ABSTRACT
The effects of triiodothyronine (T3) on the force frequency responses of isometrically contracting rabbit papillary muscles were studied in the presence of d-glucose, pyruvate or butyrate. The stimulation frequency was varied from 0.1 to 0.5 Hz, and the maximum developed tension and its maximum (Tmx) and minimum (Tmn) time derivative were measured. T3 concentrations ranged from 0.1 to 4.0 ng/ml. The addition of T3 resulted in a substrate-dependent increase in twitch tension; with the largest increases being: d-glucose 118 +/- 7%, pyruvate 143 +/- 6%, butyrate 123 +/- 11%; Tmx:d-glucose 121 +/- 8%, pyruvate 157 +/- 5%, butyrate 138 +/- 12%, and Tmn:d-glucose 150 +/- 10%, pyruvate 159 +/- 6%, butyrate 163 +/- 14%. All three measures of contractility showed a dose-dependent increase reaching a maximum value at a T3 concentration between 1 and 2 ng/ml. These data show that T3 induces an inotropic response in rabbit papillary muscles which is manifested within, approximately 30 min, and that the greatest increase is seen in Tmn.
