Partial bladder outlet obstruction alters Ca²⁺sensitivity of force, but not of MLC phosphorylation, in bladder smooth muscle

Abstract

Partial bladder outlet obstruction in the rabbit produces changes in bladder function similar to those seen clinically in patients with obstructive uropathies. Whole organ function is significantly altered, as are the smooth muscle cells inside the bladder wall. This study was designed to determine whether outlet obstruction alters smooth muscle function at the level of contractile filaments. Rabbit bladders were partially obstructed for 2 wk. Triton X-100 was used to provide a detergent-skinned bladder smooth muscle preparation that would allow control of the intracellular environment while the ability to shorten and develop force is maintained. Ca2+-force and Ca2+-myosin light chain (MLC) phosphorylation relations and maximal velocity of shortening were determined. The Ca2+ sensitivity of force was significantly lower in tissues from animals subjected to outlet obstruction compared with tissues from control animals. In contrast, no difference was noted in the Ca2+ sensitivity of MLC phosphorylation. Maximal levels of stress and MLC phosphorylation were similar in both animal groups. Maximal velocity of shortening was significantly slower in tissues from outlet-obstructed animals at all Ca2+ concentrations compared with tissues from control animals. Ultrastructurally, detergent skinning had little effect on structural integrity. Moreover, tissues from obstructed animals showed an increase in the number of sarcolemmal attachment plaque structures. We suggest that partial bladder outlet obstruction produces deleterious (e.g., decrease in Ca2+ sensitivity of force) and compensatory (e.g., increase in membrane attachment plaques) changes in bladder smooth muscle cells.

Alterations in the contractile phenotype of the bladder: lessons for understanding physiological and pathological remodelling of smooth muscle

“…Stanton et al using a skinned fibre preparation also demonstrated that there was no difference in peak force generated between the smooth muscle from control or pBOO groups [59]. In this preparation, there are also no differences in the degree of myosin light chain phosphorylation across the range of Ca 2+ concentrations.…”

Carbachol-induced rabbit bladder smooth muscle contraction: roles of protein kinase C and Rho kinase

“…However, it is unclear how these two pathways work to regulate the MLC phosphatase in agonist-stimulated bladder smooth muscle contraction. Recently, our laboratory reported an abolishment of receptor-G protein-dependent Ca 2ϩ sensitization, alterations in the MLC phosphorylation-force relationship, and loss of phorbol ester-induced contractions in a pathophysiological state of rabbit bladder smooth muscle (39,40,41). Unfortunately, the precise mechanisms underlying these important aspects of bladder smooth muscle contraction in a normal physiological state are not well understood.…”

Deletion of SM-B, the high ATPase isoform of myosin, upregulates the PKC-mediated signal transduction pathway in murine urinary bladder smooth muscle

“…Concomitant with a decrease in SM-B (the high ATPase isoform) and an increase in SM-A (low ATPase isoform), there is a decrease in maximum shortening velocity and the hypertrophied detrusor smooth muscle (DSM) reveals contractile characteristics typical of tonic smooth muscle compared with the phasic contraction shown by normal DSM (46). In addition, PBOO-induced DSM hypertrophy also shows an upregulation of Rho-kinase (5) which is implicated in calcium sensitization of muscle contraction and an increase in the resting myosin light chain (MLC 20 ) phosphorylation level (8,10,44). …”