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Characterization of purinoceptors mediating depolarization of rat isolated vagus nerve
Author(s) -
Trezise D.J.,
Kennedy I.,
Humphrey P.P.A.
Publication year - 1993
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1993.tb13920.x
Subject(s) - depolarization , purinergic receptor , adenosine , adenosine triphosphate , chemistry , vagus nerve , biophysics , membrane potential , medicine , endocrinology , biology , biochemistry , stimulation
1 As part of a broader study to characterize neuronal purinoceptors, the effects of adenosine 5′‐triphosphate (ATP) and a range of ATP analogues were investigated on the extracellularly recorded membrane potential of the rat isolated vagus nerve, using a ‘grease‐gap’ technique. 2 ATP evoked depolarization of the rat vagus nerve. The concentration‐effect curve to ATP was not monophasic: at the lower concentrations (1 × 10 −5 –1 × 10 −3 m ) the curve was shallow (<50% of the near maximal response to 5‐hydroxytryptamine (5‐HT)) whilst at higher concentrations the relationship between concentration and amplitude of depolarization was steeper (> 135% of the response to 5‐HT at the highest concentration tested, 1 × 10 −2 m ). On washout of the high drug concentrations large after‐hyperpolarizations were often observed. 3 α,β‐methylene ATP (1 × 10 −6 –3 × 10 −4 m ), β,γ‐methylene ATP (1 × 10 −6 –1 × 10 −3 m ), and 5′‐adenylylimidodiphosphate (β,γ‐imido ATP; 1 × 10 −6 –1 × 10 −3 m ) were all more potent than ATP and produced large depolarizations of the rat vagus nerve at the highest concentrations tested (> 150% of the response to 5‐HT). The overall rank order of potency was α,β‐methylene ATP > β,γ‐methylene ATP = β,γ‐imido ATP > ATP. 4 In contrast, 2‐methylthio ATP (1 × 10 −6 –1 × 10 −3 m ) produced relatively small depolarizations (< 100% of the response to 5‐HT). As was the case with low concentrations of ATP, the concentration‐effect curve to 2‐methylthio ATP was very shallow. 5 Adenosine 5′‐diphosphate (ADP), adenosine 5′‐monophosphate (AMP), adenosine and adenosine 5′‐O‐(2‐thiodiphosphate) (ADP‐β‐s; all 1 × 10 −6 –1 × 10 −3 m ) evoked only small depolarizations of the vagus nerve, amounting to 47 ± 2.5%, 40.8 ± 7.8%, 33.7 ± 3.3% and 62.4 ± 12.7% of the response to 5‐HT, respectively. Uridine 5′‐triphosphate (UTP; 1 × 10 −6 –1 × 10 −3 m ) was inactive. 6 The P 2 purinoceptor antagonist, suramin (1 × 10 −5 m –1 × 10 −4 m ), antagonized responses to α,β‐methylene ATP. The nature of this antagonism was not, however, consistent with simple competitive kinetics between agonist and antagonist. Depolarizations produced by β,γ‐methylene ATP and β,γ‐imido ATP were also attenuated by suramin (1 × 10 −4 m ), but in contrast, suramin had no effect on responses to ADP, 2‐methylthio ATP, ADP‐β‐S or 5‐HT. 7 In addition to its antagonist effects, suramin (10 −4 m ) markedly increased the maximum amplitude of the depolarization produced by ATP. 8 It is concluded that a heterogeneous receptor population mediates depolarization of the rat vagus nerve by purine nucleotides. Importantly, the large amplitude depolarizations to α,β‐methylene ATP, β,γ‐methylene ATP and β,γ‐imido ATP are mediated via receptors that share many characteristics of the classical P 2x receptor. In contrast, the relatively small depolarizing effects of ADP, ADP‐β‐S and 2‐methylthio ATP were suramin‐resistant. Although it appears that other purinoceptors are present, these data suggest that the rat vagus nerve may serve as a useful preparation for studying the pharmacology of neuronal P 2x receptors.