Influences of pressure-injected cyclic AMP on the membrane current and characteristics of an identified neuron of Aplysia kurodai.

Abstract

The ionic mechanism of the effect of intracellularly injected adenosine 3′, 5′-cyclic monophosphate (cAMP) on the membrane of identified neuron L5 of Aplysia kurodai was investigated with conventional voltage-clamp and ion-substitution techniques. The intracellular elevation of cAMP caused an inward current (I_cAMP), which was not accompanied by a significant change in membrane conductance at potentials more hyperpolarized than -60mV. The current increased over the voltage range (-50 to -30mV) associated with a conductance decrease and decreased at potentials more hyperpolarized than -60mV. Elevated intracellular cAMP was found to enhance a region of negative slope resistance in steady-state I-V relations. Duration of the I_cAMP was greatly prolonged by bath-applied isobutylmethylxanthine (50μM), but imidazole (10mM) had an opposite effect on the I_cAMP. Tolbutamide (5mM), a protein kinase inhibitor, reduced the I_cAMP. The current was not affected by the presence of bath-applied TTX (50μM), ouabain (50μM), or triaminopyrimidine (5mM). Reduction of [Na⁺]₀ reversibly decreased the I_cAMP. Li⁺ could largely substitute for Na⁺. Alterations of [K⁺]₀, and bath application of 4-AP (5mM) and TEA (30mM) did not affect the I_cAMP, In the presence of Na⁺, Cl^-, and divalent cations such as Ca²⁺ and Ba²⁺ inhibited the I_cAMP. These results suggest that fast elevation of intracellular cAMP induces a TTX-resistant slow Na⁺ inward current, and the current might be due to activation of cAMP-dependent protein kinase.