Effects of phasic and tonic activation on contraction dynamics in smooth muscle

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Abstract
Responses to isotonic quick releases by rabbit urinary bladder strips and rat portal veins activated by AC stimulation and K+ high medium were studied. The AC stimulation was adjusted to give the same tension as the K+ contractures. Releases were performed at the peak of the contractions, which was attained after 1.5-4.5 s AC stimulation or 2-3 min in K+ high solution. The response length consisted of 3 parts: elastic recoil, isotonic transient and steady shortening. Shortening velocity was determined at 100 ms after the release, and subsided after the 2nd phase. Characteristics of the initial elastic response were unaffected by the mode of stimulation for both preparations. Vmax was significantly higher for the AC stimulations than for the K+ contractures. Computer analysis separated a fast exponential shortening component comprising the larger part of the 2nd phase from the subsequent slower shortening. The amplitude of this exponential was the same for both modes of activation. The time constant was smaller for the AC stimulated preparations. In response to a sudden decrease in force the initial elastic recoil apparently is the same. The rate of transition to steady shortening and the steady shortening velocity are lower for preparations activated by K+ high medium compared to AC stimulation.