Epithelium and/or theca are required for ATP‐elicited K⁺ current in follicle‐enclosed Xenopus oocytes

Abstract

The Xenopus follicular cell membrane is endowed with ATP-sensitive K+ channels, which are operated by various transmitters. These generate the ionic response named IK,cAMP via a mechanism that involves intracellular cAMP synthesis. It is known that opening these K+ channels favors oocyte maturation. Follicle stimulation by adenosine (Ado) or ATP consistently generates a strong IK,cAMP via activation of P1 and P3 purinergic receptors; however, ATP can also inhibit IK,cAMP, apparently acting on a third receptor type. Here, we show that IK,cAMP might be elicited by ATP released within the follicle, and that current activation by ATP was entirely dependent on the presence of epithelial and/or theca layers. Morphological studies confirmed that removal of epithelium/theca in these follicles (e.t.r.) was complete, and activation of fast Cl- (Fin) currents by ATP in e.t.r. follicles confirmed that communication between oocyte and follicular cells remained unchanged. Thus, dependence on epithelium/theca was specific for ATP-elicited K+ current. Using UTP and betagamma-MeATP as specific purinergic agents for IK,cAMP inhibition and activation, respectively, it was found that inhibition of IK,cAMP elicited by ATP or UTP was robustly present in e.t.r. follicles but was absent or strongly decreased in whole follicles (w.f.). Accordingly, this indicated that in w.f., epithelium and/or theca downregulated the IK,cAMP inhibition evoked by ATP, and that this control mechanism was absent in e.t.r. follicles. We suggest that this notable action on follicular cells involves one or both of two mechanisms, a paracrine transmitter released from epithelial and/or theca layers and action of ecto-ATPases.

Purinergic signalling in the reproductive system in health and disease

“…P1 receptors on follicular cells mediate K + current responses, while P2Y 2 receptors are involved in the activation of Cl − currents [356]. These authors and others [329] suggested that these effects are consistent with intrafollicular ATP signalling that modulates oocyte maturation.…”

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

“…Inhibition of I K,cAMP by purinergic agents is up-regulated in e.t.r. preparations, as shown previously (13). This effect was further studied by comparing the potency of UTP in w.f.…”

“…For example, responses to ␤,␥-methylene-ATP (␤,␥-meATP), a P3 receptor agonist (10,11), were down-regulated in a significant fraction of e.t.r. follicles (13); however, these lower responses were inconsistent, apparently due to the well-known amplitude variability of current responses elicited by most agonists described in the Xenopus follicle. To reduce this variability, here, we compared the characteristics of the responses elicited by Ado and ATP, both in whole follicles (w.f.)…”

“…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).…”

“…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.…”