Evidence that the P2x purinoceptor of the smooth muscle of the guinea pig vas deferens is an ATP4- receptor.
In solutions containing Mg2+ and Ca2+, ATP is in equilibrium between the tetrabasic form (ATP4-) and its bidentate coordination complexes, i.e., MgATP2- and CaATP2-. We sought evidence to determine whether contractions of the smooth muscle of the guinea pig vas deferens to ATP are in response to ATP4- or its bidentate complexes. Contractions to ATP were elicited in seven modified Krebs-Henseleit solutions containing varied concentrations of free and total Mg2+ and Ca2+ to alter the concentration of ATP4- at given ATPtotal concentrations. As the concentration of Mg2+ increased the concentration of ATP required to stimulate contraction to an equivalent degree also increased. Regardless of the free or total Mg2+ and Ca2+ concentrations, response magnitude was generally correlated with [ATP4-]. This suggests that ATP4- is the agonist at the P2x purinoceptor of the guinea pig vas deferens. The potency of ATP4- is high; the threshold, occurring at approximately 1 nM ATP4-, is 1000-fold less than that for norepinephrine. The implications of ATP4- as agonist are discussed in relation to adenine nucleotide potency, metabolism and P2 purinoceptor classification.
Fedan
J S
JS
Division of Respiratory Disease Studies, National Institute for Occupational Safety and Health, Morgantown, West Virginia.
Dagirmanjian
J P
JP
Attfield
M D
MD
Chideckel
E W
EW
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology; D011983:Receptors, Purinergic / Q000145:classification / Q000187:drug effects*; D013329:Structure-Activity Relationship; D014649:Vas Deferens / Q000187:drug effects / Q000502:physiology*
D011983:Receptors, Purinergic; D000255:Adenosine Triphosphate; D008274:Magnesium