Regulation of Oxytocin Receptor Responsiveness by G Protein-Coupled Receptor Kinase 6 in Human Myometrial Smooth Muscle

May 9, 2009Molecular endocrinology (Baltimore, Md.)

Control of oxytocin receptor sensitivity by a regulatory enzyme in human uterine muscle

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Abstract

Oxytocin caused over 70% decrease in receptor responsiveness after brief exposure.

Oxytocin increased both total inositol phosphate accumulation and intracellular calcium concentration in human myometrial cells.
Responses to oxytocin were more pronounced in the ULTR cell line compared to primary myometrial cells.
Oxytocin receptor desensitization occurred rapidly within 5 minutes of exposure.
Brief exposure to oxytocin led to a significant reduction in receptor responsiveness upon subsequent stimulation.
Knockdown of GRK6 significantly reduced oxytocin-induced receptor desensitization, while depletion of other GRKs had no effect.

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Oxytocin plays an important role in the progression, timing, and modulation of uterine contraction during labor and is widely used as an uterotonic agent. We investigated the mechanisms regulating oxytocin receptor (OTR) signaling in human primary myometrial smooth muscle cells and the ULTR cell-line. Oxytocin produced concentration-dependent increases in both total [(3)H]inositol phosphate accumulation and intracellular Ca(2+) concentration ([Ca(2+)](i)); however, responses were greater and more reproducible in the ULTR cell line. Assessment of phospholipase C activity in single cells revealed that the OTR desensitizes rapidly (within 5 min) in the presence of oxytocin (100 nm). To characterize OTR desensitization further, cells were stimulated with a maximally effective concentration of oxytocin (100 nm, 30 sec) followed by a variable washout period and a second identical application of oxytocin. This brief exposure to oxytocin caused a marked decrease (>70%) in OTR responsiveness to rechallenge and was fully reversed by increasing the time period between agonist challenges. To assess involvement of G protein-coupled receptor kinases (GRKs) in OTR desensitization, cells were transfected with small interfering RNAs to cause specific > or =75% knockdown of GRKs 2, 3, 5, or 6. In both primary myometrial and ULTR cells, knockdown of GRK6 largely prevented oxytocin-induced OTR desensitization; in contrast, selective depletion of GRKs 2, 3, or 5 was without effect. These data indicate that GRK6 recruitment is a cardinal effector of OTR responsiveness and provide mechanistic insight into the likely in vivo regulation of OTR signaling in uterine smooth muscle.

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Regulation of Oxytocin Receptor Responsiveness by G Protein-Coupled Receptor Kinase 6 in Human Myometrial Smooth Muscle

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