High concentration of somatostatin immunoreactivity in chicken pancreas
High Concentration ot Somatostatin Immunoreactivity in Chicken Pancreas
G. C. Weir 0
P.C. Goltsos 0
E.P. Steinberg 0
Y.C. Patel 0
0 Diabetes Unit of Massachusetts General Hospital, and Endocrine Division, Tufts-New England Medical Center Hospital , Boston, Massachusetts USA
Summary. Extracts of discrete lobes of chicken pancreas were assayed for somatostatin (SRIF), insulin, and glucagon immunoreactivity. The distribution of hormone concentration was correlated appropriately with the known distribution of A, B, and D-cells. Concentrations of all three hormones were highest in the splenic lobe. The glucagon content of the ventral and dorsal lobes was low. Evidence is presented that the SRIF-like material in the pancreatic extracts is very similar to synthetic cyclic SRIF; parallel immunoassay displacement curves and similar chromatographic elution profiles were obtained. The SRIF concentration in chicken pancreas is 21 times higher than that found in the rat. Chicken pancreas may provide a useful model for studies of somatostatin physiology.
Chicken pancreas; somatostatin; insulin; glucagon
-
Somatostatin (SRIF) is a cyclic fourteen amino acid
peptide originally isolated from the hypothalamus [
1
].
In addition to inhibiting growth hormone secretion it
has also been found to inhibit the release of insulin,
glucagon, gastrin, secretin, ACTH, prolactin and TSH
[
2-7
]. Furthermore it is capable of inhibiting gastric
acid [3] and pancreatic exocrine secretion [
7, 8, 9
].
The presence of somatostatin in extraneural
tissues was first suggested by Dubois who demonstrated
somatostatin immunoreactivity in pancreatic islets of
mammals and birds by immunofluorescence
techniques [
10
]. Somatostatin immunoreactivity was also
found in extracts of rat islets [
11
] and of whole
pancreas [
12
] using specific radioimmunoassays.
Furthermore the gel filtration elution profiles of the rat islet
extracts were identical t o those of synthetic cyclic
somatostatin (unpublished observations of Patel,
Reichlin and Weir). Somatostatin immunoreactivity
has also been found in gastrointestinal tissue [12 and
unpublished observations of Patel, Reichlin and
Weir]. The cell of origin of somatostatin in both the
pancreas and gut appears to be the D-cell [
13
].
Because of the large number of D-cells in the avian
pancreas it was decided to evaluate the somatostatin
content of the chicken pancreas.
Materials and Methods
Five-week old White Leghorn chickens were
sacrificed and individual pancreatic lobes (ventral,
dorsal, third, and splenic) were dissected out, weighed,
and placed in iced Hanks solution. As soon as possible
they were individually homogenized with a Ten
Broeck pyrex homogenizer in 5 ml of 2 N acetic acid.
Tubes containing these homogenates were then
suspended in boiling water for five minutes to inactivate
proteolytic pancreatic enzymes and the extracts
frozen pending assay. Before dilution for assay aliquots
of the extracts were neutralized with NaOH.
The radioimmunoassay for somatostatin (method
of Patel, and Reichlin, unpublished) utilized a rabbit
antiserum obtained following immunization with
SRIF conjugated to thyroglobulin. This antiserum was
highly specific for SRIF with negligible or no
crossreactivity with insulin, glucagon, gastrin,
cholecystokinin-pancreozymin, cholecystokinin octapeptide
(C8-CCK), secretin, vasoactive peptide (VIP),
substance P and neurotensin. [1251]Tyrl-SRIF was
iodinated with chloramine T. Synthetic cyclic
somatostatin (kindly provided by the Ayerst Company,
Montreal, Canada) was used as standard. The buffered
diluent was 0.05 M PO 4 with 0.1% crystalline BSA at
a pH 7.5. The incubation volume was 0.7 ml and
dextran-charcoal separation was employed [
14
]. The
detection limit for t h e assays done in this study was
3-4 pg of SRIF per incubation tube.
Glucagon concentrations were determined with
radioimmunoassay methods previously described [
15
]
utilizing antiserum 30K provided by Dr. Roger
Unger. Pork glucagon (lot GLF 599A, EliLilly and
Company) was used as standard. The insulin
radioimmunoassay method was that of Albano et al. [
16
] and
chicken insulin (lot 615-1082B-249) was kindly
donated by Dr. Ronald Chance of Eli Lilly and
Company.
Results
The hormone concentration of individual lobes of the
chicken pancreas is presented in Table 1. The
concentration of all three hormones is greatest in the
splenic lobe, with the glucagon concentration being
strikingly high. It is noteworthy that the glucagon
concentration of the ventral and dorsal lobes is very
low. When the total hormonal content of each lobe is
determined (Table 2) it c a n b e appreciated that the
splenic lobe contributes little to total amount of
insulin or SRIF, but contains at least half of the total
a Calculations could only be made for five total pancreata because
a third lobe from one of the six chickens was lost.
glucagon. The contribution of the splenic lobe may
actually be slightly higher (...truncated)