Naloxone-reversible effect of opioids on pinocytosis in Amoeba proteus. 
A characteristic feature of induced pinocytosis in Amoeba proteus is the formation of broad channels by invagination of the cell membrane.  This process, which requires Ca2+, occurs in response to depolarising cations.  High Ca2+ levels reduce pinocytosis induced by cations such as Na+ and Tris+, whereas pinocytosis induced by K+ is less affected by Ca2+ (ref.  4).  Agents which interfere with the calcium metabolism of the amoeba will therefore either stimulate or inhibit pinocytosis induced by Na+ (ref.  5).  Among the agents which are supposed to reduce Ca2+ influx across cell membranes or otherwise decrease cellular availability of Ca2+ are the opiates and opioid peptides, high doses of which have been reported to affect the amoeba.  Accordingly, Met-enkephalin, morphine and codeine potentiate the inhibition of pinocytosis caused by Ca2+-binding agents and reverse the calcium blockade of pinocytosis mediated by caffeine.  In this report we show that pinocytosis induced by Na+ or Tris+ is suppressed by beta-endorphin, Metenkephalin and morphine.  These effects were abolished or diminished by an opiate receptor antagonist, (-)naloxone, by increasing the Na+ concentration, or by addition of Ca2+.