The ability of collecting veins to sustain sympathetic constriction

The collecting femoral vein is capable of sustaining contraction even for 15 minutes provided sympathetic stimulation frequency is low or moderate. At a frequency of 15 Hz, however, the collecting vein gradually relaxes after having reached a maximum contraction in spite of continuing stimulation. The ability of the relaxing venous smooth muscle to develop further contraction to exogenous noradrenaline has been proved. It seems probable that the relaxation is due to the decline in the amount of transmitter released from nerve terminals as a result of prolonged high rate stimulation.