Absence of acute cardiac effect of triiodothyronine in isolated, blood-perfused canine atrium.

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 (0.001 pg--0.1 microgram) did not cause a significant direct action injected into the cannulated sinus node artery. When T3 (10 pg/min, 40 pg/min, 0.1 microgram/min, or 0.4 microgram/min) was continuously infused into the sinus node artery, both positive chronotropic and inotropic actions of norepinephrine (0.03 microgram--0.3 microgram) 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.