The EFFECTS OF THE CALCIUM IONOPHORE, A23187, ON THE AXOPLASMIC TRANSPORT OF DOPAMINE Β‐HYDROXYLASE

Abstract

1The effects of the ionophore, A23187, on the intra-axonal transport of dopamine f3-hydroxylase (DBH) were investigated in the cat hypogastric nerve-inferior mesenteric ganglion preparation by monitoring, in vitro, the enzyme accumulation above a ligature, 2 to 2.5 cm distal to the ganglion.2 DBH accumulation in the proximal segment immediately above the ligature (P,) increased linearly up to 6 h, during incubation in normal Krebs solution at 37°C. The ionophore, A23187, interfered with the enzyme accumulation, but did not modify the previously accumulated DBH activity present in P1. 3 The blocking effects of A23187 on DBH transport were greatly impaired in the absence of extracellular calcium ions; an excess of calcium in the bathing solution (7.5 mM) itself blocked the enzyme transport by 50%.4 A23187 did not significantly modify the levels of adenosine triphosphate (ATP) in the segments P1 and P2 of the nerve proximal to the ligature. 5 Nerves incubated in an A23187-containing medium showed many mitochondria of normal shape and fine structure; however, typical microtubules or filaments were not seen in these preparations. 6 The results suggest that the ionophore A23187, by considerably raising the axoplasmic ionized calcium levels, interferes with the assembling of microtubules. In this manner, the ionophore would inhibit the transport of adrenergic vesicles and therefore of DBH along the axon. The results also provide additional evidence in favour of the view that for the transport system to work adequately, it is necessary to maintain the intra-axoplasmic ionized calcium concentration between certain critical levels.

Mechanism of inhibitory action of capsaicin on particulate axoplasmic transport in sensory neurons in culture

“…Ca2+ depolymerizes microtubules both in vivo and in vitro, and blocks axoplasmic transport at high concentrations such as 1 m M ( Schlaepfer, 197 1 ;Schlaepfer and Bunge, 1973;Esquerro, Garcia, and Sanches-Garcia, 1980;Kanje et al, 1981 and1982;Gotoh, Takenaka, and Shozushima, 1983). Axoplasmic transport in an extruded axoplasm from a squid was reported to be normal for Ca2+ concentrations from 0 through 10 pM (Brady, Lasek, and Allen, 1985).…”

Is protease activity involved in fast axonal transport?

“…High external Ca2+ concentrations (15-20 mM Ca2+) (Edstrom 1974), or the presence of ionophores like X-537 A and A 23187, which increase the permeability of cell membranes to Ca2+, are known to partially inhibit rapid axonal transport in this and other preparations (Esquerro et al 1980, Kanje et al 1981. Similar conditions have also been shown to disrupt microtubules and to activate proteases which degrade neurofilaments (Schlaepfer & Micko 1978, Pant & Gainer 1980.…”

“…It is known from previous experiments that conditions which increase the axonal Ca2+-concentration, i.e. exposure of nerves to high external Ca2+-concentrations or Ca2+ ionophores block AXT (Edstrom 1974, Kanje et al 1982. If this inhibition was due to increased proteolytic activity it could be expected that TLCK should counteract the effect.…”

“…The activation results in derangement of axonal structures similar to those observed during Wallerian degeneration. The same conditions are also known to inhibit AXT (Edstrom 1974, Kanje et al 1981, Esquerro et al 1980. This inhibition has usually been ascribed to a Ca'+-dependent disassembly of microtubules (MT).…”

“…This inhibition has usually been ascribed to a Ca'+-dependent disassembly of microtubules (MT). Activation of proteolysis may also be involved (Kanje et al 1982).…”