Effects of (−)-Δ9-trans-tetrahydrocannabinol on regional blood flow in anesthetized dogs

EUROPEAN JOURNAL OF PHARMACOLOGY20 (1972) 373-376. NORTH-HOLLANDPUBLISHINGCOMPANY

Short communication

EFFECTS OF (-)-A9-trans-TETRAHYDROCANNA

B I N O L O N R E G I O N A L B L O O D FLOW.

IN A N E S T H E T I Z E D D O G S I. CAVERO, R. ERTEL, J.P. BUCKLEY and B.S. JANDHYALA Department of Pharmacology School of Pharmacy, University of Pittsburgh, Pittsburgh, Pa. 15213, U.S.A. Accepted 9 October 1972

Received 25 August 1972

I. CAVERO, R. ERTEL, J.P. BUCKLEY and B.S. JANDHYALA,Effects of(-j-A9-trans-tetrahydrocannabinol on regional blood flow in anesthetized dogs, European J. Pharmacol. 20 (1972) 373-376. Distribution of cardiac output to various organs was studied utilizing 8~Sr-labelled microspheres in dogs. In the group receiving Ag-THC, 2.5 mg/kg, i.v., there was a significant reduction in the fractional blood flow accompanied by an increase in resistance in the splanchnic vasculature. Net total peripheral resistance was unchanged. A9-tetrahydrocannabinol Regional blood flow

Cardiac output 8s Sr microspheres

1. INTRODUCTION We recently reported that (-)-A9-trans-tetrahydro cannabinol (A9-THC), a constituent of marihuana, causes a significant and reproducible attenuation of blood pressure and heart rate of dogs maintained at constant arterial pO2 (Cavero et al., 1972). Hemodynamic analysis of the Ag-THC-induced hypotension showed that a reduction in cardiac output was primarily responsible for the decrease in blood pressure (Cavero and Jandhyala, 1972). The present study was conducted to investigate the effects of decreased cardiac output induced by A9-THC on the regional blood flow in the dog.

2. MATERIALS AND METHODS Mongrel dogs of either sex were anesthetized with sodium pentobarbital, 35 mg/kg, i.v., and placed on artificial respiration. Blood pressure was monitored from the abdominal aorta and heart rate obtained utilizing a tachograph.

Splanchnic vasculature Vascular resistance

Thoracotomy was performed on the left side and a Statham electromagnetic flow probe ( 1 2 - 1 4 mm internal diameter) was placed around the ascending aorta and cardiac output monitored with a Statham flowmeter (M 4001). Carbonized microspheres (50 -+ 10/a in diameter) (3M Company) labelled with 8SSr (specific activity, 10gCi/mg) were suspended in 10% dextran solution at a concentration of 10 mg/ml. A total dose of 4 - 5 mg (corresponding to approximately 48,000-60,000 spheres) was rapidly infused into the left atrium by way of a catheter which had been introduced through the auricular appendage. Ten minutes following the administration of microspheres, the animals were sacrificed with an intravenous injection of a saturated solution of potassium chloride and various organs were dissected out for determination of radioactivity content. The larger organs were divided into several sections and each specimen was placed 50 cm from a 1.75 inch crystal scintillation detector (Baird Atomic). The ratio of organ radioactivity to the net total infused radioactivity represents that part of cardiac output distributed to a particular region or tissue

374

1. Cavero et al., A9-THC on regional blood flow

Table 1 Mean blood pressure (M.B.P.), heart rate (H.R.) cardiac output (C.O.) and total peripheral resistance (T.P.R.) (2 ± S.E.M.) for two groups of dogs which received A9-THC, 2.5 mg/kg, i.v., or an equi-volume of solvent. Minutes after treatment 0 5 15 30

Solvent (n = 6)

zX9-THC (n = 5)

M.B.P. (mm Hg)

H.R. (beats/ min)

C.O. (ml/min)

T.P.R. (mm Hg/ml/min)

M.B.P. (mm Hg)

H.R. (beats/ min)

C.O. (ml/min)

T.P.R. (mm Hg/ml/min)

90.0-+ 8.4 91.6-+7.6 89.6 ± 7.6 86.6±7.7

162 ± 7 159±8 156 ± 8 155±6

1079 +- 103 1065± 99 1037 ± 100 1051± 86

0.0856 +- 0.008 0.0878±0.006 0.0882 ± 0.006 0.0836± 0.006

87.4 ± 5.2 63.4± 4.2" 59.8 ± 2.4 * 60.8± 23"

173 ± 7 142±5" 133 +- 5 * 130±5"

1008 ± 47 772+-31" 768 ± 49 * 768±49"

0.0875 ± 0.006 0.0838±0.008 0.0805 ± 0.006 0.0802±0.004

* Significant changes (p < 0.05; t-test) when compared to solvent-induced changes.

(Kaihara et al., 1969). This fraction was multiplied by the total cardiac o u t p u t to obtain absolute flow (ml/min) in a specific organ. Since the cardiac o u t p u t estimated with an electromagnetic flow probe does not include the blood flow to the heart (Guyton, 1963), the following formula was utilized: coronary flow = R h × aortic f l o w / ( l - R h) ( R h = fraction of radioactivity detected in the heart). Total cardiac output was the summation of the measured aortic flow plus the calculated coronary flow. Regional vascular resistance and total peripheral resistance (in mm Hg]ml/min) were calculated by dividing the mean aortic blood pressure by the flow to each organ or the cardiac output. In this calculation it was assumed that the venous pressure was negligible. Experimentally, 5 animals (mean body weight, 10.2 kg) received A9-THC, 2.5 mg/kg, i.v., and 6 animals (mean b o d y weight, 10.1 kg) received only an equivalent volume of the solvent (ethanol 98%; 0.025 ml/kg). 30 min following A 9-THC or solvent, labelled microspheres were injected into the left atrium and the animals were sacrified. Student's t-test was used to estimate probability levels of differences between means, whereas the non-parametric M a n n - W h i t n e y U test (Sokal and Rohlf, 1969) was used to estimate significant differences in the regional blood flow values since the distribution of these data was in many cases skewed, as noted by others ( F o r s y t h and Hoffbrand, 1970).

3. RESULTS A9-THC, 2.5 mg/kg, i.v., significantly attenuated blood pressure, heart rate and cardiac output (table 1). No effect was noticed following the administration of an equivalent volume of the solvent (table 1). The decrease in blood pressure appeared to be a result of the decrease in cardiac output since no attenuation of total peripheral resistance was induced by Ag-THC (table 1). Tables 2 and 3 report resistance and blood flow in various vascular beds in dogs receiving A9-THC or solvent, respectively. A9-THC significantly decreased the blood flow to the splanchnic area (table 2). This alteration was also seen when the flow was expressed as percent o f cardiac output. Ag-THC did not change the fractional blood flow to vital organs such as the brain, kidney and heart (tables 2 and 3). However, the absolute flow to these organs seemed to be decreased even though no statistical significance could be attached to these observations (tables 2 and 3). Calculation o f vascular resistance indicated that the decrease in splanchnic flow was accompanied by an increase in the vascular resistance in this area (table 2).

4. DISCUSSION Drugs which attenuate cardiac output often alter regional blood flow; thus, it is of primary importance to determine whether blood flow to vital organs is

375

I. Cavero et aL, A 9- THC on regional blood f l o w

Table 2 Vascular resistance and distribution of cardiac output (C.O.) 30 min after the adminstration of ~9-THC, 2.5 mg/kg, i.v. in dogs under pentoarbital anesthesia. Organ Heart Brain Kidneys Hindleg Lungs (bronchialartery) Spleen Pancreas Stomach Small intestine Splanchnic area** Liver

Absolute flow (ml/min)

% C.O.

Resistance (mm Hg/ml/min)

31.3( 20.0- 50.4)t 16.9 ( 11.5- 29.6) 164.3 (123 -216 ) 22.8 ( 17.8- 29.8) 40.7 ( 23.9- 56.7) 13.1 ( 5.8- 20.3)* 6.4 ( 3.6- 10.0) 15.3 ( 10.9- 24.3) 78.5 ( 70.9-184 ) 113.4 ( 96.4-286 )* 50.6( 11.7- 97.0)

3.9( 2.7- 5.9)t 2.5 ( 1.3- 3.5) 20.4 (18.1-24.7) 2.8 ( 2.1- 3.1) 5.2 ( 3.6- 5.9) 1.6 ( 0.9 2.2)* 0.8 ( 0.4- 1.3) 2.1 ( 1.5- 2.5) 9.9 ( 7.6-12.3)* 14.4 (12.7-16.0)* 6.0(1.6-11.8)

2.07 4.08 0.38 2.70 1.70 9.44 11.16 4.24 0.78 0.54 2.02

(1.22- 2.61) (1.75- 5.64) (0.30- 0.48) (2.18- 2.90) (1.14- 2.50) (3.45-20.31)* (5.18-17.85) (2.67- 5.94) (0.59- 0.88)* (0.41- 0.62)* (0.93- 2.99)

t Data are reported in mean and range (in parentheses). ** Splanchnic area = spleen + pancreas + stomach + small intestine. * Significantly different (p < 0.05, Mann-Whitney U-test) from solvent group (table 3). Table 3 Vascular resistance and distribution of cardiac output (C.O.) 30 min after the administration of solvent control in dogs under pentobarbital anesthesia. Organ

Absolute flow (ml/min)

% C.O.

Resistance (ram Hg/ml/min)

Heart Brain Kidneys Hindleg Lungs (bronchial artery) Spleen Pancreas Stomach Small intestine Splanchnic area * Liver

50.4 15.3 205.3 27.2 28.4 43.3 7.7 25.0 152.3 228.3 72.5

4.5 ( 3.0- 6.6)t 1.4 ( 0.9- 1.6) 19.0 (12.7-26.2) 2.4 ( 1.4- 3.3) 2.4 ( 0.7- 5.3) 3.9 ( 1 . 4 - 1 0 . 8 ) 0.7 ( 0.3 1.3) 2.3 ( 1.2- 3.4) 14.0 ( 9 . 7 - 1 9 . 2 ) 21.0 (15.9-28.0) 6.5 ( 2 . 9 - 1 2 . 5 )

2.02 6.16 0.44 3.86 5.39 3.05 16.77 3.58 0.58 0.41 1.37

( 31.9- 98.7) ( 9 . 9 - 23.4) (151 -276 ) ( 15.2- 49.2) ( 6 . 6 - 64.5) ( 22.2 109 ) ( 2.4- 14.4) ( 15.4- 32.8) (103 - 1 9 8 ) (176 -288 ) ( 34.4-126 )

(1.21- 3.53)t (3.79 8.61) (0.24- 0.53) (2.50- 7.24) (0.95-12.74) (0.77 7.09) (9.95-33.19) (2.74- 5.16) (0.39- 0.71) (0.30- 0.46) (0.67- 2.08)

t Data are reported in mean and range (in parentheses). * Splanchic area = spleen + pancreas + stomach + small intestine.

affected by these agents. F o l l o w i n g A9-THC, the fractional flow to the vital c o r o n a r y , cerebral and renal beds was essentially unchanged. Thus, the decrease in absolute b l o o d flow n o t e d in these organs was proportional to the decrease in cardiac output. Since cardiac w o r k was greatly reduced due to the decrease in b l o o d pressure as well as cardiac o u t p u t , the diminished absolute m y o c a r d i a l flow after A 9 - T H C could still be adequate to provide a sufficient a m o u n t o f o x y g e n to the m y o c a r d i u m .

Unlike the blood flow pattern in vital organs, there was a significant r e d u c t i o n in absolute as well as fractional blood flow to the splanchnic area in dogs treated with A9-THC. In addition to a decrease in cardiac o u t p u t , an increase in local vascular resistance appeared to be responsible for these effects. In contrast, a t e n d e n c y toward a decrease in resistance was noticed in the cerebral, bronchial and hindleg vasculatures. Since no significant changes were d e t e c t e d in net total peripheral resistance, it appears that admin-

376

L Cavero et al., A 9- THC on regional blood flow

istration of Ag°THC resulted in a redistribution of cardiac output due to a selective alteration of vascular resistance in certain areas. However, these data do not indicate whether these effects are directly mediated by an action of A 9-THC on specific regional vasculature or the result of reflexogenic mechanisms triggered in response to decreased cardiac output and blood pressure (Kaihara et al., 1969). The data of this study further confirmed our previous report that the hypotensive activity of A9-THC is primarily mediated through an attenuation of cardiac output (Cavero and Jandhyala, 1972). In conclusion, following the administration of A 9THC there was an alteration in the distribution of regional blood flow with a specific increase in mesenteric resistance, whereas the fractional blood flow to vital organs was not affected.

ACKNOWLEDGEMENTS This research was supported by P.H.S. Training Grant GM 1217-07. Ag-THC was kindly supplied by N.I.M.H.

REFERENCES Cavero, I. and B.S. Jandhyala, A9-tetrahydrocannabinol, (Abs.). Cavero, I., R.K. Kubena, J. Jandhyala, 1972, Certain ships between respiratory

1972, Hemodynamic effects of Federation Proc. 31, 1647

Dziak, J.P. Buckley and B.S. observations on interrelationand cardiovascular effects of (-)-/xg-trans-tetrahydrocannabinol, Res. Commun. Chem. Pathol. Pharmacol. 3, 483. Forsyth, R.P. and B.1. Hoffbrand, 1970, Redistribution of cardiac output after sodium pentobarbital anesthesia in the monkey, Amer. J. Physiol. 218,214. Guyton, A.C., 1963, Circulatory Physiology: Cardiac Output and Its Regulation (W.B. Saunders Co., Philadelphia and London) p. 120. Kaihara, S., R.B. Rutherford, E.P. Schwentker and H.N. Wagner, Jr., 1969, Distribution of cardiac output in experimental hemorrhagic shock in dogs, J. Appl. Physiol. 27,218. Sokal, R.R. and F.S. Rohlf, 1969, Biometry (W.H. Freeman and Co., San Francisco) p. 391.