertensive rats induced only minor reductions in heart rate. In contrast, in DOC-salt hypertensive rats, a significant decrease in pulse rate from 446 ± 18 to 326 ± 22 beats/min (P< 0.001) was observed with SK&F 64139. A close correlation (r - 0.80, P < 0.001) appeared between the blood pressure and heart rate changes measured at the end of the experiment in all animals given SK&F 64139. The control plasma catecholamine levels for all the rats are plotted individually in Figure 3. In the normotensive rats, plasma norepinephrine and epinephrine levels averaged 0.689 ± 0.077 and 0.539 ± 0.088 ng/ml, respectively, whereas in the two-kidney renal hypertensive rats, the same parameters were somewhat higher, at 0.912 ± 0.106 and 0.904 ± 0.149 ng/ml, respectively, but the differences were not significant. In the DOC-salt hypertensive rats, both plasma norepinephrine (1.726 ±0.115 ng/ o SKSF 29661 • SK3F 64139 -+20 o--20 TWO-KIDNEY RENAL HYPERTENSIVE °r - —0 5 -0 ; --20 0. -40 I. DOC-SALINE HYPERTENSIVE -*•—--*—-+ ^ -201- 4 -0 -"20 --40 --60 "60 r- ---80 Control 15 30 45 60 90 MINUTES 120 150 190 FIGURE 1 Blood pressure effects of PNMT inhibition with SK&F 64139 and SK&F 29661 in normotensive rats, two-kidney renal hypertensive, and DOC-salt hypertensive rats. PNMT INHIBITION IN EXPERIMENTAL HYPERTENSION/B/aeA et al. 521 5rnglV SK3F 2966) SK3F 64139 ••40 -U40 _ NORMOTENSIVE , o 5 5 5-— -40 Ho -40 ~TWO-KIONEY RENAL HYPERTENSfVE + 40 -40 DOC SALINE HYPERTENSIVE FIGURE 2 Puke rate response to PNMT inhibition with SK&F 64139 and SK&F 29661 in normotensive rats, two-kidney renal hypertensive, and DOC-salt hypertensive rats. —5-40 Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 -80 -120 -160 Centre* IS ml) and epinephrine (1.349 ± 0.173 ng/ml) levels were significantly higher (P < 0.05) than those determined in the normotensive rate. Compared to the two-kidney renal hypertensive rats, only the plasma norepinephrine levels were significantly (P < 0.05) increased in the DOC-salt hypertensive rats. Figure 4 illustrates the relationship between changes in blood pressure induced by SK&F 64139 and the control plasma catecholamine levels measured in all rate given that compound. A close correlation emerged between the magnitude of the blood pressure changes recorded at the end of the experiment and both the corresponding control norepinephrine (r = -0.795, P < 0.001) (left panel) and epinephrine (r = -0.789, P < 0.001) (right panel) levels. Discussion Recent work has focused on a possible specific involvement of the central adrenergic system in the regulation of normal blood pressure, as well as in the pathogenesis of some forms of hypertension (Saavedra et aL, 1974, 1976). The existence of epinephrine-containing neurons in the brain has been ascertained in two ways: either by demonstrating the presence of PNMT by immunofluorescence (Hokfelt et al., 1973, 1974) and biochemical techniques (Saavedra et al., 1974; Lew et al., 1977) or by measuring directly the hormonal content (Koslow and Schlumpf, 1974). Not surprisingly, since PNMT is most likely localized in the epinephrine-producing neurons, the brain PNMT and epinephrine were found to be similarly distributed (Saavedra et al., 1974; Koslow and Schlumpf, 1974). In normotensive rats, the greatest central PNMT activity has been detected in brain stem areas (Saavedra et al., 1974) such as in the A, region which represents the catecholaminergic cell bodies projecting their axons to the spinal cord and in the A2 region (Fuxe, 1965; Bolme et al., 1972) which includes the nucleus tractus solitarii, the terminal of the majority of the fibers originating from the carotid sinus (Crill and Reis, 1968; Doba and Reis, 1974). In addition, both noradrenergic nerve endings and catecholaminergic cell bodies have been described at the level of the A2 region (Fuxe, 1965; Bolme et al., 1972). Experimental evidence suggests that the same areas actively participate in blood pressure regulation (Doba and Reis, 1974; Chalmers, 1975). In the present study, we administered two different PNMT inhibitors, SK&F 29661 and SK&F 64139, to unanesthetized rats. Both compounds are potent in vitro inhibitors of PNMT activity (Pendleton et al., 1976, 1977, 1979). However, unlike SK&F 64139, which in vivo inhibits the brain stern, as well as the adrenal PNMT activity (Pendleton et al., 1976, 1977), SK&F 29661 does not cross the blood-brain barrier (Pendleton et al., 1979). Therefore, when given orally up to a dose of at least 100 mg/kg, it remains without effect on brain stem PNMT activity (Pendleton et al., 1979). The doses used, i.e., two intravenous injections of 5 mg in each rat at a 1-hour interval, were identical for the two inhibitors and are thought, on the basis of previous studies (Pendleton et al., 1976, 1977, 1979; Sauter et al., 1977), to be sufficient to block effectively the conversion of norepinephrine to epinephrine during the observation period. Although a 15-mm Hg blood pressure decrease has been demonstrated in normotensive rate as the CIRCULATION RESEARCH 522 i p<005 p<005 NS o I 24- OSK8F29661 • SK8F64139 o • • 18- 1 o i. °:° • 12o n ° ^J 0 06- ° • o o Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 G I 1 1 p<005 NS I NS 24- I 0 18- 1 S 06- 1 O o • • 8 • 0 o ° 0 to ' • o | o o • 2• 1 WRMOTENSIVE RATS TWO KIONEr DOC-SALINE '—HYPERTENSIVE RATS—' 3 Control plasma norepinephrine and epinephrine levels in normotenswe rats, two-kidney renal hypertensive, and DOC-salt hypertensive rats. FIGURE immediate consequence of adrenalectomy (deChamplain and van Ameringen, 1972), the lack of acute pressure changes observed in our normotensive rats given SK&F 29661 is not surprising, even in the face of presumably effective blockade in epinephrine generation, because it has been established that the adrenal epinephrine turnover is very slow under normal conditions (Fuller et al., 1974). Following both peripheral and central PNMT inhibition with SK&F 64139, the blood pressure decrease in our normotensive rats was small, although presumably the brain stem PNMT activity was still markedly suppressed. Indeed, other investigators, using doses of SK&F 64139 (about 12.5 VOL. 49, No. 2, AUGUST 1981 mg/rat, ip) very close to ours (10 mg/rat, iv), measured after a 3-hour interval a reduction of PNMT activity of approximately 90% in selected regions of the medulla oblongata (Sauter et al., 1977). We investigated the blood pressure response to PNMT inhibition with both compounds also in twokidney renal and DOC-salt hypertensive rats. These two experimental models of high blood pressure have been chosen because, in each one, different mechanisms seem to be primarily responsible for the development and the maintenance of the elevated blood pressure levels. Indeed, an activation of the renin-angiotensin system has been proven to be linked to t>.e increase in pressure levels observed in the early phases of renovascular hypertension (Brunner et al., 1971; Leenen et al., 1973; Carretero and Gulati, 1978), but not in the DOC-salt model of hypertension (Gavras et al., 1975). Blockade of the renin axis with saralasin, a competitive inhibitor of angiotensin II, markedly lowers arterial pressure in the former (Brunner et al., 1971; Carretero and Gulati, 1978), but not in the latter (Gavras et al., 1975). On the other hand, the central and peripheral catecholaminergic neurons, as well as the adrenal medulla, seem to be involved in the pathogenesis of DOC-salt hypertension (Nakamura et al., 1971; deChamplain and van Ameringen, 1972; Chalmers, 1975; Reid et al., 1975). Interestingly, the plasma norepinephrine levels determined in this model of hypertension were reported to be significantly higher than those measured in untreated controls (Reid etal., 1975). The time course of the blood pressure response to the two PNMT inhibitors was similar in the twokidney renal hypertensive rats to that recorded in the DOC-salt hypertensive animals. However, the magnitude of the blood pressure reduction using SK&F 64139 was far less pronounced in the renovascular hypertensive animals. In that model of high blood pressure, rats remained hypertensive despite peripheral and central inhibition of PNMT, suggesting that overactivity of the central adrenergic system is not of major importance. In our DOCsalt hypertensive rats, an impressive blood pressure fall was produced acutely by the centrally active PNMT inhibitor only. In these rats, the antihypertensive effect of SK&F 64139 was already important within the first 15 minutes of inhibition of PNMT, and blood pressure normalization was progressively achieved thereafter. Especially relevant to our study is the fact that PNMT activity has been reported to be elevated in the Ai brain stem areas of adult mineralocorticoid salt hypertensive rats and that oral administration of a PNMT inhibitor (SK&F 7698) resulted in a decrease of blood pressure toward normal levels in these rats (Saavedra et al., 1976). Unfortunately this compound has about equal affinity for norepinephrine sites on the epinephrine-synthesizing enzyme and the «-adrenergic receptors, thus confusing interpretation of its effects (Pendleton et al., 1974). ELECTROPHYSIOLOGICAL EFFECTS OF AUTONOMIC BLOCKADE/Prystowsky et at. 523 ng/ml PLASMA NOREPINEPHRINE PLASMA EPINEPHRINE r =-0 789 p< 0 001 r=p< 0001 30 24 12 • Normotenave * Two-Kidney Renol Hypertensive • OOC-Soline Hypertensive "120 -80 06 -40 0 -120 -80 -40 0 A MEAN BLOOD PRESSURE AFTER SKaF64f39(mmHg) Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 4 Relationship between the control plasma norepinephrine and epinephrine levels and the magnitude of the blood pressure response to PNMT inhibition with SK&F 64139 in normotensiue, two-kidney renal hypertensive, and DOC-salt hypertensive rats. FIGURE In the SK&F 29661-treated rats of either model, only a slight blood pressure decrease was observed from the 90th minute of the experiment onwards. This blood pressure response may not necessarily be due to the inhibited formation of epinephrine in the peripheral sympathetic nervous system, ie., in the adrenal glands, which are almost exclusively the site of epinephrine secretion. Indeed, it cannot be ruled out that SK&F 29661, which does not reach the brain in the normotensive state, may be able to cross the blood-brain barrier in some animals if the barrier has been more or less disrupted by high pressure levels (Hatzinikolaou et al., 1980). SK&F 64139 has been demonstrated in studies in vitro to have only a weak affinity for both a-adrenoceptors and monamine oxidase (MAO) (Pendleton et al., 1976). In our rats given this compound, it is unlikely that the parallel changes in blood pressure and pulse rate were due to a-blockade alone. The decelerating effect on pulse rate of SK&F 64139 contrasts with the dose-dependent tachycardia which in normotensive dogs accompanied a small decrease in blood pressure induced by another PNMT inhibitor with important a-blocking action (SK&F 7698) (Pendleton et al., 1974). On the other hand, a significant inhibition of MAO seems not to occur in intact animals since no potentiation of the pressor effect of tyramine or of tryptamine-induced convulsive activity was found in rats treated with three oral doses of 100 mg/kg of SK&F 64139 administered over a 24-hour period (Pendleton, 1979). The antihypertensive action of SK&F 64139 may be due primarily to the suppression of epinephrine synthesis in specific brain vasomotor centers. In our rats, a close correlation was found between the control plasma norepinephrine and epinephrine levels and the magnitude of the SK&F 64139-induced blood pressure drop. This correlation sup- ports the concept that central adrenergic neurons play a key role in controlling the sympathoadrenomedullary activity. In addition, the fact that the cardiac decelerating effect of SK&F 64139 was most marked in the rats with the greatest blood pressure fall, in which a tachycardia rather than a bradycardia would be expected, is consistent with an inhibition of the baroreceptor reflex by central PNMT inhibition. This tends to confirm an active participation of epinephrine as a neurotransmitter also in the central part of the baroreceptor reflex arc, most likely at the level of the nucleus tractus solitarii. A concomitant reduction in blood pressure and heart rate previously has been demonstrated to occur after microinjection of norepinephrine in the area of the nucleus tractus solitarii (DeJong et al., 1975), as well as following administration of clonidine (Kobinger and Pichler, 1974) or injection of tyrosine (Bresnahan et al., 1980). In these circumstances, the changes in both blood pressure and pulse rate generally were thought to be the consequence of a central a-catecholaminergic receptor stimulation. In our rats, there is no evidence that the cardiovascular response to central PNMT inhibition was promoted by an enhanced noradrenergic receptor stimulation. Indeed, no accumulation of norepinephrine has been detected in brain stem despite effective blockade of epinephrine formation induced by SK&F 64139 (Pendleton, 1979). Notwithstanding, it cannot be proven that this unchanged norepinephrine content corresponds to an unchanged norepinephrine turnover in the same areas. Interestingly, recent studies have also pointed out the possible role of central epinephrine in cardiovascular regulation. The reported findings have shown that one of the final effects of a-agonistic properties of clonidine on adrenoceptors is a decrease of epinephrine turnover in some brain 524 CIRCULATION RESEARCH Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 stem areas (Fuxe et al., 1979; Scatton et al., 1979), an effect which, in turn, might be important for the cadiovascular response to the drug. Although a similar relationship between central norepinephrine a-receptor stimulation and epinephxine turnover cannot be extrapolated with certainty from these studies, it is intriguing that, in the DOC-salt hypertensive rats, in which blood pressure can be normalized by the central PNMT inhibitor, a decreased turnover of norepinephrine was measured in the medulla oblongata, while in other parts of the brain, it was comparable to control values (Nakamura et al., 1971). 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Science 191: 483-484 Sauter AM, Lew JY, Baba Y, Goldstein M (1977) Effect of phenylethanolamine N-methyltransferase and dopamine-hydroxylase inhibition on epinephrine levels in the brain. Life Sci 21: 261-266 Scatton B, Pelayo F. Dubocovich ML, Langer SZ, Bartholini G (1979) Effect of clonidine on utilization and potassium-evoked release of adrenaline in rat brain areas. Brain Res 176: 197201 Blood pressure response to central and/or peripheral inhibition of phenylethanolamine N-methyltransferase in normotensive and hypertensive rats. J Black, B Waeber, M R Bresnahan, I Gavras and H Gavras Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017 Circ Res. 1981;49:518-524 doi: 10.1161/01.RES.49.2.518 Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 1981 American Heart Association, Inc. All rights reserved. Print ISSN: 0009-7330. 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