Increased retention of norepinephrine-3H in vas deferens during nerve stimulation

Suppression of sympathetic nervous system during fasting

Two days of fasting in rats significantly reduces the turnover of norepinephrine in the heart. In contrast to the effects of ganglionic blockade in fed controls, similar blockade in fasted animals is without significant effect on [3H]-norepinephrine retention or endogenous norepinephrine in the heart. These data are consistent with suppression of centrally mediated sympathetic activity in the fasted state. The decrease in norepinephrine turnover during fasting is completely reversed by 1 day of refeeding.

Adrenergic modulation of 'non-adrenergic' twitches in the rat vas deferens

1. Field stimulation of the intramural sympathetic nerves of rat vas deferens produced a biphasic contraction. Post-tetanic twitches were potentiated (PTP) and the relationship of this to the secondary component of tetanic responses was investigated. 2. A tetanus evoked by field stimulation at 4 Hz for 50 s gave a maximum increase in twitch tension of 109 +/- 5% (n = 43). The potentiation was blocked by the alpha-adrenoreceptor antagonists phenoxybenzamine, prazosin and thymoxamine. These drugs also blocked the secondary component of the tetanic response. 3. Increasing the frequency (1-8 Hz) of tetanic field stimulation enhanced the magnitude and time course of PTP. The time course of the PTP was found to depend mainly on the number of pulses in the tetanic train. 4. Reducing transmitter release in high Mg2+ solutions shortened the duration of the PTP but prevented the decline of the secondary tetanic component. Co2+ (1 mM) or Mn2+ (1 mM) mimicked the effect of high Mg2+ ions on the secondary tetanic component. Enhanced transmitter release in zero Mg2+-Krebs had the reverse effects. 5. The presynaptic alpha 2-adrenoreceptor antagonists idazoxan and yohimbine, and the neuronal uptake blockers desipramine and metaraminol, prolonged the PTP and enhanced but shortened the secondary rise in tetanic tension. 6. Bolus injections of noradrenaline before and after the tetanus gave no evidence for post-synaptic desensitization. 7. It is concluded that both the secondary rise in tetanic tension and the post-tetanic potentiation are due to an adrenergic enhancement of the 'non-adrenergic' mechanism and not, as has been supposed, to direct activation of an adrenergic tension. Thus the action of the alpha-adrenoreceptor-mediated transmission is primarily one of modulation.

A study of the post-tetanic twitch depression at the prostatic end of the rat vas deferens

The effects of different drugs on the post-tetanic twitch responses of the prostatic end of the isolation rat vas deferens were examined. Following repetitive field stimulation (4-5 Hz for 40-60 s), the post-tetanic twitches (0.2-0.5 Hz) were depressed relative to the steady-state pretetanic twitches. Maximum post-tetanic twitch depression (PTD) occurred 15-30 s after the tetanic stimulation. The PTD was not reversed but deepened and prolonged by the presynaptic alpha 2-adrenoceptor antagonists idazoxan (RX781094) or yohimbine (10(-7) M) alone or combined with the neuronal uptake blocker, desipramine (10(-6) M). The PTD was insensitive to the beta-adrenoceptor blocker, propranolol (10(-6) M), the P1-purinoceptor antagonist, 8-phenyltheophylline (10(-5) M) or the prostaglandin E synthesis inhibitor, indomethacin (10(-6) M). Pre- and post-tetanic contractions to superfused noradrenaline (NA), adenosine 5'-triphosphate (ATP), or beta, gamma-methylene ATP (beta, gamma-mATP) (bolus injections into superfusate, 1-8 X 10(-5) M) were not significantly (p less than 0.01) different. The PTD was absent in high magnesium (6.2-11.2 mM) solutions. These results are consistent with the PTD having an intrinsic presynaptic origin. They are discussed in relation to stimulation-induced modulation of transmitter release.

Suppression of sympathetic nervous system during fasting

Two days of fasting in rats significantly reduces the turnover of norepinephrine in the heart. In contrast to the effects of ganglionic blockade in fed controls, similar blockade in fasted animals is without significant effect on [3H]-norepinephrine retention or endogenous norepinephrine in the heart. These data are consistent with suppression of centrally mediated sympathetic activity in the fasted state. The decrease in norepinephrine turnover during fasting is completely reversed by 1 day of refeeding.

Effects of the inhibition of noradrenaline uptake and synthesis on the maintenance of the response to continuous nerve stimulation in the central artery of the rabbit ear

1. The central artery of the rabbit ear was perfused through its lumen in vitro, with a constant pressure technique, and stimulated continuously via its periarterial sympathetic nerves at the physiological frequency of 5 Hz.2. The vasoconstrictor response, which led initially to an almost complete cessation of intraluminal flow, deteriorated steadily over a period of hours. The involvement of presynaptic mechanisms in this effect was indicated by the finding that noradrenaline, administered extraluminally, produced a similar response before the onset of continuous stimulation and at a late stage when the constriction had decreased markedly. In addition, the noradrenaline precursor DOPA, restored the depressed responses towards their original values, indicating that failure involved depletion of mediator for release.3. Responses to continuous stimulation declined significantly faster after inhibition of tyrosine hydroxylase with alpha-methyl-p-tyrosine. However, inhibition of the uptake of noradrenaline with cocaine did not enhance the decline of the response, even when the sensitization produced by the compound was taken into account.4. It is concluded that synthesis, along with the mobilization of stored mediator, rather than uptake and re-use of noradrenaline maintain the effector response in the central artery of the rabbit ear stimulated continuously at a frequency within the physiological range.

Evaluation of mechanisms controlling the release and inactivation of the adrenergic transmitter in the rabbit portal vein and vas deferens

1. A method is described for the detection and assay of picogramme quantities of noradrenaline. This involves transferring Krebs solution containing noradrenaline to a cascade system where the catecholamine may be bioassayed on superfused preparations of the rabbit aorta and iliac artery.2. Electrical field stimulation of the rabbit vas deferens and portal vein caused the release, into the bathing medium, of a material which was identified by pharmacological and chemical tests as noradrenaline.3. Cocaine (0.3-5 mug/ml) caused a marked increase in noradrenaline output after electrical stimulation of the portal vein and vas deferens. This effect appeared to be maximal at a concentration of 2.4 mug/ml; when the cocaine concentration was increased above 10 mug/ml the noradrenaline output was greatly reduced.4. Phenoxybenzamine (5 mug/ml) caused a 4-8 times greater increase in noradrenaline output than cocaine; however, the increase in output due to phenoxybenzamine was much smaller in tissues pretreated with cocaine.5. Corticosterone (20 mug/ml) increased noradrenaline output by 30-40% in untreated vas deferentia, but caused a 300% increase in output in tissues pretreated with cocaine. Cocaine also caused a much greater increase in output in tissues pretreated with corticosterone than in untreated tissues.6. Treatment with pargyline plus tropolone caused a 100-200% increase in noradrenaline output; this effect was not modified by cocaine, but was abolished when the tissues were pretreated with either phenoxybenzamine or corticosterone.7. When tissues were stimulated for 240 pulses at 1-16 Hz, the output per pulse of noradrenaline increased linearly with the logarithm of the frequency of stimulation. This relationship between frequency and output was seen in both untreated tissues, and in tissues treated with cocaine, phenoxybenzamine, corticosterone or pargyline plus tropolone.8. It is concluded that cocaine enhances output by blocking the neuronal reuptake of noradrenaline, and corticosterone by blocking the extraneuronal uptake and subsequent metabolism of noradrenaline. Phenoxybenzamine acts by blocking both neuronal and extraneuronal uptake mechanisms. There appears to be a dynamic balance in the distribution of noradrenaline between the two uptake mechanisms after the release of the transmission from the nerve endings.9. It is calculated that more than 90% of the noradrenaline released by nerve stimulation (240 pulses at 2-16 Hz) is inactivated by neuronal and extra neuronal uptake mechanisms.10. It is calculated that the fraction of the total noradrenaline store that is released by one pulse at 2 Hz is 6.6 x 10(-5) in the portal vein and 5.6 x 10(-5) in the vas deferens; the corresponding values at 16 Hz were 15.9 x 10(-5) and 16.2 x 10(-5).