Force-velocity relations of the portal vein of hyperthyroid rats.

The effect of thyroxine on the elementary process of contraction of vascular smooth muscle was tested on 38 hyperthyroid and in 38 control rats. Hyperthyroidism was induced by i.p. injections of triiodthyronine or 1-htyroxine for 2 weeks. By means of afterloaded isotonic contractions of the tetanized portal vein force velocity relations were calculated. There was a slight increase in both the extrapolated velocity of shortening at zero load [from 0.93 +/- 0.04 ML/s (control rats) to 1.03 +/- 0.04 ML/s (hyperthyroid rats; P less than 0.05)], and the peak force generation (from 14.8 +/- 0.4 mN to 16.1 +/- 0.4 mN; P less than 0.005). The maximum of mechanical power development at a distinct extent of afterload was augmented from 1.35 +/- 0.06 muW/ML to 1.68 +/- 0.06 muW/ML (P less than 0.005). The influence of thyroxine on the elementary process of contraction in vascular smooth muscle is discussed in connection with the much stronger effect of hyperthyroidism on cardiac muscle contraction.