Absence of Acute Cardiac Effect of Triiodothyronine in Isolated, Blood-Perfused Canine Atrium

Absence of Acute Cardiac Effect of Triiodothyronine in Isolated, Blood-perfused Canine Atrium

Miyoharu KOBAYASHI, M.D., Yasuyuki FURUKAWA, M.D., and Shigetoshi CHIBA, M.D.

SUMMARY

Effects of triiodothyronine (T3) on SA node pacemaker activity, atrial contractility and on chronotropic and inotropic responses to norepinephrine were investigated, using 9 isolated, blood-perfused dog atrium

preparations. T3 (0.001pg-0.1ƒÊg) did not cause a significant direct action injected into the cannulated sinus node artery. When T3 (10pg/ min, 40pg/min, 0.1ƒÊg/min, or 0.4ƒÊg/min) was continuously infused into the sinus node artery, both positive chronotropic and inotropic actions of norepinephrine (0.03ƒÊg-0.3ƒÊg) were not influenced. From these results, it is concluded that T3 has neither acutely direct chronotropic nor inotropic effects, and also T3 does not have an acute effect on responses to norepinephrine in the isolated, blood-perfused canine atrium.

Additional Indexing Words: Isolated canine atrium Triiodothyronine Norepinephrine Chronotropic effect Inotropic effect

T has been known for many years that clinical manifestations of hyperthyroidism are similar to those caused by overactivity of the sympathetic nervous system. However, considerable controversy still remains whether the cardiovascular manifestations of hyperthyroidism are due to a direct action of thryroid hormones or due to a sensitization to catecholamines. Cairoli and Crout1) reported that tachycardia in hyperthyroidism was attributed to a direct effect of thyroid hormone on pacemaker cells. Murayama and Goodkind2) reported that the enhanced developed tension of isolated atrial muscle from hyperthyroid guinea pigs was produced by a direct effect of thyroid hormone on the heart. In clinical study, Teoh et al3) postulated that sinus tachycardia in hyperthyroid patients was due to combined effects of an increased amount of thyroid homones and increased adrenergic activity of catecholamines on the heart. In contrast, Wahlberg et al4) examined that T3 caused no direct effect on the isolated cat heart, although they acutely

From the Department of Pharmacology, Faculty of Medicine, Shinshu University, Matsumoto- 390,Japan. Received for publication July 11, 1978. Manuscript revised August 19, 1978.

219

I MJap. Heart J. 220 KOBAYASHI, FURUKAWA, AND CHIBA arch, 1979 examined its direct action. In 1962, Folkman and Edmunds5) reported that thyroid hormone caused a sensitization of the myocardium to catecholamines. Furthermore, Mc- Neil and Brody6) suggested that pretreatment with T3 potentiated effect of catecholamines on rat hearts. On the other hand, Buccino et al7) reported that hyperthyroid cat papillary muscles were not hypersensitive to exogenous norepinephrine. Wilson et al8) also confirmed that thyroid hormone did not augment the hemodynamic responses to catecholamine. The present investigation was undertaken to study direct effects of T3 and influences of T3 on responses to norepinephrine in the isolated, blood-perfused canine atria.

MATERIALS AND METHODS

Nine mongrel dogs weighing from 10 to 20Kg were anesthetized with sodium pentobarbital, 30mg/Kg i.v. After treatment with 2,000 unit of sodium heparin i.v., the right atrium was excised and immerged into a cold Tyrode solution at 4•K- 10•Ž. The sinus node artery was cannulated via the right coronary artery and perfused with arterial blood conducted from the carotid artery of the donor dog by the aid of a peristaltic pump (Harvard Apparatus Model 505-1200). The perfusion pressure was maintained at 100mmHg. The atrium was suspended in the bath filled with blood at a constant temperature of 37•Ž. The upper part of the crista terminalis of the atrium was connected directly to a force displacement transducer (Grass FTO3B) by a silk thread. The donor dogs weighing 12 to 25Kg were also anesthetized with 30mg/Kg of sodium pentobarbital i.v. Sodium heparin,

Table I. Effect of T3 on the Responeses to Norepinephrine in Pacemaker

Results are given as mean•}S.E.M. Control sinus rate was 99•}3beats/min in 5 preparations. Vol.20 No.2 T3 ON SA NODE AND CONTRACTILITY 221 500units/Kg, was intravenously administered at the beginning of the perfusion and 2,000 units were added at 1-hr intervals. The flow rate at 100mmHg of the perfusion pressure was 4.5-5.0ml/min. Bipolar platinum electrodeswere placed in contact with atrial epicardium. The atrial rate was measured with a cardiotachograph triggered by the signal from the atrial depolarization of an electrogram. The isometric tension development was recorded on an ink-writing rectigraph (Sanei Sokki Instrument) through a carrier preamplifier (Nihon Kohden RP-3), RC circuit and a high amplifier (Nihon Kohden RPH-2). Details of the isolated, blood-perfused canine atrial preparations were described in previous pa-

pers9),10). The systemic arterial blood pressure and heart rate of the donor dog were measured from a cannulated femoral artery with a pressure transducer (Nihon Kohden RP-2) and a cardiotachograph triggered by the pulse pressure, respec- tively. The drugs used in these experiments were sodium-L-triiodothyronine (Takeda, kindly furnished by Dr. M. Miyakawa, Department of Surgery, Faculty of Medicine, Shinshu University, Matsumoto), (•})-norepinephrine hydrochloride (Sankyo). The volume of drug solution injected into the sinus node artery was 0.01 to 0.03ml and given by a microsyringe (Terumo Co) in a period of 4sec. T3 was continuously infused into the sinus node artery with an infusion pump (Harvard Apparatus 901). T3 was also administered into the jugular vein of the donor dog.

RESULT

‚r 1. Absence of chronotropic and inotropic responses to T3 injected into the sinus

node artery:

T3 has neither direct chronotropic nor inotropic action on SA node when

Activity and Contractility in 5 Isolated Dog Atrial Preparations

All values were not significant when compared with control values (t-test). Jap. Heart J. 222 KOBAYASHI, FURUKAWA, AND CHIBA March, 1979

Fig. 1. Chronotropic and inotropic responses to increasing doses of triiodothyronine (T3) when injected into the sinus node artery of an isolated, blood-perfused atrium preparation of the dog.

Fig. 2. Absence of potentiation of responses to norepinephrine (NE) by an infusion of triiodothyronine (T3) on an isolated, blood-perfused atrial muscle of the dog.

T3 was injected into the cannulated sinus node artery in a dose range of

0.001ƒÊg to 0.1ƒÊg in all 6 experiments. In 2 out of 6 preparations, T3 produced a negative inotropic action over the dose of 0.3ƒÊg or 1 to 3ƒÊg (Fig. 1).

The maximum decrease was approximately 20% when compared with values of control tension. On the other hand, in chronotropism, a larger amount of T3 caused a slight negative chronotropic effect but not usually. As shown in Fig. 1, 3ƒÊg of T3 produced a clear decrease in atrial rate in this case, but in other 5 cases, 3ƒÊg of T3 did not induce so distinct negative chronotropic effect.

2. Absence of potentiation of responses to norepinephrine by a continuous infusion of T3:

When T3 was continuously infused into the cannulated sinus node artery with an infusion rate of 10pg, 40pg, 0.1ƒÊg, or 0.4ƒÊg/min, both sinus rate and developed tension did not show any change. As shown in Fig. 2, norepinephrine-induced positive chronotropic and inotropic effects were not influenced by a continuous infusion of T3. Summarized data are demonstrated in Fig. 3 and Table I. The positive chronotropic responses to norepinephrine were slightly depressed during an infusion of T3 but not significantly. On the other hand, the positive ones were rather slightly potentiated by an infusion Vol.20 No.2 T3 ON SA NODE AND CONTRACTILITY 223

Fig. 3. Effects of a continuous infusion of triiodothyronine (T3) on norepinephrine (NE) dose-response curve in 5 isolated dog atria. •œ-•œ =NE only

• -• =NE+T3 infusion (10pg/min)

•¤-•¤ =NE+T3 infusion (40pg/min) •¡-•¡ =NE+T3 infusion (0.1beg/min) =NE+T3 infusion (0.4beg/min)

Control sinus rate was 99•}3beats/min in 5 preparations. Standard errors were not shown for clarity.

of relatively larger doses of T3 but not significantly.

3. Chronotropicand isotropic responsesof isolated atria when T3 was injected into the jugular vein of the donor dog: When T3 was administered into the jugular vein of the donor dog, a slight decrease of the systemic blood pressure was observed in 2 experiments. As shown in Fig. 4, 30pg/Kg of T3 caused a slight decrease in the arterial blood pressure of the donor dog. In this case, a decrease in the developed tension of the isolated atrium occurred approximately 2min after the onset of detectable hypotension of the donor dog since the time to pass through the Jap. Heart J. M 224 KOBAYASHI, FURUKAWA, AND CHIBA arch, 1979

Fig. 4. Chronotropic and inotropic responses on an isolated, blood-

perfused atrial preparation when 30ƒÊg/Kg of triiodothyronine (T3) was administered into the jugular vein of the donor dog. circuit was 2min. Any change in atrial rate, however, did not occur.

DISCUSSION

The present results showed that, in the isolated, blood-perfused canine atrium, T3 when injected into the cannulated sinus node artery produced neither significant chronotropic nor inotropic effects, and an infusion of T3 never significantly potentiated the positive chronotropic and inotropic responses to norepinephrine. In acute experiments, there were a few available reports on effects of T3 on heart function. Wahlberg et al4) reported that T3 did not have any direct inotropic nor chronotropic effects of its own on isolated cat heart preparations when T3 was given acutely. In the present experiments, we also agreed with their results, using the isolated dog atrial preparations, although extremely larger doses of T3 occasionally produced slightly negative chronotropic and inotropic effects. On the other hand, in the observation of chronic animals or human beings, there have been a lot of reports on the effects of T3. In 1967, Cairoli and Crout1) suggested that the resing heart rate of unanesthetized rats treated with thyroxine for 10 days was higher than normal and they described that the tachycardia was due to a direct effect of thyroxine on the SA node. Similarly, Murayama and Goodkind2) reported that the developed tension of isolated atrial muscle from hyperthyroid guinea pigs treated with thyroxine for 8 to 10 days was enhanced by a direct effect of thyroxine on the heart. In clinical observation, Teoh et al3) found that the abnormal ECG changes Vol.20 No.2 T3 ON SA NODE AND CONTRACTILITY 225 reversed to normal when the patients became euthyroid after specific antithyroid thereapy but those were not corrected by an adequate adrenergic blockade except for partial reduction of heart rate. This finding provided some evidences for the concept of combined effects of an increased amount of thyroid hormones and increased adrenergic activity of catecholamines on the heart. Thus, chronic treatments with thyroid hormones might cause an en- hancement of heart rate and contractile force. Folkman and Edmunds5) demonstrated that after implantation of either thyroid grafts or T3 tablets into the myocardium of dogs, a sustained elevation in heart rate for 24 to 98 hours was produced. And they concluded that it was due to a sensitization of the myocardium to the catecholamines, rather than causing an accumula- tion of the catecholamines themselves. McNeill and Brody6) also concluded that T3 potentiated the effects of catecholamines in rats pretreated with T3 for 3 days. Whereas Buccino et al7) reported that hyperthyroidism increased the intrinsic contractile state of papillary muscles from hyperthyroid cats as compared with euthyroid controls, but the hyperthyroid muscles were not hypersensitive to exogenous norepinephrine. Wilson et al8) indicated that T3 did not augment the hemodynamic responses to isoproterenol, studying the volunteers given T3 for a couple of weeks. It has been recently suggested that the amount of norepinephrine discharged at the sympathetic nerve endings is decreased in hyperthyroidism and increased in hypothyroidism (Spaulding and North; 197511)). Banerjee and Kung12) described that T3 increased beta- adrenoceptor density in the rat heart and it caused hyperthyroid manifesta- tion. The present results indicated that a short-term infusion of T3 did not potentiated the positive chronotropic and inotropic effects of norepinephrine as reported by Wahlberg et al.4) The mechanism of altered heart function in hyperthyroidism is extremely complicated and further studies are needed to explain it by use of dogs treated chronically with T3. In conclusion, the present study showed that an adequate dose of T3 caused neither chronotropic nor inotropic effects when administered acutely into the cannulated sinus node artery and never enhanced the positive chronotropic and inotropic effects of norepinephrine in the isolated, blood-perfused canine atrium preparation.

REFERENCES

1. Cairoli VJ, Crout JR: Role of the autonomic nervous system in the resting tachycardia of experimental hyperthyroidism. J Pharmacol exp Ther 158: 55, 1967 2. Murayama M, Gookdind MJ: Effect of thyroid hormone on the frequency-force relation- ship of atrial myocardium from the guinea pig. Circulat Res 23: 743, 1968 3. Teoh PC, Cheah JS, Chia BL: Effect of adrenergic blockade and specific antithyroid therapy . Heart J. 226 KOBAYASHI, FURUKAWA, AND CHIBA March, 1979

on electrocardiographic changes in hyperthyroidism. Am J Med Sci 263: 157, 1974 4. Wahlberg P, Carlsson E, Brandt U: Lack of acute effect of triiodothyronine on noradrenaline

responses of isolated sympathectomized cat hearts. Act Endocrinol 85: 220, 1977 5. Folkman J, Edmunds LH: Endocrine pacemaker for complete heart block. Circulat Res

10: 632, 1962

6. McNeill JH, Brody TM: The effect of triiodothyronine pretreatment on amine-induced cardiac phosphorylase activation. J Pharmacol exp Ther 161: 40, 1968

7. Buccino RA, Spann JF, Pool PE, Sonnenblick EH, Braunwald E: Influence of the thyroid state on the intrinsic contractile properties and energy stores of the myocardium. J Clin

Invest 46: 1669, 1967 8. Wilson WR, Theilen EO, Hege JH, Valenca MR: Effects of beta-adrenergic receptor block-

ade in normal subjects before, during, and after triiodothyronine-induced hypermetabolism.

J Clin Invest 45: 1159, 1966 9. Chiba S, Yabuuchi Y, Hashimoto K: Comparison of the effects of norepinephrine and

acetylcholine between intraarterial and extravascular adeministration to the isolated, blood-

perfused canine atrium. Jap J Pharmacol 25: 433, 1975 10. Chiba S: Effect of pentobarbital, verapamil and manganese on the frequency-force rela- tionship of the isolated atrium and ventricle of the dog heart. Europ J Pharmacol 40: 225,

1976 11. Spaulding SW, North RH: Thyroid-catecholamine interactions. Med Clin N Am 59:

1123, 1975 12. Banerjee SP, Kung LS: ƒÀ-adrenergic receptors in rat heart. Effects of thyroidectomy

Europ J Pharmacol 43: 207, 1977

Jap