Biomedical subjects

R S Manalis

Publications and source records attributed to R S Manalis.

11 recordsLinked to original sources

PubMed1984

Cadmium: effects on transmitter release at the frog neuromuscular junction.

Cd2+ competitively antagonizes Ca2+ in the evoked release of acetylcholine from nerve terminals in the frog sciatic-sartorius neuromuscular junction. The dissociation constant between Cd2+ and its receptor sites was calculated to be 1.7 microM. In agreement with previous work we found that brief exposures to 100-500 microM Cd2+ had little or no effect on spontaneous transmitter release. However, in contrast to other reports, prolonged exposures to 100-100 microM Cd2+ or tetanic nerve stimulation during brief exposures to high concentrations of Cd2+ produced substantial increases in spontaneous transmitter release. We conclude that Cd2+, when present at low concentrations, is a specific Ca2+ channel blocker. At higher concentrations, Cd2+ is either relatively impermeable to the presynaptic nerve membrane or, if it does enter the nerve terminal, it is less effective than such metals as Pb2+, La3+ or Hg2+ in increasing the rate of spontaneous transmitter release.

Acetylcholine↗

PubMed1984

Effects of lead on neuromuscular transmission in the frog.

The acute effects of Pb2+ on synaptic transmission at the frog neuromuscular junction were measured using conventional microelectrode techniques. Experiments were performed on preparations bathed in high magnesium/low calcium Ringer solution in order to record subthreshold endplate potentials (EPPs). The effects of Pb2+ on the muscle membrane and postsynaptic membrane were minimal since relatively high doses of Pb2+ caused no significant change in the input resistance of the muscle fiber and in the amplitude of the acetylcholine (ACh) iontophoteric potential when the ACh micropipette was highly localized. However, when the ACh micropipette was moved away from the receptors, the resulting ACh potential was reduced significantly by Pb2+. Pb2+ is a potent blocker of the EPP. Extracellular recordings from motor nerve terminals showed that endplate currents (EPCs) were reduced by Pb2+ while the nerve terminal potentials were unaffected. Therefore, Pb2+ blocks evoked transmitter release at a step following the depolarization of the nerve terminal. The blocking effect on the EPP was overcome when [Ca2+]o was raised. The log-log relationship between [Ca2+]o (abscissa) and EPP amplitude was shifted to the right in the presence of 1 microM Pb2+; the mean +/- S.E. slopes were 4.16 +/- 0.12 (control) and 4.05 +/- 0.13 (Pb2+). Reciprocal plots relating [Ca2+]o-1 to (EPP)-1/5 confirmed that Pb2+ competitively antagonized the action of Ca2+. The dissociation constant between Pb2+ and the Ca2+ receptor site was found to be 0.99 microM. Pb2+ is about 3 X 10(3) times more potent than is Mg2+, about 150 times more potent than is either Mn2+ or Co2+, and about 3 times more potent than Cd2+ is in blocking evoked release of ACh. After Pb2+ decreased the EPP, the MEPP frequency began to increase; this was probably the result of intracellular Pb2+ disrupting the Ca2+ sequestering activity of mitochondria and/or other intraterminal organelles. [Ca2+]i was thereby increased and an increase in MEPP frequency followed. Decreased MEPP amplitudes were observed when the MEPP frequency had been increased by Pb2+. Pb2+ may affect most chemical synapses in a manner which is similar to its effects on the neuromuscular junction and that this may be one of its important neurotoxic effects.

Acetylcholine↗

PubMed1984

Interactions of lead and cadmium on acetylcholine release at the frog neuromuscular junction.

The interactive effects of Cd2+ and Pb2+ on evoked and spontaneous transmitter release were studied in the sciatic nerve-sartorius muscle preparation of the frog (Rana pipiens). Either Pb2+ or Cd2+ competitively inhibited the actions of Ca2+ in bringing about evoked release, as measured by the endplate potential (EPP) amplitude. Combinations of Pb2+ and Cd2+ were additive in their effects on the EPP. The rate of spontaneous transmitter release was measured as the miniature endplate potential (MEPP) frequency. In contrast to their effects on the EPP, exposure of preparations to combinations of Pb2+ and Cd2+ actually increased the MEPP frequency less than exposures of the preparations to Pb2+ alone. The degree of reduction in MEPP frequency produced by Cd2+ depended upon the relative ratio of Pb2+ and Cd2+ ions. These results suggest that Pb2+ and Cd2+ ions competed for a common presynaptic receptor site during evoked release. They also suggest that Pb2+ ions may enter the nerve terminal, possibly through the Ca2+ channel, and that this entry is inhibited by Cd2+.

Acetylcholine↗

PubMed1984

Heavy metals: effects on synaptic transmission.

The acute effects of Pb++, Cd++ and Hg++ on synaptic transmission were studied on the in vitro sciatic nerve-sartorius muscle preparation of the frog, using electrophysiological techniques. Biochemical procedures were used to examine the effects of Pb++ and Cd++ on in vitro preparations of synaptosomes. In the electrophysiological studies Pb++ was shown to be a powerful competitive inhibitor of action potential-evoked release of acetylcholine (ACh) as judged by its depressant effects on the amplitude of endplate potentials (EPPs). The dissociation constant between Pb++ and the presynaptic Ca++ receptor is about 1 microM. Pb++ also increases spontaneous transmitter release as determined by the frequency of miniature endplate potentials (MEPPs). The increase in MEPP frequency is assumed to be due to an intracellular action of Pb++ to reduce the ability of nerve terminal organelles to buffer Ca++ and thereby, increases the intracellular concentration of Ca++. Cd++ also blocks evoked ACh release by a competitive inhibitory mechanism which appears similar to that for Pb++. The dissociation constant for Cd++ is about 2.8 microM. In contrast to Pb++, Cd++, does not increase resting MEPP frequency. Hg++ is unique in that it first causes an increase in evoked ACh release and then a sudden and complete blockade; the MEPP frequency follows a similar time course. The mechanism underlying these effects of Hg++ is uncertain. In rat brain synaptosomes, Pb++ and Cd++ competitively inhibit the K+-stimulated influx of 45Ca++. The dissociation constants for the interaction of Pb++ and Cd++ with Ca++ channels is 1.1 microM and 2.2 microM respectively. These data strongly support the idea that the electrophysiological effects of Pb++ and Cd++ on the EPP are due to a reduction of voltage-gated Ca++ entry into presynaptic nerve terminals.

Acetylcholine↗

PubMed1983

Influence of heavy metals on synaptic transmission: a review.

The acute effects of Pb++, Cd++ and Hg++ have been studied on the amphibian neuromuscular junction. These heavy metal ions primarily affect those presynaptic mechanisms which underlie neurotransmitter release; no significant postsynaptic effects were observed. All experiments were performed on the isolated sciatic nerve/sartorius muscle preparation. Conventional electrophysiological techniques using intracellular recordings were used to monitor acetylcholine (ACh) release. Ringer solutions usually contained high Mg++ and low Ca++ concentrations so that endplate potentials (EPPs) could be recorded under contraction-free conditions. Pb++, Cd++ and Hg++ were added to the Ringer solutions as chloride salts. Of the two forms of transmitter release, Pb++ blocked one (evoked release or EPP amplitude) and stimulated the other (the rate of spontaneous release or MEPP frequency). When a preparation was first exposed to a moderate dose of Pb++, the EPP amplitude decreased within about 1-2 min; however, at that time, the MEPP frequency was just beginning to increase. Low concentrations of Pb++ often reduced the EPP greatly without altering the MEPP frequency. Evidence is provided for a competitive interaction between Pb++ and Ca++ ions in evoked release which is believed to occur on the extracellular side of the nerve terminal. The dissociation constant between Pb++ and the presynaptic Ca++ receptor is about 1 microM. The increase in MEPP frequency is assumed to be due to an intracellular action of Pb++ which may reduce the ability of nerve terminal organelles to sequester Ca++ and thereby increase the intracellular concentration of ionized Ca++. Cd++ also blocks evoked ACh release by a competitive inhibitory mechanism similar to that for Pb++. Cd++ is slightly less potent than Pb++, the dissociation constant for Cd++ being around 2.8 microM. In contrast to Pb++, Cd++ does not increase resting MEPP frequency. Hg++ is unique in that it first causes an increase in evoked ACh release and then a sudden and complete blockade; the MEPP frequency follows a similar time course. The mechanism underlying these effects of Hg++ is uncertain.

Acetylcholine↗

PubMed1978

Blockade of neuromuscular transmission by enzymatically active and inactive beta-bungarotoxin.

beta-Bungarotoxins have been shown to be presynaptic blockers of neuromuscular transmission. This paper reports experiments using the most positively charged beta-bungarotoxin that elutes from a CM-Sephadex C-25 column. The toxin is shown to be a single polypeptide with a molecular weight of approximately 11,000 and has phospholipase A2 activity. The application of the enzymatically active toxin to the frog sciatic nerve-sartorius muscle preparation results in an initial decrease in the average endplate potential amplitude followed by a temporary rebound in endplate potential amplitude, and finally a complete inhibition of endplate potentials. Similarly, minature endplate potential frequency is initially reduced upon toxin application but then increases dramatically. After the phospholipase A2 of the toxin is inactivated, treatment with the toxin results in only the initial decrease in transmitter release. There results suggest that this beta-bungarotoxin acts in two functionally separate steps: (i) by binding to a specific presynaptic site possibly associated with calcium entry, and (ii) by perturbing the presynaptic membrane by its enzyme action, which results in an increase and then a failure in transmitter release.

Action Potentials↗

PubMed1977