Functional significance of muscarinic receptor expression within the proximal and distal rat vagina

1 Department of Pharmacology and Physiology, Drexel University College of Medicine; Philadelphia; 2 Department of Surgery, Division of Urology, University of Pennsylvania School of Medicine, Philadelphia; and 3 Department of Obstetrics and Gynecology, Division of Urogynecology and Reconstructive Pelvic Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania/ Submitted June 18, 2008 ; accepted in final form September 3, 2009 Information regarding the role of cholinergic nerves in mediating vaginal smooth muscle contraction is sparse, and in vitro studies of the effects of muscarinic agonists on vaginal smooth muscle are discrepant. The goal of this study was to determine the expression of muscarinic receptors in the vaginal wall of the rat. In addition, we sought to determine the effect of the muscarinic receptor agonist carbachol on contractility and inositol phosphate production of the proximal and distal rat vaginal muscularis. RT-PCR analysis indicated that both M 2 and M 3 receptor transcripts were expressed within the proximal and distal rat vagina. Carbachol dose-dependently (10 –7 –10 –4 M) contracted the rat vaginal muscularis with a greater maximal contractile response in the proximal vagina ( P < 0.01) compared with the distal vagina. The contractile responses of the rat vaginal muscularis to carbachol were dose dependently inhibited by the M 3 antagonist para-fluoro-hexahydrosiladefenidol, and a pK B of 7.78 and 7.95 was calculated for the proximal and distal vagina, respectively. Inositol phosphate production was significantly increased in both regions of the vagina following 20-min exposure to 50 µM carbachol with higher levels detected in the proximal vagina compared with the distal ( P < 0.05). Preliminary experiments indicated the presence of M 2 and M 3 receptors in the human vaginal muscularis as well as contraction of human vaginal muscularis to carbachol, indicating that our animal studies are relevant to human tissue. Our results provide strong evidence for the functional significance of M 3 receptor expression in the vaginal muscularis. vagina; smooth muscle; muscarinic receptor; female sexual response Address for reprint requests and other correspondence: S. Chacko, Basic Urology Research Laboratory, Glenolden Research Bldg., 500 S. Ridgeway Ave., Glenolden, PA 19036-2307 (e-mail: chackosk{at}mail.med.upenn.edu ).