Chapter 5 - Principles of Hormonal Integration

Abstract

This chapter discusses some of the general principles of endocrine integration at the cellular and whole body level. Each hormone independently excites its receptors and initiates a cascade of transduction mechanisms, the immediate consequences of which contribute to determining the nature and magnitude of cellular responses. Therefore, target cells must integrate the various inputs at both receptor and postreceptor levels and resolve them into productive responses. One hormone may influence responses to other hormones by regulating expression of key components of transduction or signaling pathways, such as thyroid hormones increase expression of adrenergic receptors in some tissues and cortisol appears to increase expression of adrenergic receptors. Since most hormones do not simply activate a linear series of reactions, rather signal through multiple parallel intracellular pathways; the particular combination of signaling pathways that are activated, may determine the final outcome. Biological systems are dynamic; just as the amount of hormone secreted varies with changing circumstances, so too does the magnitude and duration of the response of target cells. Hormones also regulate both the sensitivity, and the capacity of target tissues to respond either to themselves or to other hormones. However, the two hormones that produce common effects may differ not only in their range of actions but also in their time constants.