Comparison of the effects of arylazido aminopropionyl ATP (ANAPP 3), an ATP antagonist, on responses of the smooth muscle of the guinea-pig vas deferens to ATP and related nucleotides

Abstract

Contractile responses of the smooth muscle of the guinea-pig vas deferens to ATP and related analogs have been suggested previously to involve stimulation of cell-surface P 2-purinergic receptors and obligatory hydrolysis, depending on the concentration and structure of the nucleotide. This hypothesis was further tested by evaluating the antagonistic effect of arylazido aminopropionyl ATP (ANAPP 3), a specific P 2-purinergic receptor antagonist, on concentration-response relationships and on the profiles of individual responses to ATP and analogs containing phosphate-chain, ribose and adenine modifications. The concentration-response curves for ATP, β, γ-methylene ATP (APPCP), β, γ-imido ATP (APPNP) and adenosine 5′-O-(3-thiotriphosphate) (ATPγS) were bimodal and suggested the existence of two receptors, one with high and one with low affinity for adenine nucleotides. The type of antagonism observed was nucleotide-specific, and concentration-response curves were monosigmoidal after ANAPP 3 treatment. For ATP, antagonism was greatest at low concentrations (< 3 × 10 −5 M) and did not affect the maximum response; for APPCP and APPNP, it was greatest at high concentrations (> 3 × 10 −5 M) and resulted in a reduction in maximum response; for adenosine tetraphosphate (APPPP) the antagonism was pronounced at low and high concentrations; for ATPγS the antagonism was moderate at all concentrations. The concentration-response curves for a second group of less potent nucleotides was monosigmoidal. Included in this group was ADP, α, β-methylene ADP (APCP), 8-bromo ATP (8-BrATP) and 2-deoxy ATP (2d-ATP). Treatment with ANAPP 3 shifted the concentration-response curves for these analogs to the right but did not reduce maximum responses. The characteristics of individual response profiles were structurally related. For most analogs responses to low concentrations of nucleotides were phasic and spike-like. Discernible tonic phases appeared with higher concentrations of ATP, ADP, ATPγS, APPPP and 2d-ATP. ANAPP 3 antagonized the initial component of responses but did not affect the tonic phase. The initial phasic component of response has been related to receptor-mediated stimulation of the preparations, while the tonic component, characteristic of nucleotides without 5′-anhydride bridge substitutions, appears to be initiated by hydrolysis of the compounds, a process which is insensitive to ANAPP 3.