Muscarinic and β-adrenergic depression of the slow Ca²⁺-activated potassium conductance in hippocampal CA3 pyramidal cells is not mediated by a reduction of depolarization-induced cytosolic Ca²⁺ transients

Combined intracellular and microfluorometric recording techniques were used to evaluate whether the inhibition by cholinergic or adrenergic transmitters of the Ca²⁺-activated potassium current (I_AHP) in hippocampal CA3 pyramidal cells was mediated by an alteration of depolarization-induced change in cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). Low concentrations of isoproterenol (1-10 μM) and muscarine (0.25-1 μM) reversibly abolished I_AHP without affecting concomitant Ca²⁺ transients or the steady-state [Ca²⁺])_i. Only after application of higher concentrations of muscarine, [Ca²⁺]_i increased; in the presence of potassium channel blockers, muscarine depressed Ca²⁺ currents and concomitant Ca²⁺ transients. These observations provide direct evidence that the inhibition of I_AHP by isoproterenol and muscarine are not mediated by an alteration of Ca²⁺ dynamics.