Absorption of alpha-MSH from subcutaneous and intraperitoneal sites in the rat.

Pentobarbitone anesthetized rats were injected with 30 nmol (50 micrograms) alpha-MSH administered intraperitoneally (IP) and subcutaneously (SC) in an acid-saline vehicle, or SC in a zinc phosphate vehicle. Concentrations of alpha-MSH in plasma were measured by radioimmunoassay. The pharmacokinetic parameters for the three modes of administration were determined by fitting a one-compartment open model to the plasma level data. The t1/2 for absorption using the saline vehicle was 7.3 and 5.6 min from the IP and SC sites, respectively. The t1/2 for absorption from the zinc phosphate complex of 17.7 min was significantly longer. Five percent of the IP dose was absorbed into the systemic circulation giving a peak plasma level of 14.1 nmol/l. The absorption of 2-3 percent was significantly lower following SC administration; peak plasma levels were 8.3 and 4.8 nmol/l for the saline and zinc phosphate vehicles, respectively. The low percentage absorption values indicated a high degree of metabolism of the peptide by peripheral tissues on its passage from the injection sites into the circulation.