The magnitude of the response to a hormone is determined by: The concentration of the hormone at the surface of the target cell, which in turn depends on the rate of production (under the control of positive and negative feedback loops); the rate of delivery (dependent on blood flow); and the rate of metabolism, degradation and elimination. The sensitivity of the target cell. The number of functional target cells. The availability of cell membrane receptors on target cells Up-regulation: stimulation of receptor synthesis;
Down-regulation: internalisation of receptors by endocytosis; suppression of receptor synthesis. Effects of a hormone on a target cell may include: An alteration in the rate of intracellular protein synthesis; An alteration in the rate of enzyme activity; Modification of plasma membrane transport; Induction of secretory activity. Regulation of hormone levels involves a balance between: Spontaneous, or basal, hormone release;
Feedback inhibition by hormones of their synthesis and/or release; Stimulation or inhibition of hormone release by substances that may/may not be regulated by the same hormones;
Positive feedback mechanisms
Oestrogens mediate a positive feedback increase in the pulsatile release of gonadotrophs releasing hormone (GnRH), luteinising hormone (LH) and follicle stimulating hormone (FSH) prior to ovulation. Ovulation terminates the positive feedback loop abruptly.
Oxytocin release is increased in a positive feedback loop by myometrial contraction during childbirth, with termination of the loop on delivery.
Oxytocin release is also increased via the milk ejection reflex by contraction of mammary duct myo-epithelial cells during suckling at the breast. This loop terminates on removal of the stimulus to the nipples.
Establishment of circadian rhythms for hormone release by systems such as brain;
Brain-mediated stimulation or inhibition of hormone release in response to anxiety, anticipation of a specific activity or other sensory inputs.
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