Functional interaction between native G protein-coupled purinergic receptors in Xenopus follicles

Purinergic receptors are expressed in the membrane of the follicular cell layer that communicates with the Xenopus oocyte. Adenosine (Ado) generates a cAMP-dependent K⁺ current (I_K,cAMP), whereas ATP activates a Cl⁻ current (F_Cl) and has a dual effect on I_K,cAMP, provoking both its activation and inhibition. Here, purinergic responses were studied electrophysiologically, first in the whole follicle (w.f.), and then in the same follicle after removal of its epithelium/theca layers (e.t.r. follicle). Responses were analyzed as the ratio of the current amplitudes (i_etr/i_wf) in the two preparations. For ATP activation of I_K,cAMP and F_Cl, the ratios i_etr/i_wf were 0.053 and 22, respectively, whereas that for Ado was 0.75. Thus, epithelium/theca removal drastically altered the ATP response, suggesting a change in the signaling pathway that correlated with changes in the pharmacological characteristics: the half-maximal effective concentration for activation of the main current in w.f. (I_K,cAMP) was 14 ± 3.8 μM Hill coefficient (nH) = 2.7 ± 0.61], and that in e.t.r. follicles (F_Cl) was 1.8 ± 0.68 μM (nH = 0.76 ± 0.09), whereas Ado-response parameters did not change. Responses to UTP and β,γ-methylene-ATP, specific agonists for I_K,cAMP inhibition and activation, respectively, indicated that in e.t.r. follicles inhibition increased and activation decreased drastically. Thus, purinergic responses were not independent; instead, they were functionally linked. We hypothesize that this property was due to direct interactions between receptors for Ado (A2 subtype) and ATP (P2Y subtype) in the Xenopus follicle.