Release, distribution and half-life of alpha-melanotrophin 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.