Ca²⁺ sensitization in contraction of human bladder smooth muscle

Purpose: The role of Ca²⁺ sensitization in the contraction of human bladder urinary smooth muscle (UBSM) was investigated. Materials and Methods: 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. Protein expressions was confirmed by Western blot analysis. Results: 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 constant [Ca²⁺]_i and produced a leftward shift in the [Ca²⁺]_i-tension relationship. RhoA, Rho-associated kinase (ROCK) I, ROCK II and CPI-17 proteins were expressed in human UBSM. In intact fura-2 loaded strips the application of 3 μM Y-27632, a ROCK inhibitor, or 3 μM bisindolylmaleimide I (GF109203X), a protein kinase C inhibitor, during the sustained phase of contraction induced by 1 μM CCh induced relaxation without changing [Ca²⁺] _i. In α-toxin permeabilized strips the application of 3 μM Y-27632 or 3 μM GF109203X during the sustained contraction induced by 0.3 μM Ca²⁺ plus 10 μM guanosine triphosphate and 1 μM CCh induced relaxation at constant [Ca²⁺]_i. Conclusions: These results indicate that in human UBSM CCh induces contraction, not only by increasing [Ca²⁺]_i, but also by increasing the Ca ²⁺ sensitivity of the contractile apparatus in a ROCK and protein kinase C dependent manner. Antagonism of Ca²⁺ sensitization pathways may represent an alternative target in the treatment of overactive bladder.