The interactions of ouabain with post-tetanic and facilitatory drug potentiations at cat soleus neuromuscular junctions in vivo

Ca-dependent regulation of the Na-K-pump by post-tetanic sensitization of extrasynaptic cholinoreceptors in common snail neurons

Studies were conducted on the role of the Na,K pump and intracellular calcium in the previously observed phenomenon of increased cholinosensitivity of the somatic membranes of neurons LPa3 and RPa3 in the common snail, induced by electrical tetanic stimulation (10.5 mA. 0.1 sec, 2 impulses/sec; 2 min) of the afferent input (the intestinal nerve). Recordings were made of integral transmembrane acetylcholine-evoked currents by two-electrode membrane voltage clamping. After intracellular administration of the Na,K pump inhibitor ouabain (70 microM), there was no post-tetanic increase in neuron cholinosensitivity. Intracellular injection of the calcium ion chelator BAPTA suppressed the sensitization of somatic cholinoreceptors after orthodromal tetanization. It was concluded that the Na,K pump is involved in the mechanism of post-tetanic potentiation of cholinosensitivity in the bodies of defensive behavior command neurons in the common snail, this involvement being Ca-dependent.

Attenuation of motor nerve terminal repetitive discharge by the 21-aminosteroid tirilazad: evidence of a neural calcium antagonist action

Pretreatment with the 21-aminosteroid antioxidant compound tirilazad mesylate has been previously shown to retard the axotomy-induced anterograde degeneration of soleus motor nerve terminals in the cat. In the present study, we examined tirilazad's effects (7.7, 13.0 or 30.0 mg/kg twice daily P.O. for 6 days) on the excitability of normal cat soleus motor nerve terminals. Low frequency (0.4 Hz) neuromuscular transmission was measured as well as the occurrence of muscle contractile potentiation in response to either a 400 Hz/10 s episode of tetanic conditioning stimulation of the soleus nerve or the administration of a 200 microg/kg i.v. dose of the neuromuscular facilitatory drug edrophonium. The mechanism of the post-tetanic potentiation (PTP) or edrophonium-induced facilitatory response involves the occurrence of a stimulus-dependent repetitive discharge of the soleus motor nerve terminals due to an exaggeration of the nerve terminal Ca2+-mediated after-depolarization. Tirilazad pretreatment caused a dose-related suppression of PTP and the edrophonium response indicative of a suppression of motor nerve terminal repetitive discharge. These effects were not shared by 6 days of oral pretreatment of cats with a high dose combination of the antioxidants vitamin E (200 I.U./day) and selenium (50 microg/day). Thus, it is unlikely that the antioxidant properties of tirilazad are involved in the suppression of motor nerve terminal excitability. Rather, it is proposed that tirilazad suppresses delayed motor nerve terminal Ca2+ conductances secondary to its ability to decrease membrane phospholipid fluidity, and that this action might in some circumstances contribute to its neuroprotective activity.

The alpha3 isoform protein of the Na+, K(+)-ATPase is associated with the sites of cardiac and neuromuscular impulse transmission

The alpha (catalytic) subunit of the Na+ pump (Na+, K(+)-ATPase) has three isoforms; alpha1 is ubiquitous, skeletal muscle expresses predominantly alpha2, and alpha3 has been localized to specific types of neurons and, possibly, to axonal processes. The alpha3 isoform mRNA is also expressed in the rat cardiac conduction system. Thus, we studied rat heart and quadriceps muscles by immunohistochemistry using isoform-specific antibodies to the Na+ pump alpha subunit and labeled alpha-bungarotoxin as a probe for the neuromuscular junction (NMJ). We found that alpha3 pump protein is localized to three sites important for impulse transmission: the junctional complex between cardiac myocytes, the heart conduction system, and the NMJ. Specifically, all levels of the conduction system expressed alpha3 immunoreactive protein, as assessed by two isoform-specific antibodies and histological conduction system markers. Specific expression at the junctional complex was confirmed by immuno-EM. Double-labeling and denervation analysis indicated that alpha3-positive areas in skeletal muscle were presynaptic and adjacent to postsynaptic bungarotoxin-positive regions, which had the classic morphology of NMJs. Thus, specific Na+,K(+)-ATPase pump isoforms may be adapted to maintenance of membrane potential and/or intracellular ion concentrations required for impulse transmission in both heart and presynaptic motor terminals contacting skeletal muscle.