DIRECT EFFECTS OF TRIIODOTHYRONINE ON HUMAN INTERNAL MAMMARY ARTERY AND SAPHENOUS VEINS

Objective: Thyroid hormone [3,5,3'-triiodo-L-thyronine) is under inves tigation as a positive inotrope and vasodilator for patients undergoin g cardiac surgery. This study determined the direct effects of triiodo thyronine on human blood vessels. Design: Prospective, controlled, in vitro study. Setting: Laboratory facility in a university teaching hos pital. Participants: Small excess segments of internal mammary arterie s or saphenous Veins were obtained from patients undergoing coronary a rtery bypass surgery. Interventions: Vessel segments were cut into rin gs to measure isometric tension development in isolated tissue baths c ontaining Krebs-Ringer bicarbonate solution at 37 degrees C. Rings wer e prestretched in vitro to resting tensions analogous to mean arterial or central Venous pressures in vivo and then constricted with potassi um or phenylephrine. Rings were exposed to increasing concentrations o f triiodothyronine (4 x 10(-12) to 1 x 10(-4) mol/L) to obtain dose-re sponse curves. Measurements and Main Results: High concentrations (gre ater than or equal to 3.3 x 10(-5) mol/L) of triiodothyronine produced dose-dependent relaxation of preconstricted rings. The relaxation was not selective for arteries or veins at arterial resting tensions, and with either potassium or phenylephrine as a vasoconstrictor. Proprano lol had little effect on subsequent triiodothyronine-induced relaxatio n of potassium-constricted rings at resting arterial tensions. Conclus ions: Triiodothyronine, in supraphysiological and suprapharmacological concentrations, dilates preconstricted rings of human blood vessels i n vitro; however, triiodothyronine had no demonstrable vasomotor effec ts on human internal mammary artery or saphenous vein in clinically re levant concentrations (10(-9) to 10(-8) mol/L). Triiodothyronine admin istration in vivo most likely has little direct effect on the tone of human vascular smooth muscle, particularly coronary artery bypass cond uits. Copyright (C) 1997 by W.B. Saunders Company.