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

Responses to isotonic quick releases of 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 th K+-contractures. Releases were performed at peak of the contractions (which was attained after 1.5-4.5 s AC-stimulation or 2-3 min in K+-high solution). The length response consisted of 3 parts: (1) elastic recoil, (2) isotonic transient, (3) steady shortening. Shortening velocity was determined at 100 ms after the release, as phase (2) had subsided by then. Characteristics of the initial elastic response was virtually unaffected by the mode of stimulation for both preparations. Vmax was significantly higher for the AC-stimulations than for the K+-contractures (bladder: 0.37 l/s vs. 0.26 l/s, portal vein: 0.46 l/s vs. 0.33 l/s). By means of computer analysis a fast exponential shortening component comprising the larger part of phase (2) could be separated from the subsequent slower shortening (see further in Hellstrand & Johansson 1979). In each preparation the amplitude of this exponential was the same for both modes of activation. The time constant was, however, smaller for the AC-stimulation preparations. Our results thus indicate that in response to a sudden decrease in force the initial elastic recoil is the same whereas the rate of transition to steady shortening, and the steady shortening velocity itself, are lower for preparations activated by K+-high medium compared to AC-stimulation.