J . Auton. Phusmucol. (1 9Y2), 12, 19 1-204

Orthostatic hypotension occurs following a,-adrenoceptor blockade in chronic prazosin-pretreated conscious spontaneously hypertensive rats J. Y. Lee, M. E. Brune, R. B. Warner & J. F. DeBernardis C'usdiovasculur Discoi~esv,Phustnuccwticui Products DiviJion, ilbbott Labosutosies. A hhott Push, Illinor, 60064, USA 1 Studies were performed to evaluate whether chronic prazosin treatment alters the a2-adrenoceptor function for orthostatic control of arterial blood pressure in conscious spontaneously hypertensive rats (SHR).

2 Conscious SHR (male 300-35Og) were subjected to 90" head-up tilts for 60s following acute administration of prazosin (0.1mg kg-' i.p.) or rauwolscine (3 mg kgl1.v.). Orthostatic hypotension was determined by the average decrease ( O h ) in mean arterial pressure (MAP femoral) over the 60-s tilt period. The basal MAP of conscious SHR was reduced to a similar extent by prazosin (--23%--26% MAP) and rauwolscine ( - 16%- - 33% MAP). However, the head-up tilt induced orthostatic hypotension in the SHR treated with prazosin ( - 16% MAP, n = 6), but not i n the SHR treated with rauwolscine (< 2% MAP, I I = 6).

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3. Conscious SHR were treated for 4 days with prazosin at 2 mg kg-I day-' i.p. for chronic a,-adrenoceptor blockade. MAP in conscious SHR after chronic prazosin treatment was 14% lower than in the untreated SHR ( n = 8 ) . Head-up tilts in these rats did not produce orthostatic hypotension when performed either prior to or after acute dosing of prazosin (0.1 mg kg-l i.p.). Conversely, administration of rauwolscine (3mgkg-'1.v.) in chronic prazosin treated SHR decreased the basal MAP by 12-31% (n=4), and subsequent tilts induced further drops of MAP by 19--23% in these rats. 4 The pressor responses and bradycardia to the a,-agonist cirazoline (0.6 and 2 pg kg-I i.v.), the a,-agonist Abbott-53693 ( I and 3pg kg-l i.v.), and noradrenaline (0.1 and 1.0pgkg-l i.v.) were determined in conscious SHR with and without chronic prazosin pretreatment. Both the pressor and bradycardia effects of cirazoline were abolished in chronic prazosin treated SHR ( n = 4 ) as compared to the untreated SHR (n=4). On the other hand, the pressor effects of Abbott-53693 were similar in both groups of SHR, but the accompanying bradycardia was greater in SHR with chronic prazosin treatment than without such treatment. Furthermore, the bradycardia that accompanied the noradrenaline-induced pressor effect in SHR was similar with and without chronic prazosin treatment despite a 47-71% reduction of the pressor effect in chronic a,-receptor blocked SHR. 5 The results indicate that chronic prazosin treatment in SHR altcrs the a,-and a*-

adrenoceptor function so that under continued a,-receptor blockade the a,-receptor mechanism(s) maintain orthostatic control of blood pressure. How an enhanced reflex bradycardia observed in chronic a , blocked SHR to the a,-receptor-mediated pressor effect is related to the a,-adrenergic mechanism(s) for postural homeostatic control of blood pressure remains to be further investigated. Correspondence: Dr Jang Y. Lee. Abbott Labs, Dept-47C, AP-9, Abbott Park, Illinois 60064, USA

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introduction Ort hostatic hypotension frequently occurs as a side-effect of antihypertensive therapy with agents that interfere with peripheral sympathetic vasoconstrictor mechanisms of a-adrenoceptors (Bendall. Baloch & Wilson, 1975: Rees & Williams, 1975; Thien, Koene & Wijdeveld, 1977; Bradshaw & Edwards, 1986). Although the exact mechanism(s) is still unclear. a failure of reflex peripheral vasoconstriction in the venous circulation due to a-adrenoceptor blockade has been suggested to be largely attributable to such an impaired cardiovascular homeostasis during the upright posture (Page ef al., 1955; Ibrahim, Tarazi & Dustan 1975; Rosendorff, 1976). Prazosin, a selective a,-blocker antihypertensive drug. has been known to cause orthostatic hypotension during its initial therapy for hypertension. The orthostatic hypotensive side-effect associated with the first dose of prazosin is greatly diminished upon subsequent medication of the drug (Graham ct al.. 1976; Turner, 1976), suggesting that upon continued a,-receptor blockade by repeated prazosin therapy, the orthostatic control of the cardiovascular system may be normalizcd via another receptor mechanism(s) to maintain the sympathetic venous tone. Recent studies (Smyth. Umemura & Pettinger, 1986: Jeffreis et a[., 1987) have shown that upon repeated prazosin treatment in rats, the a,-receptor density was increased in the kidney, and the renal sympathetic nerve stimulation-induced sodium and water retention was mediated through activation of a2-adrenoceptors.The results of these studies led to a speculation that an increase in venous a,-adrenergic receptors may maintain venous tone despite continued a,-receptor blockade, which would explain the absence of the orthostatic hypotension sideeffect with chronic prazosin therapy. In view of the above report suggesting an altered renal &,-receptor function upon repeated prazosin treatment (Smyth rt al.. 1986: Jeffreis el al., 1987), the present study was undertaken to examine whether the a2adrenoceptor function for the orthostatic control of blood pressure is altered in the genetic hypertension associated with the

spontaneously hypertensive rat (SHR) after chronic prazosin therapy. The orthostatic hypotension response to the 90” head-up tilts following acute administration of the alblocker prazosin or the a,-blocker rauwolscine (Sawyer, Warnock & Docherty. 1985; Warnock & Docherty, 1986; Lee & DeBernardis, 1990) was determined in conscious SHR with and without chronic prazosin pretreatment. In addition, the blood pressure and heart rate responses to selective agonists for the a,- and a,-receptor subtypes were examined in SHR with and without repeated prazosin pretreatment.

Materials and methods SHR (male 300-350 g) obtained from Harlan (Indianapolis, IN, USA), were anaesthetized using methoxyflurane. The left femoral artery was catheterized using polyethylene (PE 50) tubing, passed subcutaneously lo a point behind the neck and exteriorized through a skin puncture. In the case of 1.v. administration, the femoral vein on the same side was catheterized and exteriorized in the same manner.

Measurement qf mean arterial pressure und heart rate The arterial catheter was connected to a Gould Statham P-23Gb pressure transducer and the pressure recorded using a Grass 7D polygraph. Mean arterial pressure (MAP) and heart rate were determined on-line using a Buxco Electronics Hemodynamics Analyzer coupled to a Digital Micro Vax computer. Mean arterial pressure was defined as the electronic integrated average level of arterial blood pressure waveforms. Data were collected once every 5 s during the 60-s tilts. Otherwise, data were collected at oneminute intervals with an average calculatcd every 5 min. 90” head-up tilt

The rats were placed in the tilt restrainers and allowed to recover for at least 4 h after surgery. After the recovery period and a ‘training’ period consisting of 4 tilts at 5-min intervals, data collection was started. Control

ALTERED CX-ADRENOCEPTORFUNCTION IN SHR

tilts consisted of three determinations made at 1O-minute intervals prior to drug administration. After a 20-min pre-drug control period, the animals were then dosed with either prazosin at 0.1 mg kg-I i.p. or rauwolscine at 3 mgkg-' i.v., and two additional tilts were performed at 30 and 60min after dosing. The tilt experiments with either acute prazosin or rauwolscine were conducted in two groups of SHR with and without chronic prazosin pretreatment. The chronic prazosin pretreatment group received 4 days of daily i.p. injections of prazosin 2 mg kg-I. Changes (Yo) from pre-tilt baseline values of mean arterial pressure and heart rate during the last 30 s of the tilt were calculated to determine the effects to drugs on the response of mean arterial pressure and heart rate to tilting. In our preliminary report (Warner, Lee, Brune & DeBernardis, 1989) we found that rauwolscine at the 3 mg kg-I i.v. dose attenuated the pressor effects of Abbott-53693 (1 and 3 pg kg-l i.v.), a selective a,-adrenoceptor agonist, by 55-7 1Oo/ without affecting the pressor effects of the a,-adrenoceptor agonist cirazoline (0.6 and 2 pg kg-' iv), indicating that the appropriate dose of rauwolscine was used in this study for selective blockade of a,-receptors.

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Drugs Drugs used in this study were as follows: prazosin HCI (Pfizer, New York, NY, USA), rauwolscine HCI (Carl Roth, Karlsruhe, Germany), cirazoline HCI (Synthelabo, Paris, France), noradrenaline HCI (Aldrich, Milwaukee, WI, USA), and Abbott-53693 (Abbott Labs, North Chicago, IL, LJSA). All drugs were dissolved in distilled water. The doses of prazosin and rauwolscine were given in a volume of I ml kg-', whereas the other drugs were given in a volume of 0.5 ml kg-I. The chemical formula of Abbott-53693 is 2-methyl-2,3,3a,4,5,9b-hexahydro-6,7-dihydroxy- 1 H-benz(e)isoindole. Statistical analysis Data are reported as meankSEM. In some figures only mean values were used to plot curves for the purpose of clarity. Student's ttest was used to evaluate significant differences in the data between two groups of SHR with different treatments or drugs. The paired t-test was also used to analyse the data obtained within a group of animals for significant differences before and after tilts or administration of drugs. Statistical significance was accepted when P values are less than 0.05. Results

Erect of head-up tilts on MAP and H R in conscious SHR following acute a-adrenergic agonists challenge administration of prazosin and rauwolscine Cirazoline (0.6 and 2 pg kg-I i.v.) (Ruffolo & The 90" head-up tilts in conscious SHR Yaden, 1984) and Abbott-53693 ( 1 and 3 (n=6) during the control period produced no pg kg-' i.v.) (Hancock, Kyncl, Martin & De- change in the baseline MAP (184k 3 1's Bernardis, 1988; Kyncl et al., 1987; Lee, 186 +- 3 mmHg) but caused an average 14.6% in HR (baseline 3 4 3 2 13 Warner, DeBernardis & Kyncl, 1986) were increase used as selective a,- and a,-agonists, respec- beats min-I). Acute administration of prazotively. Noradrenaline (0.1 and 1 pg kg-' i.v.) sin (0.1 mgkg-I i.p.) in SHR lowered the was also employed as a non-selective agent baseline MAP b y 26% (to 137 k 4 mmHg) at that activates both subtypes of the a-adreno- 30min and by 23% (to 142k4mmH g) at ceptor (Constantine, Gunnel1 & Weeks, 60min after dosing, and HR was approxi1980; Constantine, Lebel & Archer, 1982; mately 11-13% higher than the baseline. The Lee, Walsh, Heilman & Radzialowski, 1984). head-up tilts performed at these time points Mean arterial pressure responses to these produced further drops of the MAP: an agents were determined in the groups of average of 16% to 1 16 k 8 mmHg at 30 min conscious SHR with and without chronic and an average of 15% to I23 6 mmHg at prazosin pretreatment. In addition, the 60niin post-dosing, both of which were bradycardia that occurred during the pressor significantly different from their baseline values (P Control

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in H R in response to tilts were similar during control and after prazosin administration (Fig. 1). Acute administration of rauwolscine (3.0mg kg-li.v.) in conscious SHR (n=6) also lowered the basal MAP ( 1 80 I4 mmHg) by 33'yo (to 12 1 t 5 mmHg) at 30 min and by 16O/o (to I50 k 4 mmHg) at 60 min post-dosing. and a 16% transient increase in HR occurred within the first 10 rnin after dosing.

The head-up tilts, performed for 60s at 30 and 60 min after administration of rauwolscine, did not produce any further drop in the baseline MAP from their pre-tilt control values ( I 83 2 3 vs 186 2 3 mmHg before and 1 3 2 k 5 vs 123+4mmH g at 30min and 1 5 7 k 4 lis 1 5 9 k 4 m m H g at 60min postdosing). The tilt effect on HR in SHR was similar during control and after administration of rauwolscine (Fig. 2).

ALTERED a2-ADRENOCEPTOR FUNCTION I N SHR

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Fig. 2. Effect of head-up tilts on mean arterial pressure and heart rate in conscious SHR following acute administration of rauwolscine. Rauwolscine (3 mg kg-' 1.v.) was given at 0 time. Each data point represents an average of six rats. For the purpose of clarity, no standard error bars were included. In each animal, 90" head-up tilts were performed for 60 s prior to drug administration (control), and at 30 and 60min after dosing.

Effect of head-up t i l t s on M24P and i l R in chronic prazoJin-pretreated conscious SHR following acute administration of prazosrn and rauwolscine In the S H R pretreated for 4 days with prazosin (2 mg k g - l i.p.), MAP was approximately 14% lower compared to the untreated S H R ( 1 8 3 k 3 m m H g v s 1 6 3 t 8 m m H g ; untreated vs treated, n=8 per group). In con-

trast to the SHR given prazosin acutely in which MAP was further dropped upon tilting, the baseline MAP in the chronic prazosin treated SHR ( n = 4 ) remained unchanged during the tilts (1 74 -t 2 mmHg before and I71 2 3 m m H g after tilts). Acute administration of prazosin (0.1 mgkg-'i.p.) in these chronic prazosin-treated S H R produced a minimum 5% decrease in MAP, and the

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important role in the control of the arterial, as well as the venous, tone in experimental animals and humans (Appleton et al., 1986; Elsner et a/., 1986). However, little has been known regarding the functional role of the a,-adrenoceptors with respect to orthostatic control of blood pressure in hypertension or during chronic therapy of hypertension with the a,-blocker prazosin. The data obtained in the present studies demonstrate that in the genetic hypertension of SHR ( 1 ) orthostatic hypotension can be induced by the 90" headup tilts following acute blockade of the aIreceptors with prazosin, but not following acute blockade of the a,-receptors with The M A P and H R responses to selective rauwolscine, (2) orthostatic hypotension is absent during continued blockade of the a I a-agonists in chronic prazosin pretreated receptors, but (3) could be induced by acute ronscioirs SHR blockade of the a,-receptors under chronic The selective a,-agonist cirazoline (0.6 and prazosin treatment. 2 pg kg-I i.v.) dose-dependently increased These results indicate the importance of MAP and decreased HR in conscious SHR. the a,-, as opposed to the a,-,adrenergic The effects of cirazoline at these doses were mechanism(s) for the normal orthostatic convirtually abolished in the SHR that had trol of blood pressure in SHR. It is not received prazosin 2 mg kg-' day-' i.p. for 4 certain, however, what would account for days (Fig. 5 ) . The pressor effects of the such a difference between the two subtypes of selective a2-agonist Abbott-53693 at 1 and the a-adrenoceptors for orthostatic control of 314 kg-l i.v. were similar in both groups of blood pressure. One possible explanation SHR. However, compared with the control would be that the a,-receptors located intraSHR, the bradycardia that occurred after synaptically in close proximity to the sympaadministration of Abbott-53693 at both thetic nerve terminal may play a more doses was significantly greater (P