Purpose: Four adenosine receptors (ARs), designated A1AR (A1 adenosine receptor), A2aAR (A2a adenosine receptor), A2bAR (A2b adenosine receptor), and A3AR (A3 adenosine receptor), have been cloned from various species, but the contraction mechanism via A1ARs in cat detrusor muscle cell is not well known.
Materials and methods: We examined the cellular mechanism using an A1AR agonist 2-chloroadenosine (2-CA) in cat detrusor cell isolated by enzymatic digestion. To examine which phospholipase mediates the contraction, we used phospholipase inhibitors.
Results: The adenosine analog potency order is R-N6-phenylisopropyladenosine (R-PIA) > 5'-N-ethylcarbosamine adenosine (NECA) > 2-chloroadenosine (2-CA) > S-N6-phenylisopropyladenosine (S-PIA). The ratio of equi-effective concentrations of R-PIA/S-PIA was 58.2. 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 300 nM) shifted to the right the concentration-response curves of 2-CA. These results indicate A1ARs mediate 2-CA induced contraction in cat detrusor muscle. G proteins (Gi1, Gi2, Gi3, Go, Gs, and Gq) in cat detrusor muscle were detected by immunoblot analysis. Pertussis toxin (PTX) inhibited 2-CA induced contraction. In permeabilized cells, antibodies against Galphai3 antagonized 2-CA induced contraction, suggesting that the contraction is mediated by Gi3 protein. A phosphatidylinositol-specific phospholipase C (PLC) inhibitor, neomycin, reduced 2-CA induced contraction, but a phospholipase D (PLD) inhibitor, p-chloromercuribenzoic acid, and a phospholipase A2 (PLA2) inhibitor, dimethyl-eicosa-dienoic acid (DEDA), had no effect. We found the presence of the main PLC isozymes, PLC-beta1, PLC-beta3, and PLC-gamma1. 2-CA induced contraction in permeabilized cells was inhibited by PLC-beta3 but not by PLC-beta1 or PLC-gamma1 antibody. These results imply that A1ARs are coupled to PLC-beta3 via PTX-sensitive Gi3 protein. Sr2+ medium and thapsigargin, which replaces intracellular Ca2+ and deplete intracellular calcium stores respectively, inhibited 2-CA induced contraction.
Conclusions: These results suggest that A1ARs mediating 2-CA induced contraction exist in cat detrusor muscle and the contraction depends on a PTX-sensitive Gi3 protein, PLC-beta3 and the release of intracellular Ca2+.