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+.
D000656:Amoeba / Q000187:drug effects* / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002412:Cations; D004723:Endorphins / Q000037:antagonists & inhibitors; D004745:Enkephalins / Q000037:antagonists & inhibitors; D009020:Morphine / Q000037:antagonists & inhibitors; D009270:Naloxone / Q000494:pharmacology*; D010873:Pinocytosis / Q000187:drug effects*; D012964:Sodium / Q000494:pharmacology
D002412:Cations; D004723:Endorphins; D004745:Enkephalins; D009270:Naloxone; D009020:Morphine; D012964:Sodium; D002118:Calcium