The M3 subtype predominantly mediates contraction of the normal urinary bladder as well as most smooth muscles, even though the M2 subtype has the greatest density.  The aims of this study were to define the contractile function of the M2 and M3 receptor subtypes individually in mediating carbachol induced bladder contractions in wild type mice (WT) and knock-out mice (KO) in which either the M2 or M3 receptor subtype are missing.  To determine possible species differences, M2 and M3 receptors were selectively inactivated in rat bladders to determine if the contribution of the M2 and the M3 receptor to muscarinic receptor stimulation is the same in rat and mouse bladders.

Full thickness rat and mouse bladder strips were suspended in muscle baths and the contractile response to KCl was determined.  Then a concentration response curve to carbachol was performed by the cumulative addition of carbachol.  M3 receptors were selectively inactivated by incubating the muscle strips with 4-DAMP mustard in the presence of methoctramine (to minimize M2 receptor inactivation). To inactivate M2 receptors, strips were incubated with phenoxybenzamine in the presence of darifenacin (to minimize M3 receptor inactivation).  Subtype selective immunoprecipitation was used to calculate the density of M2 and M3 receptors following receptor inactivation.

In the M2 KO mouse bladder, the M3 receptor alone mediates about 80% of the muscarinic stimulated contractile response while in the M3 KO mouse bladder, the M2 receptor mediates about 5% of the maximal response.  However, in the rat bladder, when M2 receptors are inactivated the remaining M3 receptors induce about 50% of the maximal contraction.  Similarly, when M3 receptors are inactivated, the remaining M2 receptors also induce about 50% of the maximal response.  In the mouse stomach, using M2 and M3 KO mice, the M2 receptor subtype alone can mediate about 25% of the maximal contractile response, while the M3 receptor subtype can mediate about 80% of the maximal contractile response.  In different smooth muscles from the same species, the M2 receptor subtype mediates contraction to different maximums.

The contribution of the M2 and M3 muscarinic receptors to maximal contraction are different in rat and mouse bladders.  In addition, in the mouse, the M2 receptor subtype can mediate a different maximal contraction in different smooth muscles, i.e. 5% in the bladder and 25% in the stomach.

Whether human bladder smooth muscle is more similar to the rat or mouse bladder is unknown.  Caution must be used when using results from one species as evidence for a similar function in a different species.  This caution must also be used when using results from different smooth muscles from the same species.