Reading about smooth muscle cells, I stumbled upon this sentence in Guyton & Hall, Textbook of Medical Physiology (14th ed):
When the myosin kinase and myosin phosphatase enzymes are both strongly activated, the cycling frequency of the myosin heads and the velocity of contraction are great. Then, as activation of the enzymes decreases, the cycling frequency decreases but, at the same time, the deactivation of these enzymes allows the myosin heads to remain attached to the actin filament for a longer and longer proportion of the cycling period.
I'm having trouble reconciling this with myosin phosphatase's role. Elsewhere in the book it is stated that myosin phosphatase plays a role in smooth muscle contraction cessation, and that phosphorylated myosin (i.e. the product of myosin kinase) can, by itself, repeatedly attach to and detach from the thin filament and perform the cross-bridge cycles. As in, detachment of the myosin head from the actin filament does NOT require its dephosphorylation (via myosin phosphatase); and rather, the dephosphorylation only occurs when the cycling should stop altogether so that muscle relaxation occurs.
If this is true, then why would myosin phosphatase need to be 'strongly activated' (as per the quote above) so that the cycling frequency and velocity of contraction increases? Would that not actually reduce the cycling frequency? It seems to me that myosin kinase should be strongly activated and myosin phosphatase be strongly inhibited if the cycling frequency is to increase, but I'm probably misunderstanding something.
How does myosin phosphatase activation (or as the quoted part says, strong activation of both myosin kinase and myosin phosphatase) actually increase the cycling frequency?