Alpha-adrenergic modulation of ionic currents in cultured parasympathetic neurons from rat intracardiac ganglia.

1. Modulation of ionic conductances by alpha-adrenergic agonists was investigated in cultured parasympathetic neurons from rat intracardiac ganglia. Application of norepinephrine (NE, 25-100 microM) to the soma of isolated neurons reversibly reduced both the amplitude and duration of the Ca(2+)-dependent action potential evoked by injection of depolarizing current when Na+ and K+ currents were blocked pharmacologically. 2. In the whole-cell voltage-clamp mode, application of NE reversibly reduced the amplitude and rate of activation of Ca2+ current (ICa). The amplitude inhibition was greater at the peak of the current (55%) than at the end of a 700-ms pulse (20%). Maximal doses of NE produced only approximately 60% inhibition of peak ICa amplitude. 3. Inactivation of ICa was best fit by the sum of two exponential functions in the absence of NE, but was described by a single exponential function in the presence of NE. These results suggest that NE preferentially inhibited a fast inactivating component of the Ca2+ current in these parasympathetic neurons. 4. NE reversibly reduced the amplitude of Ba2+ tail currents through open Ca channels at all voltages from -40 to +150 mV with a slight shift in the activation curve determined from the current-voltage (I-V) relationship for the tail currents. NE did not change the voltage dependence of the steady-state inactivation of the calcium channels. 5. NE inhibited Ca2+ current either in the absence or presence of nifedipine but to a lesser extent in the presence of omega-conotoxin (omega-CGTX), suggesting that the Ca channels inhibited by NE are predominantly omega-CGTX sensitive. 6. The inhibition of ICa by NE was mimicked by the alpha 1-adrenergic agonists methoxamine and phenylephrine and potentiated in the presence of the alpha 2-adrenoceptor antagonist yohimbine (10 microM). NE inhibition of ICa was antagonized by bath application of the alpha-adrenergic antagonist phentolamine (1 microM), but not by prazosin (1-10 microM), yohimbine, or the beta-adrenergic antagonist propranolol (1 microM). Taken together, these results suggest that NE inhibition of Ca2+ current in rat parasympathetic cardiac neurons is mediated by an alpha-adrenergic receptor with properties that may differ from alpha 1- and alpha 2-adrenoceptors. 7. In approximately 35% of neurons studied, NE not only reduced depolarization-activated inward Ca2+ current but also increased an outward current, with a shift of the I-V curve and reversal potential to more negative voltages.(ABSTRACT TRUNCATED AT 400 WORDS)