Pharmacologic analysis of the response to thymoxamine (6-acetoxythymoxy-ethyldimethylamine) on the sinoatrial node of the dog heart.

Using in situ constant pressure perfusion of the SA node artery at 100 mmHg, the effect of thymoxamine, which is a salt of (6-acetoxythymoxy) ethyldimethylamine, was investigated in 7 vagotomized dogs weighing 9 to 15 kg. Thymoxamine injected into the SA node artery induced a biphasic chronotropic response, i.e. a deceleration of sinus rate followed by an acceleration. The threshold dose for induction of sinus deceleration by thymoxamine was 3 to 10 ƒÊg. This deceleration response was not influenced by atropine treatment. The threshold dose for induction of sinus acceleration by thymoxamine was 10 to 30 ƒÊg. This accelerated response was inhibited by propranolol or tetrodotoxin. These results indicate that thymoxamine induces direct depressive action on the SA node pacemaker and the acceleration due to catecholamine release accompanying excitation of sympathetic nerve fibers in the SA node region. Greef and Schumann (1) first described (6-acetoxythymoxy) ethyldimethylamine, thymoxamine, as a sympathicolytic agent to reduce or abolish the rise in blood pressure by an i.v. injection of norepinephrine in anesthetized cats. Birmingham et al. (2, 3) showed that thymoxamine is a competitive antagonist at adrenergic alpha-receptors in the in vivo and in vitro experiments. Although alpha-adrenergic blocking agents induce commonly reflex tachycardia caused by peripheral blockade of vasomotor tone, i.v. administration of thymoxamine frequently induced a deceleration of heart rate followed by an acceleration. While heart rate is frequently modified by the direct effect of drug, it is also markedly influenced by adrenergic and cholinergic mechanisms, as the sinoatrial (SA) node area is densely innervated by sympathetic and parasympathetic nerve fibers (4). A selective administration of the drug into the SA node artery is a very useful method for investigating the direct effect of drugs on the pacemaker of SA node and also to perform pharmacological analysis on the mechanism. Thus, using an in situ blood-perfused SA node preparation of the dog heart (5-7), the effect of thymoxamine on the SA node pacemaker activity was investigated herein. METHODS Seven mongrel dogs of both sexes weighing 9 to 15 kg were anesthetized with sodium pentobarbital i.v. (30 mg/kg). The SA artery was perfused directly with blood led from 190 S. CHIBA, K. OGURO & K. HASHIMOTO the femoral artery. Details of the preparation have been described in previous papers (6, 7). Flow rate in the SA node artery at 100 mmHg was 2.8 •}0.6 ml/min (mean•} S.E.) in 6 experiments. Heart rate was continuously recorded using a cardiotachograph (Nihon Kohden, RT-2), triggered by the R wave of lead II. Both vagi were cut at the mid-cervical level. Drug solution was injected in a volume of 0.01 to 0.03 ml for a period of 4 sec by a microinjector into the perfusion system to the SA node artery. Drugs used were thymoxamine hydrochloride (Kohjin), dl-norepinephrine hydrochloride (Sankyo), atropine sulphate, di-propranolol hydrochloride (Sumitomo Chemicals) and tetrodotoxin (Sankyo), which were dissolved in 0.9 % saline. RESULTS 1) Effects of thymoxamine injected into the sinus node artery (Figs. 1, 2 and 3, Table 1) When a single dose of thymoxamine was injected into the SA node artery, sinus rate was initially depressed and then accelerated. Vasodilation was usually observed when thymoxamine was injected at doses above 1 ƒÊg. The threshold dose for induction of sinus deceleration was 3 to 10 ,ƒÊg and sinus acceleration was approx. 10 to 30 ƒÊg. At the dose level of 100 ƒÊg to 300 ƒÊg, a biphasic response was usually observed. The duration of the negative chronotropic response to thymoxamine was approx. 20 sec to 1 min at the dose range of 100 ƒÊg to 1 mg and positive response was approx. 3 to 5 min at the same dose range. A larger dose of thymoxamine depressed SA node pacemaker activity and sinus rhythm was replaced by nodal rhythm in one out of 4 cases at a dose of 1 mg. Sinus rate was completely depressed with 3 mg of thymoxamine and nodal rhythm appeared in 2 out of 3 cases. Typical responses to increasing doses of thymoxamine are FIG. 1. Response patterns of the SA node to increasing doses of thymoxamine injected into the canine sinus node artery. SBP : systemic blood pressure. HR : heart rate. AVNR : AV nodal rhythm. THYMOXAMINE AND HEART RATE 191 FIG. 2. Percent changes in sinus rate by increasing doses of thymoxamine. FIG. 3. Repetition of the biphasic chronotropic response to the same dose of thymoxamine, 1 mg, injected into the sinus node artery. SBP : systemic blood pressure. HR : heart rate. TABLE 1. Chronotropic responses of the SA node to thymoxamine. * Nodal rhythm was induced in one out of 4 cases . ** Nodal rhythm was induced in 2 out of 3 cases . Results are given as mean •} S.E. 192 S. CHIBA, K. OGURO & K. HASHIMOTO shown in Figs. 1 and 2. A certain dose of thymoxamine caused a similar biphasic response repetitively in the same animal, even though the degree varied somewhat in successive trials. Fig. 3 shows an experiment with 1 mg of thymoxamine, in which tachyphylaxis was not seen during repetitive intra-arterial injection with 10 to 30 min interval. Sinus deceleration induced by thymoxamine was not influenced by bilateral vagotomy. Atropine (30 μg) did not prevent sinus deceleration induced by thymoxamine 30 ƒÊg to 1 mg. The data are summarized in Table 1. 2) Effect of propranolol on thymoxamine-induced acceleration of sinus rate (Fig. 4, Table 2) Sinus acceleration with norepinephrine was inhibited by propranolol injected into the sinus node artery. Thymoxamine-induced acceleration was also blocked by propranolol. These blocking effects of propranolol disappeared after 20 min. Typical responses are shown in Fig. 4. The summarized data are in Table 2. 3) Effect of tetrodotoxin on thymoxamine-induced acceleration of sinus rate (Fig. 5, Table 3) Injection of tetrodotoxin into the SA node artery blocked the SA nodal responses to electrical stimulation of either the right vagus or the right stellate ganglion (8). TetroFIG. 4. Blocking effect of 3 ƒÊg of propranolol (PROP) on the positive chronotropic responses to 0.1 and 0.3 ƒÊg of norepinephrine (NE) and 1 mg of thymoxamine (THY). SBP : systemic blood pressure. HR : heart rate. TABLE 2. Effect of propranolol on the positive chronotropic responses to norepinephrine and thymoxamine. Results are given as mean •} S.E. THYMOXAMINE AND HEART RATE 193 dotoxin in doses of 1 to 3 ƒÊg injected into the SA node artery caused a deceleration of the sinus rate. The accelerated response to 300 ƒÊg-1 mg of thymoxamine was inhibited by treatment with 1 to 3 ,ƒÊg of tetrodotoxin. Norepinephrine-induced acceleration of sinus rate was never inhibited by tetrodotoxin. The typical responses are shown in Fig. 5. After treatment with 3 ƒÊg of tetrodotoxin injected into the SA node artery, the positive chronotropic responses to 300 ƒÊg and 1 mg of thymoxamine were completely blocked. That to 0.1 ƒÊg of norepinephrine was never inhibited by tetrodotoxin.