Rat thymocytes release a factor which inhibits muscarinic ligand binding

Abstract

Rat thymocytes incubated in RPMI medium for 120–300 min release a soluble factor of molecular weight below 10 000 Da which inhibits the binding of 3H-labeled muscarinic antagonists in an uncompetitive manner, i.e. it reduces maximal specific binding ( B max) without changing the affinity of the ligand ( K D). This factor inhibited muscarinic antagonist binding on thymocytes and rat cerebral cortex cellular membranes. Thymocytes from hydrocortisone-treated rats produced more factor per mg cell protein than thymocytes from untreated rats. The activity of the factor was unaffected by incubation with Cd 2+ (1 mM) or Zn 2+ (1 mM) or EDTA (1 mM), and its protein nature is supported by the following findings: it was trypsin sensitive, heat denaturated at 56°C and precipitated by (NH 4) 2SO 4 (40%, w/v). Inhibition by the factor was apparently irreversible after 1 h of incubation.