Ca²⁺ sensitization in contraction of human urinary bladder smooth muscle

The role of Ca²⁺ sensitization in the contraction of human urinary bladder smooth muscle (UBSM) was investigated. Simultaneous measurements of intracellular Ca²⁺ concentration ([Ca²⁺]_i) and tension in fura-2 loaded intact strips and receptor-coupled strips permeabilized with α-toxin were applied. The expressions of proteins were confirmed by western blot analysis. In intact fura-2 loaded strips, 1 μM carbachol (CCh) induced a greater contraction and a lower [Ca²⁺] _i elevation than that induced by 60 mM K⁺ depolarization. In α-toxin permeabilized strips, 1 μM CCh induced contraction at a constant [Ca²⁺]_i and produced a leftward shift in the [Ca²⁺]_i-tension relationship. RhoA, Rho-associated kinase (ROCK) I, ROCKII and CPI-17 proteins were expressed in human UBSM. In intact fura-2 loaded strips, application of 3 μM Y-27632 (a ROCK inhibitor) or 3 μM GF109203X (a protein kinase C (PKC) inhibitor) during the sustained phase of contraction induced by 1 μM CCh induced a relaxation without changing [Ca²⁺]_i. In α-toxin permeabilized strips, application of 3 μM Y-27632 or 3 μM GF109203X during the sustained contraction induced by 0.3 μM Ca²⁺ plus 10 M GTP and 1 μM CCh induced a relaxation at constant [Ca²⁺]_i. These results indicate that in human UBSM, CCh induces a contraction not only by increasing [Ca²⁺]_i but also by increasing the Ca²⁺ sensitivity of the contractile apparatus in a ROCK-and PKC-dependent manner. Antagonism of Ca²⁺ sensitization pathways may represent an alternative target in the treatment of overactive bladder.