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

Abstract

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 (IK,cAMP), whereas ATP activates a Cl ؊ current (FCl) 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 (ietr/iwf) in the two preparations. For ATP activation of IK,cAMP and FCl, the ratios ietr/iwf 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 M [Hill coefficient (nH) ‫؍‬ 2.7 ؎ 0.61], and that in e.t.r. follicles (FCl) was 1.8 ؎ 0.68 M (nH ‫؍‬ 0.76 ؎ 0.09), whereas Ado-response parameters did not change. Responses to UTP and ␤,␥-methylene-ATP, specific agonists for IK,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.Xenopus oocyte ͉ P2Y receptor ͉ A2 receptor ͉ G protein-coupled receptor oligomerization F ollicle-enclosed Xenopus oocytes are endowed with purinergic receptors located in the follicular cell membrane (1, 2). Three main subtypes have been described: (i) A2 receptors responsive to Adenosine (Ado), coupled to cAMP synthesis and opening of K ϩ channels (cAMP-dependent K ϩ current, I K,cAMP ), that are sensitive to glibenclamide (3-7); (ii) P2Y receptors responsive to ATP and UTP that operate Cl Ϫ channels generating fast and slow Ca 2ϩ -independent currents (F Cl and S Cl ) (2, 8, 9); and (iii) a purinergic receptor, referred to as a P3 or novel P1 receptor, that is sensitive to both Ado and ATP and coupled to I K,cAMP generation (10, 11). Also, there is a P2Y receptor that inhibits I K,cAMP and is probably the same as the receptor involved in F Cl and S Cl activation (2, 12, 13).Removal of epithelium and theca layers from the follicle (e.t.r. follicle) has been shown to eliminate the ability of ATP to generate I K,cAMP (13), an effect that was explained by upregulation of the ATP inhibitory action on these channels mediated by the P2Y receptor. Although this explanation accounted for most of the effects observed in e.t.r. follicles, additional observations indicated further effects on the purinergic system that suggested functional interactions between the receptors expressed in the follicle. For example, responses t...

Differential expression of the P2X7 receptor in ovarian surface epithelium during the oestrous cycle in the mouse

“…Matzuk et al 2002), whereas others were suggested based on their conspicuous expression and actions in ovarian systems of numerous species. The latter is the case for purinergic signalling, which has been demonstrated to be functionally expressed in ovarian cells of several vertebrates (Arellano et al 1998(Arellano et al , 2002(Arellano et al , 2009(Arellano et al , 2011Vázquez-Cuevas et al 2006Bintig et al 2009). Purinergic signalling is now known to be an important paracrine regulator in distinct cell systems, both in physiological and pathological processes (Burnstock 2007;Corriden and Insel 2010).…”

“…A role for purinergic signalling in ovarian function has been proposed (Arellano et al 1998(Arellano et al , 2002(Arellano et al , 2009(Arellano et al , 2011Vázquez-Cuevas et al 2006Bintig et al 2009). Purinergic signalling by ATP and its hydrolysis products is now recognised as a paracrine/autocrine system that has a role in many physiological phenomena, acting on a variety of cell types (Burnstock 2007;Corriden and Insel 2010).…”

Purinergic signalling in the reproductive system in health and disease

“…Since UTP and ATP are equipotent, this effect could be mediated by a P2Y 2 or P2Y 4 receptor subtype [136]. Moreover, it has been suggested that there are close interactions between A 2 and P2Y receptors in Xenopus follicles [19]. Cultured CHO cells express a P2 receptor equi-reactive to ATP and UTP [178], probably the P2Y 2 and/or P2Y 4 subtype.…”

Purinergic signalling – a possible mechanism for KCNQ1 channel response to cell volume challenges

“…,b, Arellano et al . , , ). Defolliculated oocytes are reputed to express a few if any purinergic receptors, and this was based on the observation that ATP failed to activate current responses.…”