20P
Medical Research Society
65. EFFECT O F HUMAN MYASTHENIC IMMUNOGLOBULIN G ON MOUSE ACETYLCHOLINE
RECEPTOR
S. G. WILSON,A. VINCEN? AND J. NEWSOM-DAVIS
The Royal Free Hospital, London
Myasthenia gravis (MG) is characterized by a decrease in the
total number of functioning acetylcholine receptors (AChR) in
the postsynaptic membrane of skeletal muscles. This appears to
follow binding of anti-AChR antibody, which can accelerate
AChR degradation (Drachman et al., 1979, New England
Journal of Medicine, 298, 1116-1 122), providing one mechanism that could underlie the decrease in AChR.
We have injected mice with MG immunoglobulin G (IgG)
and investigated the relationship between the amount of antiAChR binding and the effect of binding on the total amount of
AChR. After seven daily intraperitoneal injections of individual
myasthenic IgG, animals were killed and the diaphragms
removed and incubated in uitro in lzJI-labelled a-BuTx, which
binds specifically to AChR. The end-plate regions of each
hemidiaphragm were isolated and the specific radioactivity
associated with the end-plate region was measured. The remaining mouse carcasses were homogenized, the AChR extracted in
Triton X-100 and labelled with 12JI-labelled d-BuTx. The
percentage of total extractable receptors which had IgG bound
was then measured by immunoprecipitation with anti-IgG.
The effect of myasthenic IgG on mouse AChR was variable.
Several individuals’ sera produced a statistically significant
decrease in the total number of diaphragm end-plate AChR but
others did not. The percentage of extractable AChR with antibody bound varied from 30 to 80%. Some IgG preparations
produced 70% saturation of the AChR without affecting either
the total number of AChR or rate of degradation measured in
uiuo in other experiments. Others accelerated degradation but
there was no significant decrease in the total number of AChR.
The binding of human anti-AChR antibody per se to mouse
AChR does not necessarily result in any overall loss ofend-plate
receptor. Anti-AChR can bind without increasing the rate of
AChR degradation and AChR degradation can increase
without an overall loss of AChR. Other factors may be involved
in AChR loss in MG.
66. PRODUCTION O F THROMBOXANE B, AND THIOBARBITURIC ACID-REACTIVE SUBSTANCES BY
HUMAN BLOOD PLATELETS
P. B. B. JONES,L. C. BEST, T. K. HOLLANDAND R. G. G.
RUSSELL
Department of Human Metabolism and Clinical Biochemistry,
Universily of Shefield Medical School, Shefield, U.K.
The aggregation of human platelets is thought to be accompanied by the biosynthesis of unstable prostaglandin endoperoxides and thromboxane A, (TxA,). These substances are
potent inducers of aggregation. Owing to their instability the
endoperoxides and TxA, are usually assayed by indirect
methods. The measurement of malondialdehyde (MDA), a
stable by-product of prostaglandin metabolism, is commonly
used as an index of platelet prostaglandin and thromboxane
metabolism. Recently radioimmunoassays for TxB,, a stable
breakdown product of TxA,, have been developed. We have
measured MDA (by a thiobarbituric acid reaction) and TxB,
(by radioimmunoassay) in suspensions of human platelets, in
order to define the conditions under which these substances are
produced, and also to compare the two methods as reliable
indicators of platelet thromboxane biosynthesis.
Exogenous sodium arachidonate was converted into TxB,
and MDA in a dose-dependent manner by platelet-rich plasma,
Sepharose 2B gel-filtered platelets, and by bovine platelet microsomes. The aggregating agents, collagen, thrombin and the ionophore A23187, also produced large amounts of TxB, and MDA,
roughly in parallel. In contrast, ADP, adrenaline and a prosta-
glandin endoperoxide analogue (U46619) caused only a small
production of TxB, and MDA. Preincubation of platelets for 10
min with a specific thromboxane synthetase inhibitor, 1-Nbutylimidazole, resulted in parallel inhibition of both TxB, and
MDA, suggesting that the formation of both of these substances is catalysed by the enzyme, thromboxane synthetase.
In general, the molar quantities of MDA formed by platelets
were considerably higher than those of thromboxane B,. In
addition, thiobarbituric acid-reactive substances could be
detected when TxB, production was completely inhibited by
aspirin, particularly in the presence of exogenous arachidonate.
These findings suggest that the thiobarbituric acid method for
MDA estimation can also detect other products of lipid oxidation. This lack of specificity of the MDA assay renders it less
useful than radioimmunoassay for thromboxane B, in the
assessment of prostaglandin and thromboxane biosynthesis in
platelets. These limitations should be considered when interpreting MDA measurements in clinical disorders.
67. DIABETIC GLUCOSE CONTROL, LIPIDS AND
TRACE ELEMENTS AFTER 6 MONTHS ON GUAR
D. J. A. JENKINS,D. REYNOLDS,T. M. S. WOLEVER,R.
NINEHAM,
R. H. TAYLOR
AND T. D. R. HOCKADAY
Department of the Regius Professor of Medicine, Rodclue
Infirmary, and University Laboratory of Physiology, Odord,
Royal Postgraduate Medical School, London and Central
Middlesex Hospital, London
In short-term studies, certain forms of dietary fibre such as guar
reduce urinary glucose loss and insulin requirement. However,
there are few reports on possible long-term changes in trace
element and lipid concentrations. We therefore present data on
the first diabetic patients to complete 6 months’ dietary supplementation on guar.
Eight diabetic patients (mean f SEM: 46 f 6 years; 96 f 15%
of ideal weight; six on insulin, 49 f 7 i d d a y , two on oral
agents) took 18-26 g of guar in crispbread per day. Patients
were studied both before and on entering the seventh month on
guar and showed a mean weight loss of 2.7 f 0-6 kg (P <
0.01). No change was seen in home urine glucose tests, and
fasting blood glucose rose from 13.1 to 15.8 mmol/l. Nevertheless, the insulin dose in the six had been reduced by 26 f 6% (P
< 0.01) and oral agents reduced by 25% and 40% respectively.
Total cholesterol was reduced by 0-5 f 0.17 mmol/l (P <
0.05) from 5.0 f 0.19 mmolh, with a reduction in the LDL
fraction of 0.59 f 0.16 (P< 0.01). Serum HDL cholesterol, triglyceride, creatinine, urea and metal ions were all unchanged.
Pretreatment and 6 months guar concentrations (mmol/l) were:
calcium, 2.30 f 0.02 and 2.36 f 0.03; phosphate, 1.14 f 0.04
and 1.17 f 0.04; copper, 1.14 f 0.13 and 1.07 f 0.06; zinc,
1.20 f 0.06 and 1.15 f 0.05. Haemoglobin concentration,
mean cellular haemoglobin and mean cellular volume did not
change during treatment.
We therefore find no evidence that prolonged use of guar will
cause trace element deficiency.
68. PADRENORECEPTOR
BLOCKADE
AND
DIABETES MELLITUS: EFFECT O F OXPRENOLOL
AND METOPROLOL ON THE METABOLIC, CARDIOVASCULAR AND HORMONAL RESPONSE TO
INSULIN-INDUCED HYPOGLYCAEMIA IN NORMAL
SUBJECTS
H. KEENAND G. c. VIBERTI
Guy’s Hospital Medical School, London
In a double-blind randomized study the effects of oxprenolol, a
non-selective padrenoreceptor-blocking agent, and of
metoprolol, a selective P,-receptor-blocking agent, on insulininduced hypoglycaemia were tested in seven normal subjects.
None of the drugs potentiated the hypoglycaemic effect of