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First published online 9 October 2007
doi: 10.1242/jcs.009415

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RyR1-specific requirement for depolarization-induced Ca²⁺ sparks in urinary bladder smooth muscle

Nicolas Fritz^1,*, Jean-Luc Morel^1,2, Loice H. Jeyakumar³, Sidney Fleischer³, Paul D. Allen⁴, Jean Mironneau¹ and Nathalie Macrez^1,2,

¹ CNRS UMR 5017, Laboratoire de Signalisation et Interactions Cellulaires, Université Bordeaux 2, Bordeaux, France
² Université de Bordeaux1, CNIC, CNRS UMR 5228, Talence, France
³ Department of Biological Sciences, Vanderbilt University, Nashville, TN 37232, USA
⁴ Department of Anaesthesia Perioperative and Pain Medicine, Brigham and Women's Hospital, Boston, MA 02115, USA

Author for correspondence (e-mail: n.macrez{at}cnic.u-bordeaux1.fr)

Accepted 20 August 2007

Ryanodine receptor subtype 1 (RyR1) has been primarily characterized in skeletal muscle but several studies have revealed its expression in smooth muscle. Here, we used Ryr1-null mice to investigate the role of this isoform in Ca²⁺ signaling in urinary bladder smooth muscle. We show that RyR1 is required for depolarization-induced Ca²⁺ sparks, whereas RyR2 and RyR3 are sufficient for spontaneous or caffeine-induced Ca²⁺ sparks. Immunostaining revealed specific subcellular localization of RyR1 in the superficial sarcoplasmic reticulum; by contrast, RyR2 and RyR3 are mainly expressed in the deep sarcoplasmic reticulum. Paradoxically, lack of depolarization-induced Ca²⁺ sparks in Ryr1^–/– myocytes was accompanied by an increased number of cells displaying spontaneous or depolarization-induced Ca²⁺ waves. Investigation of protein expression showed that FK506-binding protein (FKBP) 12 and FKBP12.6 (both of which are RyR-associated proteins) are downregulated in Ryr1^–/– myocytes, whereas expression of RyR2 and RyR3 are unchanged. Moreover, treatment with rapamycin, which uncouples FKBPs from RyR, led to an increase of RyR-dependent Ca²⁺ signaling in wild-type urinary bladder myocytes but not in Ryr1^–/– myocytes.

In conclusion, although decreased amounts of FKBP increase Ca²⁺ signals in Ryr1^–/– urinary bladder myocytes the depolarization-induced Ca²⁺ sparks are specifically lost, demonstrating that RyR1 is required for depolarization-induced Ca²⁺ sparks and suggesting that the intracellular localization of RyR1 fine-tunes Ca²⁺ signals in smooth muscle.

Key words: Ryanodine receptors, Ryr1^–/– mice, Ca²⁺ sparks, FK506-binding protein, Smooth muscle