Adenosine triphosphate-evoked currents in cultured neurones dissociated from rat parasympathetic cardiac ganglia
1. The excitatory response of cultured neurones of rat parasympathetic cardiac ganglia to extracellular adenosine 5'-triphosphate (ATP) was examined using the whole-cell isolated membrane patch recording configurations of the patch clamp technique. The short latency between ATP application and activation of the membrane current (less than 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. 2. 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 approximately 60 pS. 3. The order of agonist potency for the purinergic receptor-mediated response was 2-methylthioATP = ATP greater than ADP greater than AMP greater than adenosine = alpha,beta-methylene ATP greater than beta,gamma-methylene ATP, a sequence consistent with a P2y receptor subtype. ATP-evoked currents were attenuated by alpha,beta-methylene ATP (IC50 approximately 10 microM) and reversibly inhibited in a dose-dependent manner by Reactive Blue 2 (Kd = 1 microM). 4. 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) greater than Na+ (1.0) greater than Cs+ (0.67). Anions were not measurably permeant. 5. 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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