Facilitation of acetylcholine release from cardiac parasympathetic nerve endings. Effect of stimulation pattern and Mn ions

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Effects of micromolar concentrations of Mn, Mo, and Si on alpha1-adrenoceptor-mediated contraction in porcine coronary artery

We studied the effects of trace elements, Mn, Mo, and Si, on vasoconstriction induced by norepinephrine (NE) or electrical field stimulation in isolated porcine right coronary arteries. Alpha1-adrenoceptor (AR) antagonist prazosin dose-despondently suppressed vasoconstriction in response to NE or field stimulation indicating an alpha1-AR mediated response. Mn, Mo, and Si at 0.3-3 micromol/L dose-despondently inhibited NE mediated contraction (all p < 0.05). In contrast, Mn, Mo, and Si at the same concentrations (0.3-3 micromol/L) enhanced the maximal contractile response to field stimulation in a dose-dependent manner (all p < 0.05), but these elements at 10 micromol/L suppressed the vasoconstrictive response. The results indicate that in porcine right coronary arteries, the alpha1-AR-mediated vasoconstriction by NE or electrical field stimulation was affected differently by micromolar concentrations of Mn, Mo, and Si and that the elements might facilitate NE release presynaptically but inhibit the contractile response postsynaptically.

Dependence of release of [3H]noradrenaline from rabbit pulmonary artery on internal sodium

1. [3H]Noradrenaline ([3H]NA) release from the isolated main pulmonary artery of the rabbit has been measured in the presence of uptake blockers (cocaine, 3 x 10(-5) M, and corticosterone, 5 x 10(-5) M) and after blocking the monoamine oxidase enzyme by pargyline (1.2 x 10(-4) M). 2. In normal Krebs solution Mn2+ (2 mM) significantly inhibited both [3H]NA release (approximately 80%; P < 0.001) and the contraction following 2 Hz field stimulation. 3. In Ca(2+)-free, EGTA (1 mM)-containing solution, the Na+ pump was inhibited by removal of K+ from the external medium. In Na+ pump-inhibited arteries, 2 mM Mn2+ (free Mn2+, 1 mM) increased the spontaneous release of [3H]NA according to the time of Na+ loading. TTX (10(-7) M) did not inhibit significantly the Mn(2+)-induced [3H]NA release from Na(+)-loaded preparations (percentage inhibition, approximately 24; P > 0.30). 4. Without Na+ loading (Ca2+ free, EGTA alone), Mn2+ failed to promote 3H release from arteries. 5. With constant Na+ loading (120 min 'K(+)-free' perfusion in Ca(2+)-free, 1 mM EGTA-containing solution), the release of 3H was also directly dependent on free Mn2+ concentration (0.2, 0.6 and 1 mM). 6. The Mn2+ (2 mM; free Mn2+, 1 mM)-induced 3H release from Na(+)-loaded nerves (120 min 'K(+)-free', perfusion) was further enhanced, when external Na+ was simultaneously reduced from 139.2 to 26.2 mM (choline+ or sucrose substitution). 7. Diphenylhydantoin (DPH, 10(-4) M) significantly reduced the Mn(2+)-evoked 3H release (approximately 44%; P < 0.02) when it was present during 'K(+)-free', perfusion. 8. Mn2+ was ineffective in releasing 3H if the Na+ pump was previously reactivated by readmission of K+ to Na(+)-loaded arteries. 9. It is concluded that in Ca(2+)-free solution Mn2+ releases neurotransmitter in a manner which depends on the degree of loading with internal Na+. The results suggest this depends at least partly on a block of Ca2+ efflux.

Strontium, barium, and manganese metabolism in isolated presynaptic nerve terminals

To gain insight into the mechanisms by which the divalent cations Sr, Ba, and Mn affect neurotransmitter release from presynaptic nerve terminals, we examined the sequestration of these cations, in comparison to Ca, by mitochondrial and nonmitochondrial [presumably smooth endoplasmic reticulum (SER)] organelles and the extrusion of these cations from isolated nerve terminals. Sequestration was studied in synaptosomes made leaky to small ions by treatment with saponin; efflux was examined in intact synaptosomes that were preloaded with the divalent cations by incubation in depolarizing (K rich) media. The selectivity sequence for ATP-dependent mitochondrial uptake that we observed was Mn much greater than Ca greater than Sr much greater than Ba, whereas that for the SER was Ca greater than or equal to Mn greater than Sr much greater than Ba. When synaptosomes that were preloaded with divalent cations were incubated in Na- and Ca-free media, there was little efflux of Ca, Ba, Sr, or Mn. When the incubation was carried out in media containing Na without Ca, there was substantial stimulation of Ca and Sr efflux, but only slight stimulation of Ba or Mn efflux. In Na-free media, the addition of 1 mM Ca promoted the efflux of all four divalent cations, probably via Ca-divalent cation exchange. In summary, the sequestration and extrusion data suggest that, with equal loads, Mn will be buffered to the greatest extent, whereas Ba will be least well buffered. These results may help to explain why Mn has a very long-lasting effect on transmitter release, while the effect of Sr is much briefer.

Manganese fluxes and manganese-dependent neurotransmitter release in presynaptic nerve endings isolated from rat brain

The uptake and efflux of 54Mn and 45Ca, and the release of dopamine (DA) were measured in pinched-off presynaptic nerve endings (synaptosomes) isolated from rat brain. The uptake of Mn and Ca was increased when forebrain or striatal synaptosomes were incubated in a depolarizing, K-rich solution. The time courses of K-stimulated Mn and Ca entry were similar: there was initially a high rate of ion accumulation, lasting 1-3 s, that gradually levelled off. The initial uptake of Mn, like that of Ca, was greatly diminished by a 10 s pre-incubation in K-rich solution prior to the addition of radiotracer. Several Ca channel blockers, including Ni (0.03 mM), Sr (2.0 mM), Co (0.04 mM), Ba (1.5 mM) and La (0.2 mM), suppressed the K-stimulated uptake of Mn and of Ca to a similar extent. The K-stimulated uptake of Mn increased as a function of the external Mn concentration, and saturated at high external concentrations of Mn. These high concentrations of Mn also blocked the K-stimulated uptake of Ca. There was a decreased efflux of Ca, but not of Mn, from the synaptosomes when the external Na concentration was reduced. The Na-dependent efflux of Ca was diminished by external Mn, but was unaffected when the synaptosomes were loaded with Mn. The rate of [3H]DA release from striatal synaptosomes was less than 0.001 s-1 in non-depolarizing, low-K solutions, in the absence or presence of Mn and Ca (1 mM). The rate of release was also unchanged in depolarizing, K-rich solutions in the absence of these divalent cations. The addition of 1 mM-Mn to a K-rich solution increased the rate of DA release by about 40%, and the time course of release was linear for at least 30 s. The addition of 1 mM-Ca increased the rate of release nearly 100-fold during the first second, and thereafter the rate of release rapidly declined. Ni (1 mM) and, to a lesser extent, Mg (10 mM) reduced the rate of K-stimulated DA release that is dependent on either Mn or Ca. The pattern of inhibition of DA release resembled the pattern of inhibition of K-stimulated uptake of Mn and Ca. The addition of Mn to K-rich solutions stimulated the release of the neurotransmitters 5-hydroxytryptamine and gamma-aminobutyric acid, but not acetylcholine, from striatal synaptosomes.(ABSTRACT TRUNCATED AT 400 WORDS)

A simple technique to study the effect of drugs on the negative inotropic response elicited by postganglionic cholinergic nerve stimulation in guinea pig atria

A simple technique is described for evaluating the effects of drugs on postganglionic cholinergic nerve stimulation in guinea pig atrial tissue. Raising the voltage of stimulation tenfold (high voltage stimulation, HVS) in the left atrium produced a positive inotropic response. Propranolol abolished this effect but failed to reveal a negative inotropic response. However, in atria obtained from 6-hydroxydopamine (6-OHDA) pretreated animals, HVS produced a negative inotropic effect. The magnitude of the response was frequency-dependent and was potentiated by the anticholinesterase dyflos. Tetrodotoxin abolished the response but mecamylamine had no significant effect. Drugs which inhibit cardiac muscarinic receptors caused a parallel shift of the frequency-response relationship. In the right atrium, HVS caused a biphasic inotropic response. Propranolol or 6-OHDA pretreatment inhibited the positive inotropic effect, thereby enhancing the negative inotropic response. The effect did not show as great a dependence on the frequency of stimulation as in the left atrium and was less useful for comparing the effectiveness of muscarinic receptor antagonists.