Myoplasmic [$Ca^{2+}$], Crossbridge Phosphorylation and Latch in Rabbit Bladder Smooth Muscle

Tonic smooth muscle exhibit the latch phenomenon: high force at low myosin regulatory light chains (MRLC) phosphorylation, shortening velocity (Vo), and energy consumption. However, the kinetics of MRLC phosphorylation and cellular activation in phasic smooth muscle are unknown. The present study was to determine whether $Ca^{2+}$-stimulated MRLC phosphorylation could suffice to explain the agonist- or high $K^+$-induced contraction in a fast, phasic smooth muscle. We measured myoplasmic [$Ca^{2+}$], MRLC phosphorylation, half-time after step-shortening (a measure of Vo) and contractile stress in rabbit urinary bladder strips. High $K^+$-induced contractions were phasic at both $22^{\circ}C$ and $37^{\circ}C$: myoplasmic [$Ca^{2+}$], MRLC phosphorylation, 1/half-time, and contractile stress increased transiently and then all decreased to intermediate values. Carbachol (CCh)-induced contractions exhibited latch at $37^{\circ}C$: stress was maintained at high levels despite decreasing myoplasmic [$Ca^{2+}$], MRLC phosphorylation, and 1/half-time. At $22^{\circ}C$ CCh induced sustained elevations in all parameters. 1/half-time depended on both myoplasmic [$Ca^{2+}$] and MRLC phosphorylation. The steady-state dependence of stress on MRLC phosphorylation was very steep at $37^{\circ}C$ in the CCh- or $K^+$-depolarized tissue and reduced temperature flattend the dependence of stress on MRLC phosphorylation compared to $37^{\circ}C$. These data suggest that phasic smooth muscle also exhibits latch behavior and latch is less prominent at lower temperature.