Regulation of Contractile Force

Activated myosin light chain kinase phosphorylates the 20-kDa light chain of myosin on serine 19. This phosphorylation is associated with an increase in the actin-activated myosin ATPase activity (Ikebe et al., 

RELATIVE IMPORTANCE OF THE ELECTROMECHANICAL AND PHARMACOMECHANICAL CONTRACTILE MECHANISMS 

We evaluated the relative importance of these electromechanical and pharmacomechanical mechanisms in the rat tail artery by measuring , [Ca +Jj, and force. Both 0.1 and 0.3 pM phenylephrine depolarized the smooth muscle, increased Fura 2 estimated [Ca +J, and induced a contraction (Fig. 2). This increase in [Ca2+]j was partially induced by depolarization increasing Ca + influx through L-type Ca + channels, because the Ca + channel blocker diltiazem decreased both Fura 2 estimated [Ca +l and force to near resting 

FIG U R E 2 Comparison of E , Fura 2 fluorescence, and isometric force in de-endothelialized rat tail artery segments stimulated with phenylephrine or high (in the presence of 3 pM phentolamine to block the effects of endogenous norepinephrine release). Tissues were stimulated for 5 min with the following stimuli 0.1 pM phenylephrine at 5 min, 0.3 pM phenylephrine at 15 min, 20 pM [K+] at 25 min, 30 mM [K+] at 35 min, and40mM [K+] at45min. The top panel shows mean 1 SEM (symbols without error bars imply that the SEM is smaller than the symbol size). The middle panel shows the normalized mean Fura 340/380 ratio (solid line) 1 SEM (dotted lines). The bottom panel is normalized force in grams from the Fura 2 experiments (solid and dotted lines). Force from the experiments was similar to the force measured in the Fura 2 experiments. The Fura 340/380 ratio was normalized to the measured response 5 min after stimulation with 90 mM [K+] . Data are replotted from Chen and Rembold (1995). 

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Regulation of contractile force by changes in is called "electromechanical coupling." This type of regulation primarily involves changes in [Ca +Jj. 

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