The excitatory response of cultured neurones of rat parasympathetic cardiac ganglia to extracellular adenosine 5'-triphosphate (ATP) was examined using the whole-cell and isolated membrane patch recording nfigurations of the patch clamp technique. The short latency between ATP application and activation of the membrane current (< 20 ms) suggests a direct coupling between purinergic receptor and ion channel. The response was maintained during exposure to ATP suggesting that receptor desensitization is not a factor in current decay. The current-voltage (I-V) relationship for macroscopic ATP-evoked currents showed strong inward rectification in the presence and absence of external divalent cations and a reversal potential of +10 mV (NaCl outside, CsCl inside). Unitary ATP-activated currents in cell-attached membrane patches exhibited a linear (ohmic) I-V relationship with a slope conductance of ~ 60 pS. The order of agonist potency for the purinergic receptor-mediated response was 2-methylthioATP = ATP > ADP > AMP > adenosine = α,β-methylene ATP > β,γ-methylene ATP, a sequence consistent with a P(2y) receptor subtype. ATP-evoked currents were attenuated by α,β-methylene ATP (IC50 ~ 10 μM) and reversibly inhibited in a dose-dependent manner by Reactive Blue 2 (K(d) = 1 μM). The amplitude of the ATP-evoked current was dependent on the extracellular Na+ concentration. The direction of the shift in reversal potential when NaCl was replaced with mannitol indicated that the purinergic receptor channel is cation selective. The cation permeability relative to Na+ followed the ionic selectivity sequence Ca2+ (1.48) > Na+ (1.0) > Cs+(0.67). Anions were not measurably permeant. ATP and ACh-evoked responses in rat intracardiac neurones are mediated by distinct receptor channels. The ATP-activated channels in cardiac neurones may contribute to non-cholinergic, non-adrenergic neurotransmission and mediate, in part, the vagal innervation of the mammalian heart.
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