Naloxone-reversible effect of opioids on pinocytosis in Amoeba proteus

Abstract

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^1–4. 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^6–9, high doses of which have been reported to affect the amoeba¹⁰. 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 caffeine¹¹. In this report we show that pinocytosis induced by Na⁺ or Tris⁺ is suppressed by β-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⁺.