Interaction between injected Ca²⁺ and intracellular Ca²⁺ stores in Xenopus oocytes

Upon two repetitive deep injections of Ca²⁺ into Xenopus oocyte (200-300 μn under the membrane), the amplitude of the transient Cl^- current induced by the second injection is several-fold higher than that of the first one. This 'potentiation' persists even at 60-90 min intervals between injections. However, in oocytes permeabilized to Ca²⁺ by the ionophore A23187 in a Ca²⁺-free solution, the potentiation completely disappears after 30 min. It is proposed that the injected Ca²⁺ is largely taken up by the stores, whereas following the second injection, a higher proportion of Ca²⁺ reaches the membrane, since the stores are already loaded. In ionophore-treated oocytes, the stores lose the accumulated Ca²⁺ over several minutes and are then ready to take up Ca²⁺ again, hindering its arrival at the membrane.