A STUDY OF STIMULATION‐EVOKED ACTIVATION OF α₂‐ADRENOCEPTORS IN THE RAT ISOLATED VAS DEFERENS

Abstract

Experiments were performed with prostatic and epididymal segments of rat vas deferens to determine whether alpha 2-adrenoceptors in this organ were activated during field stimulation of sympathetic terminals with short trains of pulses applied at low frequencies. In prostatic segments the magnitude of twitches evoked by trains of ten field pulses at 1 or 2 Hz declined after 2-3 s of stimulation. In contrast, facilitation of twitches and fusion of contractions occurred when similar stimulation was applied to epididymal segments. In prostatic segments from rats treated with reserpine (2.5 mg/kg i.p.) 24 h previously, there was no decline in the magnitude of twitches produced by successive impulses in a train of stimulating pulses. In epididymal segments from reserpine-treated rats facilitation of twitches in response to successive impulses in each train still occurred. In prostatic segments cocaine (5 and 10 mumol/l) enhanced twitch fade with stimulation at 1 and 2 Hz without altering the time for onset of this effect. In epididymal segments cocaine led to enhancement and prolongation of contractile responses. In prostatic segments yohimbine (0.01-0.06 mumol/l) reduced or reversed the effect of cocaine in enhancing twitch fade. In preparations where the reversal of the effect of cocaine by yohimbine was incomplete, subsequent addition of phentolamine (1 mumol/l) produced complete reversal. In epididymal segments yohimbine (0.01 mumol/l) produced a further enhancement in the twitch responses to stimulation at 1 and 2 Hz. Subsequent addition of phentolamine (1 mumol/l) reversed the facilitatory effects of cocaine and yohimbine. Propranolol (10 mumol/l) was without effect upon responses to stimulation in either segment of rat vasa deferentia. These experiments indicate that noradrenaline, released by short trains of impulses applied at low frequency to hypogastric nerve terminals activates prejunctional alpha 2-adrenoceptors in the prostatic segment of the vas deferens. In the epididymal portion the effects arising from activation of prejunctional alpha 2-adrenoceptors are outweighed by the consequences of activation of extrajunctional alpha 1-adrenoceptors located on longitudinally arranged muscle.

Variation in nitric oxide synthase-immunoreactive nerve fibres with age and along the length of the vas deferens in the rat

“…The abdominal end of the vas deferens contracts with greater sensitivity to exogenous ATP than does the scrotal end of the vas deferens (French and Scott 1983), while the reverse is true for exogenously applied noradrenaline (Kasuya and Suzuki 1979;French and Scott 1983). The abdominal portion of the vas deferens is also more sensitive to the prejunctional inhibitory effects of drugs acting at α 2 -adrenoceptors than is the scrotal portion of the vas deferens (Pennefather 1983), while field stimulation-induced contractions of the cauda epididymidis are not inhibited at all by α 2 -adrenoceptor agonists nor are they enhanced by α 2 -adrenoceptor antagonists Pennefather 1992, 1994). This regional variation appears to aid in the unidirectional transport of sperm.…”

Influence of Lowered Temperature Upon the Response of Prostatic and Epididymal Portion of the Rat Isolated Vas Deferens to Field Stimulation

“…One notable observation arising from this study is that there is a difference in the responsiveness of the two ends of the vas deferens to cooling. Regional variation in the response of this tissue to field stimulation and to noradrenaline has been suggested to be of functional significance (Pennefather et al 1975;Pennefather 1983). The twitch responses to field stimulation at both ends of the tissue were less affected by cooling.…”

Changes in Prejunctional Potency of B‐ht 933 During Aging in the Rat Vas Deferens

“…If such a change occurred in man it could explain, at least in part, the high plasma levels of noradrenaline found in the elderly (Ziegler et al 1976). The purpose of the present investigation was to investigate using the rat vas deferens as a model (Pennefather 1983;Docherty 1984). Prejunctional cw2-adrenoceptors in the preparation were stimulated with the selective a2-adrenoceptor agonist B-HT 933 (Ruffolo & Zeid 1985), the selectivity of the agonist being assessed with the selective az-adrenoceptor antagonist rauwolscine (Docherty et al 1981).…”