Intracellular calcium in hypertrophic smooth muscle from rat urinary bladder

To explore whether infravesical outlet obstruction is associated with alterations in calcium activation of detrusor smooth muscle.

Outlet obstruction was created by partial ligature of the urethra in female rats. Western blotting was performed using an antibody against the cytoplasmatic region of the alpha1c subunit of the L-type Ca2+ channel. Intracellular calcium was measured using Fura-2 in detrusors that had been obstructed for 10 days and activated by high K+ concentrations at different extracellular Ca2+ concentrations. The rate of force development after rapid opening of L-type Ca2+ channels was measured in contractions initiated by flash photolysis of nifedipine in Ca2(+)-containing depolarizing solution.

Bladder weight increased from 62 +/- 3 to 254 +/- 43 mg after 10 days of obstruction. Expression of the alpha1c subunit increased after 3 days and continued to increase until it was about fourfold greater after 10 days; however, it had not increased further at 6 weeks. This change was reversible after removal of obstruction. Activation with K+ produced a stable force at different extracellular Ca2+ concentrations, with no difference in response between controls and rats that had been obstructed for 10 days. Intracellular Ca2+ concentrations were lower in the obstructed group, showing that the calcium sensitivity of the contraction force had increased. The delay between the opening of L-type channels and the onset of contraction was longer in obstructed detrusors.

Growth of detrusor muscle following obstruction is accompanied by attenuated calcium transients following activation, despite upregulation of L-type Ca2+ channels. The Ca2+ sensitivity of contraction was increased in obstructed detrusors. We suggest that the decreased surface: volume ratio in hypertrophic smooth muscle cells is partly involved in the lowered Ca2+ transients. The increases in L-type calcium channels and in calcium sensitivity may be compensatory mechanisms.