Function of M3 muscarinic receptors in the rat urinary bladder following partial outlet obstruction

Partial outlet obstruction of the rat urinary bladder leads to hypertrophy and alteration in contractility of the detrusor muscle involving changes in muscarinic receptors. m3 muscarinic receptor subtype has been known to play a predominant role in contractility of normal urinary bladder. The purpose of the present study was to assess the role of m3 receptors in contractility of the obstructed bladder.

In male rats, partial outlet obstruction of the urinary bladder was performed by surgically tying a 6-0 suture around the bladder neck, reducing the diameter of it by 2/3 of the original size. Four weeks after the surgery, the bladders were removed and thin strips were microdissected. Similarly, bladder strips from age matched unoperated normal rats were obtained. Sets of four strips from four normal or four obstructed rats were mounted in an in vitro multi-muscle chamber containing normal physiological solution at 37C. The tension responses evoked by optimal electrical field stimulation at 1, 10, 30, 50, and 100 Hz, and the contracture responses evoked by 120 mM potassium and 0.01 to 300.0 microM carbachol were recorded using a Nicolet digital oscilloscope. Similar responses were recorded in different sets of four strips following exposure to 10 and 100 nM 4-DAMP, which is a muscarinic antagonist with a high affinity for m3 and m1 receptor subtypes.

The obstructed bladders showed 119% increase in weight. In control physiological solution, the obstructed bladder strips did not show significant difference in electrically-evoked tension or carbachol contractures, but showed significantly lower potassium contractures compared with normal bladder strips. 4-DAMP at 10 to 100 nM significantly reduced the electrically evoked tension responses by about the same degree in normal and obstructed bladders, without affecting the potassium contractures. It significantly increased the EC50 values for carbachol contractures in normal bladder, and to a significantly lesser extent in obstructed bladder. Schild plots using the Hill transformed EC50 values showed that the pA2 value for 4-DAMP was not significantly different in normal and obstructed bladders.

Significantly smaller potassium contracture in the obstructed bladder indicates that depolarizability of the detrusor muscle membrane, and consequently the activity of voltage-gated Ca2+ channels may be reduced in the detrusor after partial outlet obstruction. Lack of a significant difference in the effect of 4-DAMP on the electrically evoked tension responses and in the pA2 values for 4-DAMP assessed by carbachol contractures, in normal and obstructed bladder strips, indicates that m3 muscarinic receptors still play a predominant role in causing detrusor contractility in the obstructed bladder, as in the normal bladder.