From the Department of Physiology, Unirersiiy of Lund.

The Vasodilator Action of Adenosine T riphosyha8te.

BY

BJORN FOLKOW.

Received 5 Narch 1949.

Adenosine compounds have been credited to dilate the coro- naries (DRURY and SZENT-GYORGYI 1929-30, BENNET and DRURY 1931 and GILLESPIE 1934), the muscular vessels (FLEISCH and WEGER 1937, SCHOEDEL 1935, KALCKAR and LOWRY 1947), the intestinal vessels (MARCOU 1932, DE WAELE and v. D. VELDE 1945) and the cutaneous vessels (BENNET and DRURY 1931). I n lower concentrations adenosine compounds are reported to dilate the puhnonary vessels (GADDUM and HOLZ 1933), and to constrict them in high concentrations (GADDUM and HOLZ 1933, EMMELIN and FELDBERG 1948).

The relative dilator potency of the different adenosine com- pounds has not been quantitatively established. according to FLEISCH and WEGER (1937) the dilator action of adenosine tri- phosphate (ATP) is 100-150 times stronger than that of adenylic acid. According to GILLESPIE (1934), SCHOEDEL (1935) and KALC- KAR and LOWRY (1947) the compounds are approximately equal in potency, while DRURY (1932) states that the activity increases somewhat with phosphorylisation.

Adenosine compounds have a wide distribution in the body, especially striated muscles contain large amounts, predominantly as ATP. Because of its wide distribution and high biological ac- tivity, i t has been suggested that ATP might play a r61e in periph- eral vascular regulation.

In most of the previous experiments large amounts of adenosine compounds were given to produce vasodilator effects, from 50-

212 B.JORN FOLKOW.

100 y up t o niany nig rrere generally administered. SCHOEDEL (1935) a n d PLEISCH a n d WEGER (1937) observed vasodilatation in t h e hind limb of dogs on intraarterial injection of ATP calcu- lated t o give a concentration in t h e blood of approximately 1 : 1400000.

FOLKOW, H A E G E R and RAHLSOS (1948) observed that intra- arterial injection of 0.1 y ATP was sufficient to liroduce a signif- icant vasodilatation in t h e hind limbs of t h e cat. They stressed that ATP possesses potentialities as a normal vasodilator nietab- olite, a suggestion previously made by BIT~DIX'GS and M ~ E G R A I T H

(1937) and others. RIGLER (1932) bronght some evidence t o indi- cate that ATP or other adenosine compounds play a r81e i n t h e local regulation of blood flow during muscular activity.

It is t he purpose of t he present s tudy t o investigate t h e vaso- dilator action of -4TP in various regions.

Method.

The esperimeuts were performecl on cats and dogs narcotized wit.11 chloralose (70 mgjkg), chloralose-urethane (50 mg i; 500 mg/kg) or nctnlJutha1 (30 iiigikg).

A. Mrtsctclar and cuta)ieou.s vessels of the hiwJ limbs in cats. The ani- mals were eviscerated. The blood flow was separately recorded in the abdoniinal c a d vein and in one of the saphenal veins. The caval blood flow was directed to a GADDL-M recorder (GADDUM 1929). The flow from the saphenel vein wa,s registered by a drop recorder. Intraarterid in- jectsions of ATP were niade in the caiuiulated stump of the inferior niesenteric artery. Intrnvenons injections were made in the cannulated brachial rein. Some of the experiments were performed on animals v i th their hind linibs syri~~~athectomizecl or de-afferentinted 3 weeks previously.

B. I~t~sti,caZ vessels. These experinicnts were performed on cats. The technique has beeii described by FOLKOW, FROST and UVNAS (1918). The out,flow froin the superior nieseiiteric vein was measured witah a Gaclduin recorder and the blood pressure was registered froin a brachial artery. Tntraarterial iriject'ions were niade into the rubber t,ubing inserted in the dirided superior niesenteric artery.

C. Coro)tary vessels. These experiments were performed on clogs with a cross circulation technique previously described liy FOLKOW, FROST and Uvxis (194s). The coronaries of the fibrillating heart of a recipient dog were perfused from a donor dog. The outflow from the coronary sinus was measured with a Gaddum recorder. ,4TP was given intra- arterially into t.he rubber tubing connecting the carotid artery of tha donor dog with the perfused coronaries of the recipieut.

THE VASODILATOR ACTION OF ADENOSINE TRIPHOSPHATE. 313

Fig. 1. A . Cat, 3.5 kg. Chloralose-urethane. Atropine 0.1 mg/kg given. Intraarterial

From ubove: Blood pressure, blood flow through the hind limbs, signal, time mark. B. Cat 4 kg. Chloralose-urethane. Registration of skin blood flow. The volume

of the flow is calculated from the number of drops registered per minute. Intra- arterial and intravenous injections of ATP. At “11” 1 7 ATP is mixed with blood and injected 2 minutes later. Compare the action with “I”.

and intravenous application of ATP.

Brom above: Blood pressure, skin blood flow, signal, time mark.

Results.

ATP was observed to elicit a pronounced vasodilatation in all vascular regions studied. The threshold-dose for the vasodilator effect amounted t o 0.05-0.1 y. Total dene.rvation and consequent degeneration of the vasomotor nerves did not significantly in- fluence the vasodilator responses to ATP. Neither atropine nor neoantergan, given in doses sufficient to completely abolish the vasodilator action of moderate doses of acetylcholine or histamine, antagonized the vasodilator action of ATP. Quantitatively, ex- pressed in equimolar concentrations, the vasodilator potency of ATP amounted to about 1/5-1/15 of the potency of acetyl- choline.

Fig. 1 illustrates the vasodilator action of ATP on muscular and cutaneous vessels. Intraarterial injections of 0.1-10 y elicits a pronounced vasodilatation. The figure further illustrates the remarkable fact, that intravenous injection of ATP in amounts exceeding the threshold dilator dose 200-300 times does not exert any significant depressor action. Contrastingly acetylcholine and histamine cause pronounced depressor responses when given intravenously, even in minute amounts. Evidently ATP is rapidly destroyed in the circulation. This destruction does not appear to

314 BJORK FOLKOW.

Fig. '7. A . Cat, 3.5 kg. Chloralose-urethane. Blood flow through the intestines. Intra-

From nbore: Blood prcssure, intestinal blood flow, signal, time mark. B . Dogs, cross circulation. Donor 11 kg, recipient 7 kg. Nembuthal. Atropine

From c c b o ~ ~ ' Blood pressure donor, coronary blood flow of recipient, signal,

arterial injections of AT€' and acetylcholine.

0.1 mg'kg is given. Intraarterial injections of ATP.

time mark.

occur in the blood. At inark I1 in fig. 1 1 y ATP is mixed with 1 ml blood and the mixture allowed to stand for 2 minutes in 37 "C. When injected intraarterially the dilator response to ATP remains unchanged as compared with mark I in figure 1. Pig. 2 A shows the vasodilator effects of ATP and acetylcholine on the intestinal vessels of a cat. It is difficult to compare the dilator action of ATP and acetylcholine on the intestinal vessels, since even small amounts of acetylcholine cause rather strong contractions of the intestines. These contractions interfere v i th the circulation me- chanically by temporarily increasing the vascular peripheral resistance. The amounts of ATP used in these experiinents (not more than 20 y ) had no perceptible action on the intestinal smooth niuscles. Therefore i t seeins as if the increase of flow following administration of ATP is due t o a pure vasodilator action.

In fig. 2 B the dilator effect of ATP on the coronaries of a dog is shown. Amounts of ATP from 0.1-20 y were observed to induce a marked dilatation. The effect was not influenced by administration of atropine. S o significant action on the heart muscle was observed by these minute amounts.

Intravenous application of XTP in amounts of 10-30 y were never with certainty observed to have any effects on respiration or heart rate.

THE VASODILATOR ACTION OF ADENOSINE: TRIPBOSPHATE. 315

Discussion.

ATP proved to be a very potent dilator substance in all vascular regions investigated. I t s dilator potency is comparable to that of acetylcholine and histamine. It was previously observed (POLKOW, HAEGER and KAHLSON 1948) that the dilator action of ATP was not blocked by atropine or neoantergan. This observation which is confirmed in the present experiments indicates that the dilator action of ATP is not produced by the release of any of these potent dilator agents. Nor are any nervous structures engaged since the dilator action of ATP remains approximately unchanged after denervation of the vessels.

It was previously stressed (POLKOW, HAEGER and KAHLSON 1948) that recording the arterial blood pressure alone is a very poor indicator of vasodilator responses. This is especially true when ATP is given intravenously, since the substance seems to be rapidly destroyed in the lungs. Further, when given in larger amounts ATP constricts the pulmonary vessels and induces a reflex bradycardia (EMMELIN and FELDBERG 1948). The fall in blood pressure observed when ATP is injected will then a t least partly be due to these mechanisms and the peripheral dilator action difficult to observe.

Neither can the dilator action of ATP occurring in intact ani- mals be quantitatively estimated in perfusion experiments as the artificial perfusion considerably reduces the vasodilator responses of the perfused vessels.

RIGLER (1932) concludes from experiments on frogs that adeno- sine compounds might play a part in eliciting local vasodilatation in contracting muscles. BILDINGS and MAEGRAITH (1937) claim that similar substances appear in the venous blood on the release of arterial ischaemia. There is some evidence to indicate that adenosine compounds are released from traumatized tissues (BENNET and DRURY 1931). Further GREEN (1943) showed that most symptoms seen in the crush injury syndrome could be closely imitated by injections of muscle extracts, the active agent of which seemed to be ATP. It seems desirable to investigate more closely the possible r81e of ATP in peripheral vascular regulation.

Summary.

ATP is a potent vasodilator agent, the minimal active doses amounting to 115-1115 of those of acetylcholine when calculated

316 MORX FOLKOW.

as equiniolar concentrations. The vasodilator action seems to be due to a direct action on the vessels. On intravenous injection ATP is rapidly destroyed, presumably in the lungs.

References.

BESSET, B. W. and A. S. DRURT, J. Physiol. 1931. 72. 285. BILIIISGS, F. T. a i d B. G. ~IBEGRAITH, Quart. J. esp. Physiol. 1937-

I )R~RY, -1. S., J. Physiol. 1933. 71. 147. URURT, A. S. and d. SZEST-GTORGYI, J. Physiol. 1939-30. 65. 213. ENXELIS, K. and W. FELDBERG, Brit. J. Pharm. Chem. ther. 1948. 3.

FLEISCH, -1. a i d P. WEGER, Pfliig. Arch. ges. Physiol. 1937. 239. 362. YOLKOW, U., J. FROST and B. UvsXs, hcta Yhysiol. Scand., 1948. 15.

38. 27. 249.

373.

412. - - _ , , , Ibid. In press. FOLROIT, R., K. HAEGEX. and G. KAHLSOS, Ibid. 194s. 25. 264. GADDUM, J. €I., J. Physiol. 1929. 67. Proc. XVI. GADDL-31, J. H. and P. HOLZ, J. Physiol. 1933. 77. 139. ~ ~ I L L E S P I E , J. H., lbicl. 1931. 80. 345. GREEX, H. X., The Lancet 1043. 2. 147. KALCKAR, H. JI. and 0. H. LOWRT, dmer. J. Physiol. 1947. 149. 240. XIRCOV, I., C. R. Soc. Biol. 1932. 109. 778, 985. RIGLER, It., Arch. exp. Path. Pharmak. 1932. 167. 54. SCHOEUEL, W., Pflug. Arch. ges. Physiol. 1935. 236. 93. WAELE, I€. m and J. r. D. VELDE, Arch. Int. Pharmacodyn. 1945.

LXS. 3-4.