Final answer:
Thyroid hormones increase the number of epinephrine receptors in a permissive hormone interaction, enhancing the cell's response to epinephrine, particularly in cardiac rate and contractility.
Step-by-step explanation:
The interaction described where thyroid hormones increase the number of receptors available for epinephrine at its target cell is an example of a permissive hormone interaction. Permissive effects occur when the presence of one hormone enables another hormone to act. Thyroid hormones, such as T3, enter cells and can up-regulate the number of beta-adrenergic receptors, which are targets for epinephrine, thus increasing cellular sensitivity and response. This up-regulation ensures that cells are more responsive to epinephrine, leading to more pronounced effects such as increased cardiac rate and contractility.