Cyclic AMP-dependent phosphorylation modifies the gating properties of L-type Ca2+ channels in bovine adrenal chromaffin cells

We investigated the effects of cAMP-dependent phosphorylation on the voltage- and time-dependent gating properties of Ca²⁺ channel currents recorded from bovine adrenal chromaffin cells under whole-cell voltage clamp. Extracellular perfusion with the membrane-permeant activator of cAMP-dependent protein kinase, 8-bromo(8-Br)-cAMP (1 mM), caused a 49%, 29%, and 21% increase in Ca²⁺ current (I_Ca) amplitudes evoked by voltage steps to 0, +10, and +20 mV respectively (mean values from eight cells, p0.05). Analysis of voltage-dependent steady-state activation (m) curves revealed a 0.70±0.27 charge increase in the activation-gate valency (z_m) following 8-Br-cAMP perfusion. Similar responses were observed when Ba²⁺ was the charge carrier, where z_m was increased by 1.33±0.34 charges (n=8). The membrane potential for half activation (V_1/2) was also significantly shifted 6 mV more negative for I_Ba (mean, n=8). The time course for I_Ba (and I_Ca) activation was well described by second-order m² kinetics. The derived time constant for activation (_m) was voltage-dependent, and the _m/V relation shifted negatively after 8-Br-cAMP treatment. Ca²⁺ channel gating rates were derived from the (_m) and m²values according to a Hodgkin-Huxley type m² activation process. The forward rate (_m) for channel activation was increased by 8-Br-cAMP at membrane potentials 0 mV, and the backward rate (_m) decreased at potentials +10 mV. Time-dependent inactivation of I_Ca consisted of a slowly decaying component (_h 300 ms) and a non-inactivating steady-state component. The currents contributed by the two inactivation processes displayed different voltage dependences, the effects of 8-Br-cAMP being exclusively on the slowly inactivating L-type component.