Final answer:
Injected TSH increases the metabolic rate in a normal rat by stimulating the thyroid to release T4 and T3. It has no effect in a thyroidectomized rat since the thyroid gland is absent. In a hypophysectomized rat, injected TSH may increase the metabolic rate if the thyroid is still present and functional.
Step-by-step explanation:
The effect of injected thyroid-stimulating hormone (TSH) on the metabolic rate of each of the three types of rats can be described as follows: Normal rat: In a normal rat with a functional thyroid gland,
the injection of TSH would typically stimulate the thyroid to release more thyroxine (T4) and triiodothyronine (T3), hormones that are key regulators of the basal metabolic rate (BMR). As a result, there would be an increase in the rat's metabolic rate.
Thyroidectomized rat: In a rat that has undergone thyroidectomy (removal of the thyroid gland), injecting TSH would have no effect on metabolic rate. This is because the organ that responds to TSH and produces thyroid hormones (T3 and T4) has been removed, and thus, there can't be any up-regulation of metabolic activity without T4 and T3.
Hypophysectomized rat: This rat has had its pituitary gland removed. Since TSH is produced by the anterior pituitary, this rat would normally not produce any TSH, and therefore, its thyroid gland would not be normally stimulated to produce thyroid hormones.
Injecting TSH into a hypophysectomized rat would therefore bypass the lack of pituitary function and might cause an increase in metabolic rate, assuming the thyroid is still present and functional.
The above scenarios are assuming the rest of the endocrine system is functioning normally, and there are no other underlying conditions affecting the BMR.