Influence of the length of the stimulus period and frequency of sympathetic stimulation on the response of the guinea-pig isolated vas deferens to bretylium, guanethidine and amphetamine

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Abstract
The height of contraction of guinea-pig isolated vas deferens preparations in response to pre- or postganglionic sympathetic nerve stimulation at various stimulus frequencies was shown to vary with varying length of stimulation period. Short stimulation periods, as used by some workers, produced suboptimal contractions especially at the lower frequencies. It is suggested that variations in the length of the stimulation period could account for some of the contradictory results reported by various workers using this preparation. Thus the effect of low concentrations of dexamphetamine was qualitatively changed as the length of the stimulation period was increased; with short stimulation periods dexamphetamine impaired the responses to sympathetic stimulation but enhanced them if stimulation was continued until contractions had reached the maximum height obtainable. Higher concentrations of dexamphetamine impaired the contractions, the effect being most marked against the higher stimulus frequencies. Using stimulation to maximal effect a qualitative difference in the blocking actions of bretylium and guanethidine was demonstrated. Bretylium reduced the responses to the higher stimulus frequencies to a greater extent than it reduced the responses to the lower frequencies. Guanethidine reduced the responses to all frequencies so that there was a parallel shift of the frequency/response curve to the right. The possible significance of these findings is discussed.