title: [Acute negative chronotropic effect of L-thyroxine on the everted sinus-atrium preparation of the bullfrog].
abstract: An application of L-thyroxine (T4) to bullfrog everted sinus-atrium preparations showed an acute negative chronotropic effect instead of a positive one in the chronic phase. The acute effect was dose dependent (7-70 microM) and the maximal negative chronotropic effect (about + 20% interval prolongation) was observed at 40 microM T4. The acute T4 effect on the cardiac interval consisted of two stages: an initial peak (about 5 min) accompanied by a transient increase in contractility, and a delayed peak (about 50 min) accompanied by a decrease in contractility to the control level. Pretreatment with TEA (15 mM) suppressed the initial T4 effect on the cardiac interval but enhanced the delayed one. In contrast the metabolic inhibitors NaCN (0.5 mM) or DNP (10 microM) abolished the delayed T4 effect while the initial one remained. Furthermore, pretreatment with Cd2+ (10 microM) suppressed antagonistically the initial T4 effect and inhibited the delayed one. These results suggest that the initial effects of T4 were accompanied by a transient increase in intracellular Ca2+ and the delayed effects of T4 were accompanied by acceleration of intracellular metabolism.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D003864:Depression, Chemical; D006321:Heart / Q000187:drug effects*; D006325:Heart Atria; D006339:Heart Rate / Q000187:drug effects*; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects; D011892:Rana catesbeiana; D012849:Sinoatrial Node / Q000187:drug effects; D012850:Sinus of Valsalva / Q000187:drug effects; D013974:Thyroxine / Q000494:pharmacology*
chemical_list: D013974:Thyroxine; D002118:Calcium