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.
