A hormone is a regulatory substance and in order to achieve a coordinated physiological effect there are a number of control points at which its effectiveness can be regulated. These are, the regulation of the rate of synthesis, storage, and secretion from the tissue of origin; the regulation of the receptors for the hormone at the target tissues; and perhaps the regulation of the mechanisms for the degradation of the hormone.Relaxin measurement in plasma by radioimmunoassay (RIA) has been routine in a number of laboratories for several years, but surprisingly little has emerged on the control of relaxin secretion. The problem may be partly due to limitations of the homologous model for the RIA-the pig-and because most attention has been paid to the relaxin produced by the corpus luteum of pregnancy. Progress with studies on the identification of relaxin receptors and the factors controlling them has shown some promise. The sequence of actions and mechanisms of degradation for secreted relaxin, as for other peptide hormones, is virtually unknown, but we have circumstantial evidence suggesting that these may be rapid since the half-life of the clearance of endogenous immunoreactive relaxin is rapid and plasma immunoactivity is heterogeneous. Recent work from our laboratory has been aimed primarily at linking investigations of the control of relaxin secretion and the control of relaxin receptor number and affinity in order to achieve an integrated physiological role for relaxin.Hormone secretion may readily be unlinked from action experimentally; the secretion of a hormone may thus be stimulated but the released hormone may be physiologically inactive; for example, fluphenazine given to normal men will markedly increase prolactin secretion,' but in the absence of mammary gland or ovarian receptors the hormone is unable to act physiologically and may simply be degraded. Of course, the hormone may be acting on unknown target tissues with receptors and producing effects not sought through ignorance.There has been considerable controversy whether plasma oxytocin levels increase as gestation progressed, but there is no doubt that the myometrial oxytocin receptor concentration changes dramatically at a time when oxytocin is required to exert its maximal physiological effect on this target tissue.2 In such cases as this the control of the receptors for the hormone must be considered to be of equal importance to the control of the hormone's secretion.
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