The effect of length on the sensitivity to phenylephrine and calcium in intact and skinned vascular smooth muscle

The length dependency of the sensitivity to activators of the smooth muscle of different blood vessels is not yet fully understood. Muscle preparations of the aorta, the femoral artery and the portal vein of the rabbit were investigated for the length dependency of the sensitivity to phenylephrine and calcium in both intact and triton X- 100 skinned preparations. For intact smooth muscles we found that at increased preparation length, the sensitivity of contraction was increased. The femoral artery showed the largest effect and the portal vein the smallest. In the skinned preparations of the three preparations the calcium sensitivity was not dependent on the preparation length. We conclude that the changes of the sensitivity in intact preparations are not caused by changes of the calcium sensitivity of the contractile proteins.     

Tension responses of frog sartorius muscle to quick ramp-shaped shortenings and some effects of metabolic inhibition

The dynamic properties of the contractile mechanism of skeletal muscle were investigated in electrically stimulated sartorius muscle of the frog at 0°C by analysis of the force responses to quick changes in length.

1. The tension responses to ramp-shaped shortenings at different rates of shortening have been recorded. The tension course during the ramp is described in terms of a damped elastic element. In addition a maximum value for an additional series elastic element is estimated.
2. The tension responses to pulse-shaped shortenings have been recorded. At least 15ms after the pulse a minimum in tension is found, which depends on amplitude and duration of the pulse. This effect is optimal at a pulse duration of about 6ms. It is concluded that within 15ms after the pulse detachment of crossbridges occurs.
3. Muscles were incubated with the metabolic inhibitors IAA and FAA or IAA and FDNB. After repeated stimulation, but before the muscle passes into the rigor state, the fast recovery and the plateau in the responses to quick shortenings are either diminished or abolished, depending on the amplitude of the shortening. This can be explained by assuming that in the unpoisened muscle, ATP enables the crossbridges to exert temporarily a larger force.