Characterization of the muscarinic receptor in isolated uterus of sham operated and ovariectomized rats

Abstract

1 The pharmacological characteristics of muscarinic receptors in rat isolated uterus were studied in ovariectomized (ov.) and sham operated (sh.) animals. 2 Competition radioligand binding studies, using uterine membranes and [ 3 H]-NMS, were undertaken with several muscarinic receptor antagonists. Most of the antagonists indicated a onesite ®t with apparent a nity estimates (pK i ) unchanged by ovariectomy. The selective M 2 antagonist, tripitramine revealed high (representing 33+8 and 38+2%) and low (67+8 and 62+2%) a nity binding sites in both sh. and ov. rat uterus, respectively. These sites likely represented muscarinic M 2 and M 3 receptors and the proportions were not signi®cantly di erent in the two conditions. 3 Carbachol induced concentration-dependent contractions which were surmountably antagonized by several muscarinic receptor antagonists (pK B , sh.; ov.): zamifenacin (9.19; 9.18), p-F-HHSiD (8.50; 9.06), tripitramine (7.23; 7.54), himbacine (7.21; 7.41), methoctramine (6.79; 7.49), pirenzepine (6.48; 7.21), AF DX 116 (6.26; 6.61), MTx 3 (57.00; 57.00) and PD 102807 (57.00; 57.00). 4 The apparent a nity values obtained in functional studies using the uteri from both sh. and ov. animals correlated most closely with values reported at human recombinant muscarinic M 3 receptors. This suggests that the muscarinic M 3 receptor mediates contraction under both conditions. 5 Radioligand binding experiments indicate the presence of M 2 receptors, in addition to M 3 receptors, which probably explains the discrepancies between functional and binding a nities. These data further suggest that the pharmacological pro®le and proportions of the two populations of muscarinic receptors are una ected by ovariectomy.

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

“…11). This finding is consistent with studies on nonvascular smooth muscles that indicate contraction is mediated predominantly via M 3 receptor stimulation (1,9,10,18,23,38).…”

“…Five subtypes of muscarinic receptors (M 1 , M 2 , M 3 , M 4 , M 5 ) have been pharmacologically described and molecularly identified (8). Although M 2 receptors outnumber M 3 receptors within smooth muscle, pharmacological studies have indicated that M 3 receptor signaling is responsible for the majority of muscarinic agonist-induced smooth muscle contraction of the lower urogenital tract including the bladder (2) and uterus (1,10,23). Recent studies with M 3 receptor knockout mice have provided further evidence that the M 3 receptor plays the predominant role in mediating smooth muscle contraction (25).…”

Multiple functional defects in peripheral autonomic organs in mice lacking muscarinic acetylcholine receptor gene for the M 3 subtype

“…For example, the vas deferens (38), prostate gland (39), and corpus cavernosum of penis (40) express mAChRs of the M 3 profile. In addition, M 3 receptors are responsible for uterine smooth muscle contraction (41). However, our result of crosses between homozygous mice suggests that M 3 receptors are dispensable for the normal reproduction of the mouse.…”

Differences in muscarinic-receptor agonist–, oxytocin-, and prostaglandin-induced uterine contractions

“…Uterus contraction in response to acetylcholine or in response to stimulation of cholinergic neurons has been observed in several mammals, including rats (17), pigs (18,19), and humans (20). This effect is mediated by interaction of acetylcholine with muscarinic receptors (4).…”

“…Leiber et al (22) studied the effects of different muscarinic-receptor antagonists on carbachol-induced contractions and measured accumulation of inositol phosphates and inhibition of cyclic adenosine monophosphate production in the guinea pig uterus. In rat uterus membranes, M2 and M3 muscarinic receptors have been detected using radioligand-binding studies (17). In the porcine uterus, the presence of M3 but not M2 muscarinic receptors has been revealed by functional and radioligand-binding studies (18).…”