Alpha 1-adrenergic activation inhibits beta-adrenergic-stimulated unitary Ca2+ currents in cardiac ventricular myocytes.
We have previously shown that whole-cell L-type Ca2+ current that was stimulated through beta-adrenergic receptors was negatively modulated by alpha 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 Ba2+ 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 beta-adrenergic stimulation (beta effect), cells were superfused with norepinephrine (10 mumol/L) in the presence of prazosin (10 mumol/L), an alpha 1-adrenergic blocker. beta-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 alpha 1-receptors while maintaining the beta-adrenergic stimulation, cells were superfused with norepinephrine alone (alpha 1 + beta effects). alpha 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. alpha 1-adrenergic activation also inhibited ensemble-averaged currents stimulated by a low concentration (10 mumol/L) of 8-bromo-cAMP but not by (-)Bay K 8644 (1 mumol/L). Calphostin C (1 mumol/L), a specific inhibitor of protein kinase C, attenuated alpha 1-adrenergic inhibition on beta-adrenergic-stimulated unitary currents. We conclude that alpha 1-adrenergic activation exerts an inhibitory effect on beta-adrenergic-stimulated unitary Ba2+ 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 Ca2+ current previously reported. This inhibition is in part mediated through the protein kinase C pathway.
Chen
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L
Department of Medicine, Veterans Administration Medical Center, Brooklyn, NY 11209, USA.
el-Sherif
N
N
Boutjdir
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D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology; D004095:Dihydropyridines / Q000494:pharmacology; D004553:Electric Conductivity; D007700:Kinetics; D009206:Myocardium / Q000166:cytology; D011493:Protein Kinase C / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D016276:Ventricular Function
D015220:Calcium Channels; D004095:Dihydropyridines; D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; D015124:8-Bromo Cyclic Adenosine Monophosphate; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D011493:Protein Kinase C; D002118:Calcium