Effect of Quin-2 on 45Ca2+ uptake mediated by Na+i/Ca2+o exchange and 45Ca2+ efflux in rat brain synaptosomes: a requirement for [Ca2+]i

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Chemical hypoxia increases cytosolic Ca2+ and oxygen free radical formation

'Chemical hypoxia' was produced in isolated rat hepatocytes. The cells were immobilized in agarose gel threads and perfused with Krebs-Henseleit bicarbonate buffer equilibrated with 95% O2 + 5% CO2 or 95% air + 5% CO2. During 'chemical hypoxia', 2 mM KCN + 0.5 mM iodoacetate (CN-IAA) were added to the perfusate for 30 min. Cytosolic ionized Ca2+ (Cai2+) was measured with aequorin, the formation of oxygen free radicals by lucigenin-enhanced chemiluminescence and cell injury by the rate of LDH released by the cells in the effluent perfusate. As soon as the cells were exposed to CN-IAA in the presence of 95% O2 + 5% CO2, Cai2+ increased rapidly to reach 1.5 microM within 10 min, and oxygen free radical formation increased 5-fold. The increase in LDH release was temporally delayed and occurred only during the recovery phase. The results were not significantly different when the cells were perfused with KHB equilibrated with 95% air + 5% CO2, except that oxygen free radical formation increased 13-fold. These results suggest that both a rise in Cai2+ and a formation of reactive oxygen species could be responsible for the cell injury and the cell death induced by CN-IAA poisoning.

Sodium inhibits hormone release and stimulates calcium efflux from isolated nerve endings of the rat neurohypophysis

1. We studied the effects of extracellular sodium on the secretion of vasopressin (VP) and oxytocin (OT) and the efflux of 45Ca from isolated, perfused nerve endings of the rat neurohypophysis (neurosecretosomes). 2. Upon removal of sodium from the perfusing medium, basal release of VP and OT increased by 3.95 +/- 0.23- and 3.71 +/- 0.22-fold, respectively, followed by a decline to about double the levels in normal (150 mM) sodium (P less than or equal to 0.1). 3. Compared to neurosecretosomes perfused in normal (150 mM) sodium, omission of sodium from the medium augmented ionomycin-induced VP and OT secretion by 66 +/- 5- and 20 +/- 3-fold, respectively, and A23187-induced secretion was increased 1.3 +/- 0.4- and 1.3 +/- 0.1-fold (P less than or equal to 0.01 for both ionophores). 4. The inhibition of ionomycin-induced secretion by sodium was concentration dependent (P less than or equal to 0.01 for sodium greater than or equal to 5 mM); the IC50 was about 10 mM sodium for both hormones, and the Hill slope was close to -1. 5. The rate of 45Ca efflux from neurosecretosomes showed 2.7 +/- 0.1-fold stimulation upon increasing sodium from 4.5 to 150 mM (P less than or equal to 0.01). 6. Our results suggest that sodium inhibits basal and stimulated secretion at the nerve terminal, possibly by reducing intraterminal calcium through sodium/calcium exchange.