Fast decrease of the peak current carried by barium ions through calcium channels in the somatic membrane of mollusc neurons.

In experiments on nonidentified intracellularly perfused snail neurons the effects of replacing external divalent cations on the function of potential-dependent Ca channels have been studied. Ba substitution for Ca in the external medium caused a rapid decline (with half-times of about 2-3 min) in peak inward current amplitude when the current was activated from holding potential levels close to the resting potential. The decline could be reversed by membrane hyperpolarization. Barium current declined to a steady-state level which resembled in both relative amplitude (10-30% of the initial current amplitude) and insensitivity to intracellular introduction of exogenous cAMP the steady-state Ca current reached during the "wash out" process. It is suggested that two populations of Ca channels exist in snail neuronal membrane, one of which is dependent on cAMP metabolism and is reversibly switched off by the passage of Ba ions.