Rigor and resistance to stretch in vertebrate smooth muscle

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Abstract
Depletion of the APT content of the rabbit taenia coli muscle to 0.07 mumol/g caused an increase in force equal to 11% of maximum active tension (Po) when the muscle was bathed in a calcium-containing Krebs solution at 21 degrees C, but no increase in force in a calcium-free solution. Stimulation of the muscle in calcium-containing medium during the loss of ATP increased the force maintained and permitted demonstration of an increased stiffness by shortening of the muscle. There was no increase in resistance to stretch in the ATP-depleted state compared with the resting state. There was, however, a calcium-dependent resistance to stretch with properties that suggested the presence of attached crossbridges in the resting state. Bathing of the resting muscle in a calcium-free medium decreased the resting resistance to stretch and permitted demonstration of an increased resistance to stretch in the rigor state. It is concluded that the rigor state exists in vertebrate smooth muscle.