5. THE ENDOCRINE PANCREAS
291
111. FUNCTION AND METABOLISM OF THE ISLET CELLS IN FISHES
A. Synthesis, Storage, and Release of Insulin
Studies on the mechanism of protein biosynthesis particularly require
tissues of great viability, synthesizing preferably a protein of known structure. Moreover, the tissue must be available in considerable amounts.
These requirements are met by the Brockmann bodies. After Lazarow
and co-workers had explored this possibility (cf. Lazarow et al., 1964a),
studies on the islet tissue of teleosts yielded important general information on protein synthesis and particularly on the biosynthesis of insulin.
When islet tissue of teleosts is incubated in vitro in the presence of
radioactive amino acids or glucose, there is a progressive incorporation
of radioactivity into protein. After incubation with radioactive amino
acids, Bauer and Lazarow (1961) found that the specific radioactivity of
the insulin fraction of the islet tissue of the goosefish, Lophius piscatorius,
was three times greater than that of the other tissue proteins. Likewise,
goosefish islets incorporated radioactivity from glu~ose-U-~~C into many
constituent amino acids of the highly purified insulin fraction (Bauer
et al., 1965). Similarly, Hellman and Larsson ( 1961) observed incorporation of radioactivity from labeled glucose into amino acids of the islet
tissue of Cottus quadricornis. Corresponding findings on the uptake of
glucose carbon and leucine into the insulin fraction of the islets of the
toadfish, Opsanus tau, were obtained by Humbel and co-workers (cf.
Humbel and Renold, 1963). Using radioactive isoleucine for in vitro studies on toadfish islets, Humbel (1963) also obtained preferential labeling
of the A chain of insulin. This would be expected from the absence of
this amino acid in the B chain.
The effect of tolbutamide on the incorporation of glu~ose-U-~~C and
l e ~ c i n e - ~ H
into the insulin fraction of goosefish islets in vitro was studied
by Bauer and associates (cf. Lazarow, 1965). Contrary to its antidiabetic
action in mammals, it inhibited the incorporation of glu~ose-U-~~C, while
there was no consistent effect on l e ~ c i n e - ~ H
incorporation.
In extensive studies, Lazarow and co-workers investigated the relationship between insulin synthesis and insulin storage in goosefish islets
in vivo and in vitro. Their findings, especially the results of pulse-chase
experiments, suggest the microsomes as primary sites of insuIin synthesis,
and a subsequent transfer of the hormone to the secretion granules (cf.
Lazarow, 1963, 1965; Lazarow et al., 1964a; Bauer et al., 1965). During
these experiments, Bauer et al. (1966) obtained evidence that the secre-
291
111. FUNCTION AND METABOLISM OF THE ISLET CELLS IN FISHES
A. Synthesis, Storage, and Release of Insulin
Studies on the mechanism of protein biosynthesis particularly require
tissues of great viability, synthesizing preferably a protein of known structure. Moreover, the tissue must be available in considerable amounts.
These requirements are met by the Brockmann bodies. After Lazarow
and co-workers had explored this possibility (cf. Lazarow et al., 1964a),
studies on the islet tissue of teleosts yielded important general information on protein synthesis and particularly on the biosynthesis of insulin.
When islet tissue of teleosts is incubated in vitro in the presence of
radioactive amino acids or glucose, there is a progressive incorporation
of radioactivity into protein. After incubation with radioactive amino
acids, Bauer and Lazarow (1961) found that the specific radioactivity of
the insulin fraction of the islet tissue of the goosefish, Lophius piscatorius,
was three times greater than that of the other tissue proteins. Likewise,
goosefish islets incorporated radioactivity from glu~ose-U-~~C into many
constituent amino acids of the highly purified insulin fraction (Bauer
et al., 1965). Similarly, Hellman and Larsson ( 1961) observed incorporation of radioactivity from labeled glucose into amino acids of the islet
tissue of Cottus quadricornis. Corresponding findings on the uptake of
glucose carbon and leucine into the insulin fraction of the islets of the
toadfish, Opsanus tau, were obtained by Humbel and co-workers (cf.
Humbel and Renold, 1963). Using radioactive isoleucine for in vitro studies on toadfish islets, Humbel (1963) also obtained preferential labeling
of the A chain of insulin. This would be expected from the absence of
this amino acid in the B chain.
The effect of tolbutamide on the incorporation of glu~ose-U-~~C and
l e ~ c i n e - ~ H
into the insulin fraction of goosefish islets in vitro was studied
by Bauer and associates (cf. Lazarow, 1965). Contrary to its antidiabetic
action in mammals, it inhibited the incorporation of glu~ose-U-~~C, while
there was no consistent effect on l e ~ c i n e - ~ H
incorporation.
In extensive studies, Lazarow and co-workers investigated the relationship between insulin synthesis and insulin storage in goosefish islets
in vivo and in vitro. Their findings, especially the results of pulse-chase
experiments, suggest the microsomes as primary sites of insuIin synthesis,
and a subsequent transfer of the hormone to the secretion granules (cf.
Lazarow, 1963, 1965; Lazarow et al., 1964a; Bauer et al., 1965). During
these experiments, Bauer et al. (1966) obtained evidence that the secre-