5. THE ENDOCRINE PANCREAS
293
Pihl (1968) suggests that zinc may be implicated in storage and release
of insulin rather than in the production of the hormone. He considers the
almost complete absence of heavy metal in the islets of the guinea pig,
nutria, and Myxine as evidence supporting his conclusion. He furthermore
points out that the lack of heavy metals in the B cells of these species may
result from the absence of certain zinc-affinic histidine residues of the
B chains of their insulins. If this conclusion is correct, the insulin of
Petromyzon must be different from that of Myxine (see Section 11, D, 1).
B. Metabolic Pathways of the Islet Tissue
As in the liver, glucose freely diffuses into the islet tissue of teleosts
( Goetz and Cooperstein, 1962) and mammals (cf. Grodsky and Forsham,
1966). Thus the possibility exists that insulin release is directly influenced
by glycemia and that accumulation of intermediates of glucose metabolism and/or changes in the contribution to these pathways are involved
(cf. Hostetler et aZ., 1966). Both, the Embden-Meyerhof chain and the
pentose pathway are found in the islet tissue of both mammals and teleosts (cf. Grodsky and Forsham, 1966). When goosefish islets are incubated in vitro in the presence of radioactive glucose, at a “normal” glucose
level (25 mg %) the fraction of glucose metabolized via pentose pathway
is higher than under hyperglycemic conditions (200 mg X) . This eightfold
increase of glucose concentration in the medium is followed by a sevenfold increase of the disappearance of glucose from the medium. The estimated amount of glucose oxidized via the Krebs cycle increases fivefold,
while little or no increase in the utilization via pentose cycle was observed
(Hostetler et al., 1966). Much of the glucose which disappears from the
incubation medium may be used for the synthesis of fat and proteins
(Lazarow, 1965), and some is stored as glycogen (Hostetler et al., 1966).
Possibly, the pentose phosphate cycle is the prevalent pathway in
mammalian islet tissue (cf. Field and Lazarow, 1960; Brolin and Berne,
1967).
C. Enzymes of the Islet Tissue
The search for glucose metabolites which might be specifically involved in either insulin synthesis or insulin release, stimulated a number
of studies on the “enzyme fingerprint” of both mammalian and teleost
islet tissue (cf. Lazarow, 1963, 1965; Lazarow et al., 1964b; Grodsky and
293
Pihl (1968) suggests that zinc may be implicated in storage and release
of insulin rather than in the production of the hormone. He considers the
almost complete absence of heavy metal in the islets of the guinea pig,
nutria, and Myxine as evidence supporting his conclusion. He furthermore
points out that the lack of heavy metals in the B cells of these species may
result from the absence of certain zinc-affinic histidine residues of the
B chains of their insulins. If this conclusion is correct, the insulin of
Petromyzon must be different from that of Myxine (see Section 11, D, 1).
B. Metabolic Pathways of the Islet Tissue
As in the liver, glucose freely diffuses into the islet tissue of teleosts
( Goetz and Cooperstein, 1962) and mammals (cf. Grodsky and Forsham,
1966). Thus the possibility exists that insulin release is directly influenced
by glycemia and that accumulation of intermediates of glucose metabolism and/or changes in the contribution to these pathways are involved
(cf. Hostetler et aZ., 1966). Both, the Embden-Meyerhof chain and the
pentose pathway are found in the islet tissue of both mammals and teleosts (cf. Grodsky and Forsham, 1966). When goosefish islets are incubated in vitro in the presence of radioactive glucose, at a “normal” glucose
level (25 mg %) the fraction of glucose metabolized via pentose pathway
is higher than under hyperglycemic conditions (200 mg X) . This eightfold
increase of glucose concentration in the medium is followed by a sevenfold increase of the disappearance of glucose from the medium. The estimated amount of glucose oxidized via the Krebs cycle increases fivefold,
while little or no increase in the utilization via pentose cycle was observed
(Hostetler et al., 1966). Much of the glucose which disappears from the
incubation medium may be used for the synthesis of fat and proteins
(Lazarow, 1965), and some is stored as glycogen (Hostetler et al., 1966).
Possibly, the pentose phosphate cycle is the prevalent pathway in
mammalian islet tissue (cf. Field and Lazarow, 1960; Brolin and Berne,
1967).
C. Enzymes of the Islet Tissue
The search for glucose metabolites which might be specifically involved in either insulin synthesis or insulin release, stimulated a number
of studies on the “enzyme fingerprint” of both mammalian and teleost
islet tissue (cf. Lazarow, 1963, 1965; Lazarow et al., 1964b; Grodsky and