Post-tetanic potentiation of the vascular neurogenic response in rats

We have previously shown that, at least at frequencies of 4 Hz and above, a previous stimulation of the intramural vasoconstrictor nerves facilitates the subsequent response so that even maximal contractions can develop during periods of stimulation as short as a few seconds. In the present study the facilitating effect of a continuous nerve stimulation was quantitatively tested on 'single-twitch' responses to individual nerve impulses. Small arteries and veins (150-500 micron diameter) from rats were mounted in a myograph at known wall tension, and the intramural nerves were activated at frequencies between 0.1 and 32 Hz. After a period of continuous stimulation the single-twitch responses could be amplified 10-20 times, depending on frequency and duration of the continuous stimulation. No such amplification was observed after application of exogenous noradrenaline. The results therefore reflect a prejunctional frequency-dependent potentiating mechanism which can strongly reinforce sympathetic neuroeffector control. Furthermore, combined with previous data concerning adrenergic transmitter release, the present results seem to be more compatible with the view that the quantum of transmitter released from each varicosity upon activation is less than an entire granule.