7.8: Target Cell Response

Receptor Upregulation

Upregulation signifies a hormone's capacity to enhance the abundance or avidity of its receptors, either for itself or other hormones. For instance, within the ovary, estrogen orchestrates a symphony of receptor upregulation, augmenting its own receptor population and elevating the receptivity of luteinizing hormone (LH) receptors. This harmonious interaction amplifies the ovarian response to hormonal cues.

Receptor Downregulation

Conversely, the downregulation of receptors represents a counterpoint to the symphony of hormonal modulation. Here, hormones curtail the quantity or affinity of their receptors, often in response to prolonged exposure to elevated hormone levels. As an illustrative example, progesterone orchestrates the downregulation of both its receptors and those of estrogen. This strategic modulation preserves the delicacy of hormonal balance.

Hormonal Interactions

Hormones seldom operate in isolation, engaging in complex interactions that sculpt cellular responses. There are three primary modes of interaction:

The permissive effect is where the presence of one hormone sets the stage for another hormone's action. For instance, thyroid hormones orchestrate intricate permissive relationships with specific reproductive hormones. A deficiency of iodine, a fundamental thyroid hormone component, can reverberate throughout the reproductive system, impacting its development and functionality.

The synergistic effect is when two hormones wielding similar effects collaborate. At times, the presence of both hormones is requisite for a robust response. For example, FSH from the anterior pituitary and ovarian estrogens nurture the maturation of ova.

The antagonistic effect is where opposing forces vie for dominance. The interplay of two pancreatic hormones, insulin, and glucagon, serves as an example where insulin takes the lead in glucose storage as glycogen in muscle fibers and hepatocytes, lowering blood glucose levels. In contrast, glucagon stimulates hepatocyte glycogen breakdown and raises blood glucose levels.