GEOMETRY, KINETICS AND PLASTICITY OF RELEASE AND CLEARANCE OF ATP ANDNORADRENALINE AS SYMPATHETIC COTRANSMITTERS - ROLES FOR THE NEUROGENIC CONTRACTION
L. Stjarne et E. Stjarne, GEOMETRY, KINETICS AND PLASTICITY OF RELEASE AND CLEARANCE OF ATP ANDNORADRENALINE AS SYMPATHETIC COTRANSMITTERS - ROLES FOR THE NEUROGENIC CONTRACTION, Progress in neurobiology, 47(1), 1995, pp. 45-94
The paper compares the microphysiology of sympathetic neuromuscular tr
ansmission in three model preparations: the guinea-pig and mouse vas d
eferens and rat tail artery. The first section describes the quantal r
elease of ATP and noradrenaline from individual sites. The data are pr
oposed to support a string model in which: (i) most sites (greater tha
n or equal to 99%) ignore the nerve impulse and a few (less than or eq
ual to 1%) release a single quantum of ATP and noradrenaline; (ii) the
probability of monoquantal release is extremely non-uniform; (iii) hi
gh probability varicosities form 'active' strings; and (iv) an impulse
train causes repeated quantal release from these sites. Analogy with
molecular mechanisms regulating transmitter exocytosis in other system
s is proposed to imply that coincidence of at least two factors at the
active zone, Ca2+ and specific cytosolic protein(s), may be required
to remove a 'fusion clamp', form a 'fusion complex' and trigger exocyt
osis of a sympathetic transmitter quantum, and that the availability o
f these proteins may regulate the release probability. The second sect
ion shows that clearance of noradrenaline in rat tail artery is basica
lly greater than or equal to 30-fold slower than of co-reIeased ATP, a
nd that saturation of local reuptake and binding to local buffering si
tes maintain the noradrenaline concentration at the receptors, in spit
e of a profound decline in per pulse release during high frequency tra
ins. The third section describes differences in the strategies by whic
h mouse vas deferens and rat tail artery use ATP and noradrenaline to
trigger and maintain the neurogenic contraction.