α 1 -Adrenergic Activation Inhibits β-Adrenergic–Stimulated Unitary Ca 2+ Currents in Cardiac Ventricular Myocytes

Abstract

We have previously shown that whole-cell L-type Ca 2+ current that was stimulated through β-adrenergic receptors was negatively modulated by α 1 -adrenergic activation. In the present study, we investigated the kinetic basis of this modulation at the single-channel level in adult rat ventricular myocytes using Ba 2+ as the charge carrier. Unitary current sweeps were evoked by 300-ms depolarizing pulses to 0 mV, from a holding potential of −50 mV at 0.5 Hz. During control conditions, the ensemble-averaged current amplitude was 0.18±0.01 pA (n=7). To achieve β-adrenergic stimulation (β effect), cells were superfused with norepinephrine (10 μmol/L) in the presence of prazosin (10 μmol/L), an α 1 -adrenergic blocker. β-Adrenergic stimulation enhanced ensemble-averaged current (from 0.18±0.01 to 0.75±0.04 pA, P <.05, n=7), increased the open-time constants, and decreased the closed-time constants. To activate α 1 -receptors while maintaining the β-adrenergic stimulation, cells were superfused with norepinephrine alone (α 1 +β effects). α 1 -Adrenergic activation reduced ensemble-averaged current (from 0.75±0.04 to 0.41±0.03 pA, P <.05, n=7), decreased open-time constants, and increased closed-time constants. α 1 -Adrenergic activation also inhibited ensemble-averaged currents stimulated by a low concentration (10 μmol/L) of 8-bromo-cAMP but not by (−)Bay K 8644 (1 μmol/L). Calphostin C (1 μmol/L), a specific inhibitor of protein kinase C, attenuated α 1 -adrenergic inhibition on β-adrenergic–stimulated unitary currents. We conclude that α 1 -adrenergic activation exerts an inhibitory effect on β-adrenergic–stimulated unitary Ba 2+ current at the single-channel level. The shortening of the open-time and the lengthening of the closed-time constants and the increase in blank sweeps may explain the inhibition of the Ca 2+ -channel activity and the reduction in whole-cell Ca 2+ current previously reported. This inhibition is in part mediated through the protein kinase C pathway.