α-Adrenergic modulation of ionic currents in cultured parasympathetic neurons from rat intracardiac ganglia
1. Modulation of ionic conductances by α-adrenergic agonists was investigated in cultured parasympathetic neurons from rat intracardiac ganglia. Application of norepinephrine (NE, 25-100 μM) to the soma of isolated neurons reversibly reduced both the amplitude and duration of the Ca2+-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 (I(Ca)). 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 ~60% inhibition of peak I(Ca) amplitude. 3. Inactivation of I(Ca) 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 ω-conotoxin (ω-CGTX), suggesting that the Ca channels inhibited by NE are predominantly ω-CGTX sensitive. 6. The inhibition of I(Ca) by NE was mimicked by the α1- adrenergic agonists methoxamine and phenylephrine and potentiated in the presence of the α2-adrenoceptor antagonist yohimbine (10 μM). NE inhibition of I(Ca) was antagonized by bath application of the α-adrenergic antagonist phentolamine (1 μM), but not by prazosin (1-10 μM), yohimbine, or the β-adrenergic antagonist propranolol (1 μM). Taken together, these results suggest that NE inhibition of Ca2+ current in rat parasympathetic cardiac neurons is mediated by an α-adrenergic receptor with properties that may differ from α1- and α2-adrenoceptors. 7. In ~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. Replacement of the Cs+ in the pipette by a larger sized cation, arginine, attenuated the NE-induced increase in outward current. This NE-activated current appeared to be a time-independent background current carried by small cations. 8. The inhibition of I(Ca) and activation of background current by NE were mimicked by intracellular application of guanosine-5'-O-(3-thiotriphosphate) (100 μM) and antagonized by either intracellular application of guanosine-5'-O-(2-thiodiphosphate) (100 μM) or pretreatment of the neurons with pertussis toxin. Thus activation of α-adrenergic receptors by NE and modulation of I(Ca) and background current is coupled by a pertussis toxin-sensitive G-protein(s). 9. NE-induced inhibition of I(Ca) and activation of background current were not mimicked by activators of adenylate cyclase nor protein kinase activators or inhibitors, suggesting that neither adenosine 3',5'-cyclic monophosphate- protein kinase A nor diacylglycerol (DAG)-protein kinase C second messenger pathways mediate NE modulation of the ionic currents. 10. This modulation of ionic conductances in rat intracardiac neurons by NE may contribute to the positive chronotropic effect observed in the mammalian heart on stimulation of sympathetic nerve terminals.
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