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alpha-Melanotrophin was detected by radioimmunoassay in the pituitary glands of fetal rats from day 17 of gestation. The pituitary content of alpha-melanotrophin increased regularly, at a gradually decreasing rate, throughout gestation and in the postnatal period. Concentrations of alpha-melanotrophin in the plasma of fetal and newborn rats were below the detection limit of the radioimmunoassay (10 pmol/l). Detectable concentrations were first found in young rats on day 3 after birth and did not differ significantly from those in their mothers throughout the period of suckling. Plasma concentrations of alpha-melanotrophin were raised in pregnant rats during the last 4 days of gestation and after parturition. They returned to basal levels in the 2 weeks after delivery. After weaning at 3 weeks of age, a large increase in the plasma concentration of alpha-melanotrophin was detected in juvenile rats. Plasma levels had returned to the normal adult range by 6 weeks of age. The increases in alpha-melanotrophin in the blood were thought to be the result of non-specific stress effects. The data did not provide evidence for a role of alpha-melanotrophin in reproductive processes in the rat.

Concentrations of alpha-melanotrophin in the plasma of anaesthetized rats were measured by radioimmunoassay at intervals of 2-10 min. Although significant increases of three to ten times in the concentration of alpha-melanotrophin occurred over periods of 15-35 min, levels of hormone could also be stable for periods of greater than 1 h. The data did not indicate separate secretory episodes but rather a continuous release of alpha-melanotrophin by the pituitary gland. A very rapid pulsatile secretion operating over periods of less than 5 min cannot be excluded. Intravenous injection of synthetic alpha-melanotrophin was followed by a biexponential decline in levels of hormone in plasma indicating a rapid distribution to the extracellular fluid and a slower elimination by metabolism. The apparent volume of distribution of alpha-melanotrophin was 210 ml/kg, the half-life for elimination 7.1 min and the estimated clearance 1.55 nmol/day per kg.

Radioimmunoassay measurements of alpha-melanotrophin in plasma have identified a diurnal rhythm in male rats. Animals maintained on a 12 h light : 12 h darkness photoperiod had raised levels of plasma alpha-melanotrophin during the dark phase. Time-series analysis gave a fitted mean level of alpha-melanotrophin of 52.4 pmol/l, an amplitude of 12.1 pmol/l and peak levels 2.2 h before dawn. Measurements throughout the oestrous cycle in female rats showed that similar variations between the dark and light phases occurred on the 2 days of dioestrus. The raised levels during the dark period were, however, absent on the nights of pro-oestrus and oestrus. During this pro-oestrous/oestrous period, plasma alpha-melanotrophin levels were below average but higher than the normal minimum levels found during the light period.

The half-life for the disappearance of immunoreactive alpha-melanotrophin in plasma following intravenous injection of synthetic hormone was measured before and after occlusion of the blood supply to certain organs in the anaesthetized rat. Occlusion of the blood supply to the liver, gut, spleen and pancreas, and of the renal circulation caused non-significant increases in half-life of 2.8 and 10.7% respectively. The importance of peripheral tissues in the clearance of alpha-melanotrophin was demonstrated by the significant 63.4% increase in half-life caused by occlusion of the blood supply to sections of skeletal muscle, fat and skin.