Membrane protein mobility and exocytosis

Effects of vinblastine in the frog neuromuscular junction during repetitive stimulation

Vinblastine (10(-5) to 10(-4) M) applied in a Mg-blocked sciatic nerve-sartorius muscle preparation of the frog at the beginning of 20 min of repetitive stimulation of the nerve at 10 Hz, markedly inhibited the facilitation period. There was a smaller increase of end-plate potential amplitude, of quantal content, and of miniature end-plate potential (MEPP) frequency in comparison with that found in experiments without the drug. This depressant effect was more evident when the preparation was preincubated in vinblastine: synaptic responses often disappeared after a few minutes, whereas MEPP frequency attained a maximum at the 5th min. The results suggest a multiple site of action for vinblastine. First, an alteration of both vesicle and plasmalemma membrane, as revealed by the increase of MEPP frequency and by the change of the binomial release features. Second, the drug could damage the intracellular transport system, interfering with the mobilization of the vesicles toward the nerve endings, as shown by the very low or absent facilitation.

Cytochemical localization of ouabain-sensitive, K+ -dependent p-nitrophenylphosphatase and Ca++-stimulated adenosine triphosphatase activities in human parotid and submandibular glands

K+ -dependent p-nitrophenylphosphatase (pNPPase) and Ca++ -stimulated adenosine triphosphatase (ATPase) activities were studied in human parotid and submandibular glands using cytochemical methods at the ultrastructural level. In both glands, only the striated-duct epithelium showed K+ -pNPPase reaction product, thereby indicating the localization of Na+, K+ -ATPase. The precipitate was concentrated on the deep invaginations of the basolateral plasma membranes, in close association with their cytoplasmic surface. Ca++ -ATPase activity was also found on the basolateral plasma membranes, but two striking differences from the K+ -pNPPase distribution were observed: firstly, Ca++ -ATPase appeared in both acinar and ductal cells, and secondly, it was localized on the outer side of the plasma membranes.

Role of calcium in triggering the release of transmitters at the neuromuscular junction

Calcium ions have a key role in triggering the release of packaged transmitter at the amphibian neuromuscular junction and of the chromaffin granules at the adrenal medulla. It is suggested that (i) proteins on the vesicle and plasma membranes are of particular importance in promoting membrane fusion and exocytosis (ii) they may be divalent cation-stimulated ATPases, which form the calcium-binding sites or have a specific calcium-binding protein in close molecular apposition (iii) these ATPases in synaptic vesicles and chromaffin granules also generate a protonmotive force which is associated with the uptake of transmitter (iv) the osmotic properties of the vesicle may be important during fission, but it is not suggested that chemiosmotic effects are involved in Ca2+-triggered fusion (v) the action of calcium is markedly co-operative (vi) the adrenal medullary cell and the n.m.j. may differ in the Ca2+-binding site; there is evidence for the involvement of calmodulin in granule-plasmalemma fusion in the chromaffin cells, but not at present (surprisingly) for a role of this Ca2+-binding protein at the n.m.j. (vii) exocytosis requires MgATP (viii) phosphorylation of the ATPase may well be involved; phosphorylation via cAMP does not seem to be involved in fusion in either system (ix) the ATPase may undergo configurational changes during exocytosis and is markedly sensitive to the physical state of its phospholipid environment and to the oxidation of its -SH groups.

Inhibition of spontaneous transmitter release by the membrane ligand cationized ferritin

Exposure of the frog neuromuscular junction to cationized ferritin (200 micrograms ml-1) results in a reduction in MEPP frequency of approximately 25% within 10 min. This effect is not dependent on extracellular Ca2+, and identical results are obtained in salines buffered with EGTA. It is suggested that cationized ferritin does not inhibit transmitter release via a reduction in the intracellular Ca2+ concentration, but exerts its effect by a direct action on exocytosis. A mode of action is proposed. Unlike Con A, cationized ferritin is unable to suppress the stimulation of spontaneous release caused by high osmolarity.