Dual vasoactive effects of tolazoline on rabbit pulmonary arteries

D u a l V a s o a c t i v e E f f e c t s of T o l a z o l i n e on R a b b i t P u l m o n a r y A r t e r i e s Yai-Shion Lee, MD, and Xmhua Hou, MD Objectives: To determine the vasoactive effects of tolazoline on isolated rabbit pulmonary arteries. Design: Prospective, in vitro, randomized, controlled trial. Setting: Experimental laboratory in a university-affiliated hospital. Participants: New Zealand White Rabbits. Interventions: The pulmonary artery rings were obtained via thoracotomy. Their vasoactive responses were assessed in the presence and absence of intact endothelium and with or without precontraction by norepinephrme (NE, 3 x 10 -6 M) or potassium chloride (KCI, 3 x 10 -2 M). Using a tissue bath preparation, cumulative concentration response curves of tolazoline were obtained at different concentrations (10 -9 to 10 -4 M) after a period of stabilization. Measurements and Main Results: Tolazoline caused vasoconstriction of isolated pulmonary arteries without any pretreatment. The magnitude of the constriction was dose E C E N T R E P O R T S , b o t h chmcal a n d experimental, of favorable results with nitric oxide o n the p u l m o n a r y circulation have b e e n encouraging. However, t h e ideal p u l m o n a r y vasodilator for the t r e a t m e n t of persistent p u l m o n a r y hypertensaon r e m a i n s elusive. 1 Tolazoline was once c o n s i d e r e d to b e an amportant, effecnve, chmcally available p u l m o n a r y vasodilator a n d was used to r e d u c e elevated p u l m o n a r y vascular resistance caused by pulmonary h y p e r t e n s i o n of variable ehologlesf1-1° particularly persistent p u l m o n a r y h y p e r t e n s i o n of the n e w b o r n (persist e n t fetal circulation). 11 It was b e h e v e d t h a t t o l a z o h n e might have a direct r e l a x a n o n effect o n vascular s m o o t h muscle or a primary a l p h a - a d r e n e r g i c r e c e p t o r blocking effect. 3,1°,12-~6 However, t h e exact m e c h a m s m is not clear. O n the o t h e r h a n d , climcally as well as experimentally, paradoxical responses of the p u l m o n a r y carculanon to tola z o h n e have b e e n r e p o r t e d 10,13,14,17,18 In most instances, p u l m o n a r y arterial p r e s s u r e a n d p u l m o n a r y vascular resastance are usually d e c r e a s e d after tolazoline a d m i m s t r a n o n . N o n e t h e l e s s , it has b e e n r e p o r t e d t h a t in some cases, such as p a n e n t s w~th mltral stenosls, t o l a z o h n e may cause e i t h e r an increase or d e c r e a s e or n o effect at all on p u l m o n a r y artery pressure a n d total p u l m o n a r y resistance. 19 To date, the direct effect of tolazoline on p u l m o n a r y arteries has not b e e n clearly defined. T h e p u r p o s e of this study was to m v e s n g a t e the darect vasoactive effects of tolazoline on isolated r a b b i t p u l m o n a r y arteries In o r d e r to f u r t h e r u n d e r s t a n d the m e c h a n i s m , p u l m o n a r y arteries with or w~thout intact e n d o t h e l i u m were p r e t r e a t e d with p o t a s s m m chloride (KC1) or n o r e p l n e p h r i n e (NE).

R

From the Department of Anestheszology, Halbor-UCLA Medtcal Center, Torrance, CA Address reprint requests to Tal-Shton Lee, MD, Department of Anesthestology, Harbor-UCLA Medzcal Cento. 1000 W Carson St, Tormnce, CA 90509-2910 Copyright © 1996 by W B Saunders Company 1053-0770/96/1003-001253 00/0

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related and reached 300 g / g wet tissue at a concentration of 10 4 M. On KCI-precontracted pulmonary arteries, tolazoline caused significant dose-related vasoconstriction. On the NE-precontracted vessel rings, it elicited significant dosedependent vasodilation up to 60% relaxation at 10 -5 M. All the above effects were endothelium independent Conclusions: Tolazoline has dual endothelium-independent vasoactive effects, causing vasoconstriction on isolated rabbit pulmonary arteries, either untreated or precontracted with KCI, and vasodilation on those precontracted with NE. Tolazoline may act as a competitive alpha-adrenoceptor blocking agent.

Copyright © 1996by W.B. Saunders Company KEY WORDS: tolazoline, pulmonary vasoconstrictor, pulmonary v a s o d i l a t o r

METHODS

Seven New Zealand White Rabbits, weighing 2 to 2 5 kg, were used for the experiment The rabbits were anesthetized with sodium pentobarbltal, 40 mg/kg A segment of the main pulmonary artery just distal from the bifurcation was removed immediately after respiratory arrest through a mldsternotomy Precaunon was exercised to avoid damage of the intimal surface The vessels were freed from the connective tissue and cut Into 3-mm wide rings. Each ring of pulmonary artery was Immersed in a 10-mL nssue bath containing a continuously oxygenated (95% 02, 5% CO2) Krebs soluhon at 37°C and pH 7 4. The solunon consisted of 118 mmol NaC1, 0.8 mmol KC1, 2 5 mmol CaC12, 25 mmol NaHCO3, 1 18 mmol KHzPO4, 1 19 mmol mgCO4, and 11 mmol glucose The ring was suspended between a fixed hook and an isometric transducer (Grass FT03, MA). The contracnons were recorded on a polygraph (Beckman R612, CA) The resting tension of each ring was adjusted to 3.75 gm based on the prior search of the opumal resting tension (Lma×) in rabbit pulmonary arteries. At least 1 hour was allowed for eqmhbranon and stabflxzation Two groups of vessels were prepared one with and one without intact endothelium Both were tested under the same condmons for comparison The de-endothehalization was performed by gently rubbing the mtimal surface with forceps and confirmed pharmacologically by standard test with NE (3 × 10 -7 M) and acetylchohne (10 6 M) Two (one intact and one denuded) pulmonary artery rings were obtained from each animal Seven rings were prepared for each set of the experiment (total of 14 rings used) Protocol Step 1 After stabilization, cumulative concentration-response curves of tolazohne were constructed by adding increasing concentrations of tolazohne in logarithmic increments (10 -s to 10 -4 M) Step 2 The rings were washed after step 1 until complete recovery of the resting tension was reached Another hour was allowed for equilibration. Then submaxlmal precontractlon of the vessels was induced by KCI (3 × 10 -~- M). When contractile responses reached a plateau, cumulatwe concentration-response curves of tolazoline (10 -9 to 10 -5 M) were obtained m the same manner as in step 1 Step 3 The procedures were repeated as m step 2, replacing the KCI (3 × 10 2 M) with NE (3 × 10 6 M) The order of using non-pretreated, KC1- or NE-pretreated pulmonary artery rings m each step was randomized

Journal of CardJothoractc and Vascular Anesthesia, Vol 10, No 3 (April), 1996 pp 364-367

VASOACTIVE EFFECT OF TOLAZOLINE

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After each assay, each pulmonary artery ring was removed from the organ bath, blotted dry, and weighed All results were expressed as the mean _+SEM The difference between mean values was analyzed by analysis of variance and Student's t test. The difference was considered stanstlcally slgmficantwhen P < 0.05

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RESULTS

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Effects of tolazohne on isolated untreated pulmonary arteries with or without Intact endothelium: W~thout any pretreatment, tolazoline alone caused vasoconstriction of isolated pulmonary arteries (Fig 1). There was no difference in groups with or without intact endothelium; it was dose related. The force of contraction reached to 300 g/g wet tissue when the concentration was 10 .4 M. Effect of tolazohne on isolated KCl-precontracted pulmonary arteries with or without intact endothelium: Tolazoline caused vasoconstriction in the KC1 (30 mM)precontracted pulmonary arteries in a dose-related manner (Fig 2). At 10 .5 M, it increased constriction by almost 18%. This vasoconstriction was endothehum-independent Effect of tolazohne on isolated, NE (3 x 10.6 M)precontracted pulmonary arteries with or without Intact endothehum' Tolazohne elicited significant dose-dependent vasodllatanon in NE-precontracted pulmonary arteries (Fig 2). At 10 -5 M, it produced about 60% relaxation from the controlcontraction by NE (3 x 10 .6 M). There was no difference between groups wlth or without intact endothehum. Tolazoline appeared to have a dual vasoactive effect.

8

DISCUSSION

Tolazohne was once widely used in infants and children for the management of pulmonary hypertensive crises caused by a variety of etIologms.2°-23It has also been used m children as well as adults to treat postbypass pulmonary hypertension in cardiac surgery}3,24 The use of tolazollne may result in a rapid and sustained decrease in the ratio of systolic pulmonary mtery pressure to systemic artery pressure and prevent a fatal outcome from acute elevation of pulmonary vascular resmtance.24 However, the results of climcal experience with tolazo-

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Fig 2 Comparatlve effects of tolazoline on isolated, KCI- or NE-precontracted rabbit pulmonary arterms Dual effect (vasoconstrictmn on KCI-precontracted and vasodilation on NE-precontracted vessels) IS shown P values of all E(+) versus E ( - } are greater than 0 O5

hne are mconsmtent and confusing, and its efficacy is in doubt. 1°,13,14,17.1sIn fact, this present study in isolated rabbit pulmonary arteries shows that tolazohne has dual vasoactive effects. It caused vasoconstriction on non- or KC1precontracted pulmonary arteries and vasodllatlon on NEprecontracted pulmonary arteries. Both are dose related and endothelium-independent. Tolazoline is pharmacologically recognized as an alphaadrenergic antagomst and H2 agonlst agent. 324 However, it has also been shown to have histaminerglc, cholinergic, and alpha-agonist actions. 25 It may also have a direct relaxant effect on vascular smooth muscle.24Tolazoline is an lmidazoline compound that has structural similarities to both sympathomimetlc amines and histamine3,22as well as clonldine and phentolamme. 2 Conventxonally, it is believed that tolazoline acts primarily as an alpha-adrenergic blocker, dilating both systemic and pulmonary vascular smooth muscle. However, Goetzman et al have shown that the pulmonary vasodllator action of tolazoline is histamine mediated, Involving both histamine HI and H2 receptors, and can be blocked by anUhlstamines.12 On the other hand, Its vasoactive effects appear to be more complex and unclear. Tolazolane has also been reported to have sympathomimetic, chronotropic and inotropm acUon on cardiac muscle 17 and may elicit marked hypertension, tachycardla, cardiac arrhythmias, angina, and myocardial Infarction It appears to be contraindicated in patients with a history of coronary artery disease or after a cerebrovascular accident} Tolazoline has also been shown to not cause pulmonary vasodilatatlon in healthy, normoxlc lambs.25 In vitro, on an molated preparation, this study showed that tolazoline is a vasoconstrictor with dose-related vasoconstriction in both non- or KCl-precontracted pulmonary artery. KCI reduces contraction of the blood vessel by increasing Ca 2+ influx through voltage-operated channels after membrane depolarization. Further contraction by tolazohne may, indeed, Indicate that It acts as an alpha-1

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LEE AND HOU

agonist by releasing Ca 2+ from sarcoplasmic reticulum lntracellularly or enhancing Ca 2+ influx from outside the cells. This vasoconstrictive effect of tolazoline may be responsible for the ineffective responses of pulmonary arterial pressure in previous studies. Furthermore, tolazolme infusion has been reported to reduce systemic arterial pressure with little or no change in pulmonary artery pressure and improvement m arterial oxygenation in meconium aspiration.2I In a study of pulmonary hypertension in lambs with congemtal diaphragmatic hernias, tolazohne was found to be anconsistent and not as good as prostaglandln D2 and isoproterenol in reducing pulmonary artery pressure and increasing pulmonary blood flow. 1 On the other hand, tolazohne effectively dilated pulmonary blood vessels during hypoxla.3,22 One study showed that medium-dose tolazoline alone does not elicit significant changes in the circulatory status, although this same dose of tolazoline caused significant alteration in the circulatory responses to simultaneously administered dopamine.25 it appears that various basal tones in the pulmonary circuit might modify both the magnitude and direction of the pulmonary vascular response. 3.z5 The results of the current study demonstrated that tolazoline caused a significant dose-related vasorelaxatlon in pulmonary arteries precontracted by NE. At 10-5 M, tolazolme causes 60% vasorelaxataon in NE-precontracted pulmonary artery rings; whereas the same dose of tolazohne causes about 20% vasoconstriction an KCl-precontracted ones The fact is that the hemodynamlc responses to tolazoline may be related to the level of preexisting sympathetic tone or other factors such as changes in blood pH, PO2, or PCO2 .2 The attenuatmn of the contraction by tolazohne on NE-precontracted pulmonary arteries suggests that either tolazohne blocks the release of Ca 2+ lntracellularly or simply competes with NE for alpha-1 receptors on the membrane Because it caused vasoconstriction on pulmonary arteries without any pretreatment and those pretreated with KC1 in the current

experiment, it is very unlikely that tolazohne blocks the release of Ca 2+ from sarcoplasmic retlculum, and there is no direct alpha-adrenerglc blocking effect. In fact, tolazoline acts hke an alpha-adrenergic agonist. It is consistent with the findings that tolazohne increases pulmonary vascular resistance after histamine receptor blockade, which might be caused by the unmasking of an adrenergic vasoconstrictor action, t2,26 It has been reported that large doses of tolazoline administered concurrently with eplnephrlne or NE may cause a paradoxical decrease in blood pressure followed by an exaggerated rebound hypertension.2 All these suggest that tolazoline may not be an alpha-blocker but rather an alpha-agonist, competing with NE for both alpha-1 and alpha-2 receptor sites. 3.t7 Despite the widely accepted use of tolazoline in patients with pulmonary hypertension, its effectiveness appears to be inconsistent and has had very little influence on the overall survival rate. I3,14,1s Although it is inappropriate to extrapolate in vitro animal data directly to humans, nevertheless the authors are in agreement with Meadow et al in calling for reevaluation of the role of tolazohne in pulmonary hypertension and its administration to hypoxlc human newborns.IS The authors conclude that tolazoline is not a direct vasodllator nor an alpha antagonist. It is more hkely an alpha-agonist acting competitively for the alpha-adrenergic receptors. Its pharmacologic properties should be reassessed and redefined. The posslbihty of interaction with lmidazoline receptors is worth further investigation. In summary, this study demonstrates that tolazoline has a dual effect on isolated rabbit pulmonary arteries. It causes vasoconstriction on non- and KCl-precontracted vessels but vasodllation on NE-precontracted ones. All these are dose dependent and endothehum independent. Because of the availability of other new potent pulmonary vasodllators, tolazoline may no longer be a desirable agent to be used in acute pulmonary hypertension.

REFERENCES

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pulmonary hypertension in patients with cystic fibrosis Observations on the effect of tolazollne hydrochloride Pediatrics 39 24-35, 1967 9 Cotton EK The use of pnscoline in the treatment of the bypoperfuslon syndrome Pediatrics 36.149, 1965 10 Heath RE Jr: Vasospasm in the neonate. Response to tolazoline infusion Pediatrics 77'405-408, 1986 11 Korones SB, Eyal FB Successful treatment of "persistent fetal circulation" with tolazollne Pedlatr Res 9,367, 1975 12. Goetzman BW, Mllsteln JM. Pulmonary vasodilator action of tolazoline Pedlatr Res 13 942-944, 1979 13. Bloss RS, Turmen T, Beardmore HE, et al' Tolazohne therapy for persistent pulmonary hypertension after congenital diaphragmatic hernia repair. J Pediatr 97"984-988, 1980 14 Goetzman BW, Sunshine P, Johnson JD, et al Neonatal hypoxlaand pulmonaryvasospasm' Response to tolazohne J Pedlatr 89'617-621, 1976 15 Grover RF. Reeves JT, Blount SG Tolazollne hydrochloride (Prlscohne) An effective pulmonary vasodllator Am Heart J 61 5-15, 1961

VASOACTIVE EFFECT OF TOLAZOLINE

16. Drummond WH, Gregory GA, Heymann MA, et al. The independent effects of hyperventllatlon, tolazollne, and dopamme on infants with persistent pulmonary hypertension. J Pedlatr 98:603-611, 1981 17. Stevens DC, Schreiner RL, Bull MJ, et al. An analysis of tolazoline therapy in the critically 111 neonate. J Pediatr Surg 15:964-970, 1980 18. Meadow W, Benn A, Giardini N, et al' Clinical correlates do not pre&ct PaO2 response after tolazoline administration m hyporac newborns. Crlt Care Med 14'548-551, 1986 19. Yoran C, Glassman E: The paradoxlc effect of tolazollne hydrochloride on pulmonary hypertension of mltral stenosis. Chest 63:843-846, 1973 20. Bush A, Busst CM, Knight WB, et al. Comparison of the haemodynamlc effects of epoprostenol (prostacychn) and tolazoline. Br Heart J 60:141-148, 1988 21. Truog WE, Lyrene RK, Standaert TA, et al: Effects of PEEP

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and tolazollne infusion on respiratory and inert gas exchange m experimental meconlum aspiration. J Pe&atr 100:284-290, 1982 22 Bressack MA, Bland RD' Intravenous infusion of tolazohne reduces pulmonary vascular resistance and net fluid filtration in the lungs of awake, hypoxlc newborn lambs. Am Rev Resplr Dis 123:217-221, 1981 23. Bush A, Busst CM, Knight WB, et al: Cardiovascular effects of tolazohne and ranitidine Arch Dis Child 62.241-246, 1987 24. Jones ODH, Shore DF, Rlgby ML, et al: The use of tolazohne hydrochloride as a pulmonary vasodllator in potentially fatal episodes of pulmonary vasoconstriction after cardiac surgery in children. Circulation 64:134-139, 1981 (suppl 2) 25. Drummond WH, Williams BJ: Effect of continuous tolazoline infusion on cardlopulmonary response to dopamme in unanesthetized newborn lambs. J Pedlatr 103:278-284, 1983 26 Benfey BG, Varma DR: Vasoconstrictor action of tolazohne. Br J Pharmacol 22:66-71, 1964