PURINE ANTAGONISTS IN THE IDENTIFICATION OF ADENOSINE‐RECEPTORS IN GUINEA‐PIG TRACHEA AND THE ROLE OF PURINES IN NON‐ADRENERGIC INHIBITORY NEUROTRANSMISSION

Abstract

1 To test the possibility that adenosine receptors exist within the trachea of the guinea-pig, an attempt has been made to identify a compound with adenosine antagonist activity in this tissue. 2 Quinidine, phentolamine, phenoxybenzamine, 2–2′-pyridylisatogen tosylate (PIT) and caffeine were tested for antagonism of spasmolytic responses to adenosine, adenosine 5′-triphosphate (ATP) and adenine on the guinea-pig isolated trachea. 3 Quinidine (10 and 25 μg/ml), phentolamine (10 and 30 μg/ml) and phenoxybenzamine (10 μg/ml) had little or no effect on response to adenosine, ATP and adenine. PIT (21 μg/ml) potentiated responses to adenosine, ATP and adenine by an unexplained mechanism. 4 Caffeine (25 μg/ml) partially relaxed the trachea and inhibited spasmolytic responses to both adenosine and ATP, but not to adenine, isoprenaline, aminophylline or prostaglandin E₂ (PGE₂). 5 A number of compounds related to caffeine (xanthine, hypoxanthine, theophylline and theobromine) were tested for adenosine antagonist activity. Xanthine (300 μg/ml) and hypoxanthine (300 μg/ml) did not relax the trachea or antagonize spasmolytic responses to adenosine. Both theophylline (10 μg/ml) and theobromine (30 μg/ml) partially relaxed the trachea; theophylline, but not theobromine, antagonized spasmolytic responses to adenosine. 6 pA₂ values for caffeine and theophylline as antagonists of adenosine were 4.3 and 4.7 respectively. However, the slopes of the Schild plot regressions were significantly less than 1.0 for both compounds. 7 Four compounds, adenine, AH 8883, M30966 and ICI 63197, which like caffeine and theophylline, have phosphodiesterase inhibitory activity were tested for adenosine antagonist activity in the trachea. Adenine and AH 8883 had no effect and M30966 and ICI 63197 caused significant potentiation. 8 The effects of caffeine and theophylline were also investigated on the non-adrenergic inhibitory response to nerve stimulation (NAIR). Both caffeine (100 μg/ml, n = 4) and theophylline (30 μg/ml, n = 4) enhanced the NAIR (20 Hz) while virtually abolishing matched responses to exogenous adenosine. 9 The results support the existence of adenosine receptors in the guinea-pig trachea.