Purinoceptor-stimulated phosphoinositide hydrolysis in Madin-Darby canine kidney (MDCK) cells

Extracellular nucleotides, acting through P₂-purinoceptors, have been implicated in the regulation of ion transport in epithelia, including Madin-Darby canine kidney (MDCK) cells. In this study, experiments were conducted to characterize the P₂-purinoceptor subtype on MDCK cells responsible for stimulating inositol phosphate (U?) accumulation using a range of nucleotide analogues. In Ca²⁺- and Mg²⁺-free Krebs-Henseleit solution (KHS), ATP, UTP, and ATPγS caused an increase in IP accumulation as a function of concentration with comparable kinetics. The order of potency for the nucleotide analogues was UTP = ATPγS > ATP = 2-chloro ATP (Cl-ATP) >> α,β-methylene ATP (α,β-MeATP) = 2-methylthio ATP (2MeSATP). Selective agonists for P₁-, P(2X)- and P(2Y)-purinoceptors, such as N⁶-cyclopentyl adenosine, AMP, α,β-MeATP, and 2MeSATP, had little effect. Stimulation of MDCK cells with maximally effective concentrations of ATP and UTP showed no additive effect and furthermore, ATP, UTP, and ATPγS induced cross-desensitization of the IP response, suggesting that ATP and UTP act upon a common nucleotide receptor, i.e. a P(2U)-purinoceptor. In Ca²⁺- and Mg²⁺-containing KHS, the concentration-response curves of ATP, UTP, and ATPγS were Shifted to the right of those obtained in Ca²⁺- and Mg²⁺-free buffer, and asymptotic maxima were not reached, indicating that ATP^4- and not MgATP^2- or CaATP^2- was the active agonist. Pretreatment of MDCK cells with pertussis toxin (PTX) inhibited ATP- and UTP-induced IP accumulation in a concentration-dependent fashion but did not completely abolish the UP accumulation, indicating that a PTX-sensitive G protein was partially involved in the IP response. In conclusion, ATP- and UTP-stimulated IP accumulation in MDCK cells appears to be mediated through the activation of P(2U)-purinoceptors coupled to a G protein that is partially sensitive to PTX. A form of nucleotide uncomplexed with divalent ions such as ATP^4- seems to be the preferential agonist form for the purinoceptors on MDCK cells.