Changes in intracellular calcium concentration and P2X(1) receptor expression in hypertrophic rat urinary bladder smooth muscle

Aims: Contractile responses to purinergic activation in the urinary bladder are altered in outflow obstruction (O). We determined if the lowered contractile response to adenosine triphosphate (ATP) in obstructed rat urinary bladder was due to changes in calcium handling or in P2X(1) purinoceptor density. Materials and Methods: O was created in rat by partial ligature of the urethra, with non-obstructed rats as controls (C). Force and intracellular calcium were measured in bladder strips activated with ATP. Tissue was sectioned for light and electron microscopy and analyzed with Western blot using a P2X(1) antibody. Results: Bladder weight increased from 66 +/- 3 (C) to 206 +/- 17 mg (O) (n = 6). ATP gave a transient contractile response which was decreased in the obstructed strips (C: 161 +/- 20; O: 63 +/- 16% of high-K+ force). Intracellular calcium concentration after ATP activation in the obstructed bladder muscle was about 50% of that in the control preparations (C: 669 +/- 110; O: 335 +/- 59 nM). Half-time for calcium influx was increased in the O group. P2X(1) immunoreactivity per unit bladder weight was similar in the two groups. Cell membrane area per unit wet weight was decreased in the O group. Conclusions: Attenuated contractile responses to ATP in obstructed rat urinary bladder are due to a lowered rate of calcium influx and maximal peak calcium concentration. This change in Ca2+ transients is not due to a decrease in P2X1 receptor density in the smooth muscle cell membranes. Possibly, the increase in cell volume buffers the rapid and transient influx of Ca2+ following purinoceptor activation in the obstructed bladder. (C) 2004 Wiley-Liss, Inc.