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

The temperature sensitivity of motor units in rat soleus

Temperature has a significant impact on the performance of the neuromuscular system and motor control processes. The smallest functional components of these systems are motor units (MUs), which may differ significantly between different muscles. The influence of temperature on the contractile properties of slow-twitch (S) MUs from soleus (SOL) muscles in rats was investigated under hypothermia (25 °C), normothermia (37 °C), and hyperthermia (41 °C). Hypothermia prolonged the twitch time parameters, decreased the rate of force development, increased the twitch-to-tetanus ratio, enhanced twitch force, and abolished post-tetanic depression. In contrast, hyperthermia did not alter twitch time parameters. Moreover, there was no effect on force despite the noted increase in post-tetanic depression and the twitch-to-tetanus ratio. Therefore, hypothermia induced more profound changes in S MUs compared with hyperthermia. The temperature effects in SOL MUs were compared to the effects previously reported for S MUs in the medial gastrocnemius (MG). The major differences between the S MUs of both muscles were the effects of temperature on twitch force, post-tetanic force modulation, twitch-to-tetanus ratio, and the slope of the force-frequency curve under hypothermia. Hyperthermia shortened twitch time parameters solely in the MG. In contrast, post-tetanic depression, twitch-to-tetanus ratio, and the slope of the force-frequency curve were influenced by hyperthermia only in SOL MUs. The different temperature effects of S MUs probably corresponded to differences in muscle architecture and their diverse functional tasks and enzyme activity. In summary, S MUs in SOL are more thermal-sensitive than their counterparts in MG.

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.