Vasomotor nerve control of isolated arteries and veins

In order to compare neuro-effector function in different blood vessels, frequency-response relationships were determined for the following preparations: 1)Isolated rings of the proximal saphenous, distal saphenous and ear arteries, the parietal branch of the internal iliac vein and the small saphenous vein from the rabbit, 2) spiral strips of the rabbit pulmonary artery and 3) longitudinal preparations of the rat portal vein. In each rabbit tissue only one low (less than or equal to 4 Hz) and one high (larger than or equal to 8 Hz) transmural nerve stimulation frequency was applied until steady state responses were obtained and these were expressed as a percentage of a maximum response to exogenous noradrenaline (NA) applied in each experiment. The general shape of the frequency-response curves was similar, but differences in steepness and amplitude of the maximum neurogenic response relative to exogenous NA were found. The steepness of the frequency-response relationships of the veins tended to be greater than those of the arteries. It appears that factors such as close neuro-muscular contacts, presence of terminal nerve fibres within the media and the operation of mechanisms for myogenic propagation of activity contribute to the effectiveness of neurogenic vascular control as revealed by frequency-response curves. In vivo, geometrical factors can greatly augment the hemodynamic significance of the observed differences.