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Pituitary growth hormone release is under dual hypothalamic control, stimulated by growth hormone releasing hormone (GHRH) and inhibited by somatostatin. Childhood growth hormone deficiency (GHD), leading to dwarfism and metabolic dysfunction¹, can result from a failure of hypothalamic GHRH production or release, from maldevel-opment of the pituitary somatotrophes, and from genetic disorders of growth hormone synthesis. Some familial cases of isolated GHD have been attributed to mutations in the growth hormone gene itself² but in other families GHD is not linked to this locus². The mouse dwarfism trait, little, is due to a recessively inherited missense mutation (lit) in the extracellular domain of the GHRH receptor (Ghrhr)^3–6: Pituitary glands of the little mouse are deficient in growth hormone and are unresponsive to GHRH in vivo and in vitro⁷. Somatic growth is increased by systemic administration of …