“…However, such a shift from Vi to V3 has not yet been demonstrated in hypothyroid newborn guinea pigs and is worth investigating. In the present study we have confirmed that the positive inotropic and toxic effects of oua bain are age-related, namely the responsive ness increases with age [2,3,9], As pre viously reported, we also found that hypo thyroidism amplifies the myocardial re sponse to ouabain [6,7,17], A possible explanation for the inverse re lationship between the thyroid state and the (age-related) responsiveness to ouabain, is the modulation of Na+-K+,ATPase by thy roid hormones, which is brought about by increasing the number of enzyme molecules as well as their activity [10,11,13]. The involvement of Na+-K+,ATPase is further supported by studies demonstrating devel opmental changes in myocardial Na+-K+,ATPase activity, such that enzyme activ ity is larger in newborns than in adults [15,23].…”