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Muscarinic receptor heterogeneity in follicle‐enclosed Xenopus oocytes
Author(s) -
Arellano Rogelio O.,
Garay Edith,
Miledi Ricardo
Publication year - 1999
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1999.00409.x
Subject(s) - muscarinic acetylcholine receptor , oxotremorine , endocrinology , medicine , muscarinic acetylcholine receptor m1 , acetylcholine , chemistry , agonist , muscarinic agonist , muscarinic acetylcholine receptor m3 , biology , muscarinic acetylcholine receptor m5 , receptor
1 Ionic current responses elicited by acetylcholine (ACh) in follicle‐enclosed Xenopus oocytes (follicles) were studied using the two‐electrode voltage‐clamp technique. ACh generated a fast chloride current ( F in ) and inhibited K + currents gated by cAMP (I K,cAMP ) following receptor activation by adenosine, follicle‐stimulating hormone or noradrenaline. These previously described cholinergic responses were confirmed to be of the muscarinic type, and were independently generated among follicles from different frogs. 2 Inhibition of I K,cAMP was about 100 times more sensitive to ACh than F in activation; the half‐maximal effective concentrations (EC 50 ) were 6.6 ± 0.4 and 784 ± 4 n m , respectively. 3 Both responses were blocked by several muscarinic receptor antagonists. Using the respective EC 5 0 concentrations of ACh as standard, the antagonist 4‐diphenylacetoxy‐ N ‐methylpiperidine methiodide blocked the two effects with very different potencies. F in was blocked with a half‐maximal inhibitory concentration (IC 50 ) of 2.4 ± 0.07 n m , whilst the IC 50 for I K,cAMP inhibition was 5.9 ± 0.2 μ m . 4 Oxotremorine, a muscarinic agonist, preferentially stimulated I K,cAMP inhibition (EC 50 = 15.8 ± 1.4 μ m ), whilst F in was only weakly activated. In contrast, oxotremorine inhibited F in generated by ACh with an IC 50 of 2.3 ± 0.7 μ m . 5 Fin elicited via purinergic receptor stimulation was not affected by oxotremorine, indicating that the inhibition produced was specific to the muscarinic receptor, and suggesting that muscarinic actions do not exert a strong effect on follicular cell‐oocyte coupling. 6 Using reverse transcription‐PCR, transcripts of a previously cloned muscarinic receptor from Xenopus (XlmR) were amplified from the RNA of both the isolated follicular cells and the oocyte. The pharmacological and molecular characteristics suggest that XlmR is involved in I K,cAMP inhibition. 7 In conclusion, follicular cells possess two different muscarinic receptors, one resembling the M 2 (or M 4 ) subtype and the other the M 3 subtype. These receptors are coupled to distinct membrane mechanisms leading to independent regulation of two membrane conductances.