title: Influence of altered thyroid state on the interval-strength relationship of paced left atria of rats.
abstract: Rats were treated with methimazole or thyroxine to render them either hypothyroid or hyperthyroid. Left atria from these animals and from control, untreated rats were isolated, placed in organ baths, and paced by direct electrical stimulation delivered via a punctate electrode. Contractile responses to trains of stimuli at 0.01-6 Hz were recorded isometrically. Atria from methimazole-treated rats required less voltage to elicit submaximal contractions; these were greater in magnitude and duration than those recorded from control preparations. The reverse was observed for preparations from hyperthyroid rats. Thyroid state had a marked influence upon the pattern of contractions elicited by trains of stimuli at varying frequencies. Atria from control rats displayed a typical negative interval-strength relationship, i.e., a decrease in contractile amplitude as the interval between stimuli in a train was decreased. This negative relationship was more pronounced in left atria from hypothyroid rats. In preparations from hyperthyroid rats, increases in stimulation frequency resulted in elevations in contractile amplitude. These observations are discussed in the light of known actions of thyroid hormone upon cardiac contractile mechanisms.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D004558:Electric Stimulation; D006325:Heart Atria / Q000503:physiopathology; D006980:Hyperthyroidism / Q000503:physiopathology*; D007037:Hypothyroidism / Q000503:physiopathology*; D066298:In Vitro Techniques; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D011433:Propranolol / Q000494:pharmacology; D051381:Rats
chemical_list: D011433:Propranolol; D002118:Calcium