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

Extracellular nucleotides, acting through P-2-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 P-2-purinoceptor subtype on MDCK cells responsible for stimulating inositol phosphate (IP) accumulation using a range of nucleotide analogues. In Ca2+- and Mg2+-free Krebs-Henseleit solution (KHS), ATP, UTP, and ATP gamma S caused an increase in IP accumulation as a function of concentration with comparable kinetics. The order of potency for the nucleotide analogues was UTP = ATP gamma S > ATP = 2-chloro ATP (Cl-ATP) >> alpha,beta-methylene ATP (alpha,beta-MeATP) = 2-methylthio ATP (2MeSATP). Selective agonists for P-1-, P-2X- and P-2Y-purinoceptors, such as N-6-cyclopentyl adenosine, AMP, alpha,beta-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 ATP gamma S 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 ATP gamma S 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 IP 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.