16. ACTIONS OF HORMONES
595
did not inhibit mitosis if either L-lactate or fructose were used, a finding
which suggests that the growth hormone selectively inhibits the
glucokinase reaction. Growth hormone and insulin proved to be
antagonists under the experimental conditions used. The lowering
of the mitotic rate by the growth hormone was partially offset by large
doses of insulin, and mitotic stimulation due to insulin could be inhibited by large doses of the growth hormone.
Bullough's findings and conclusions have been challenged by Gelfant
(1960a, b ) ; in particular, the theory that insulin and growth hormone
mediate their mitotic effects in mouse ear epidermis through the glucokinase reaction could not be confirmed by this author.
Convincing evidence that insulin also influences other pathways of
carbohydrate metabolism in culture was presented by Sidman (1956b)
who grew adipose tissue of the rat in the presence of this hormone.
Brown adipose tissue differentiates in culture in the absence of nervous
or endocrine influences and is determined while still part of the loose
mesenchyme (Sidman, 1956a). Sidman explanted mesenchyme of the
interscapular region of late foetal rats. After a few days the cells withdrew their amoeboid processes and aggregated into clearly demarcated
islands. The cells increased in size and deposited in their cytoplasm
small droplets of lipid which enlarged and coalesced; eventually,
typical adipose cells were formed. This occurred faster and more
profusely in cultures grown in serum than in defined medium.
The addition of insulin to the medium (0-04-4 /xg/ml) promoted
the synthesis of glycogen, accelerated the deposition of lipids and
prolonged the survival time of the tissue in culture (Figs. 1 and 2). This
effect was greater in cultures grown in serum than in synthetic medium
and the author contends that the serum may contain substances which
potentiate the action of the hormone. After 3-4 days in culture, glycogen
synthesis decreased even if fresh insulin was added to the medium, but
this fall could be mitigated by addition of glucose. The author claims
that this decrease in glycogen synthesis was not due to exhaustion of
essential metabolites but represents a shift in synthetic activity in the
direction of lipogenesis. The tissue retains the ability to synthesize
glycogen beyond a growth period of 4 days in vitro, since insulin still
stimulates glycogen synthesis when added to cultures after various
periods of growth in normal medium.
Haugaard, Haugaard and Stadie (1954) and Haugaard and Marsh
(1952) demonstrated the rapid binding of isotope-labelled insulin and
Chayen and Smith (1954) that of fluorescein-labelled insulin to many
tissues, including adipose tissue. Sidman (1956b) confirmed their
results and obtained evidence that the hormone was not only rapidly
bound but also remained effective for several days in vitro. Thus, cul-
595
did not inhibit mitosis if either L-lactate or fructose were used, a finding
which suggests that the growth hormone selectively inhibits the
glucokinase reaction. Growth hormone and insulin proved to be
antagonists under the experimental conditions used. The lowering
of the mitotic rate by the growth hormone was partially offset by large
doses of insulin, and mitotic stimulation due to insulin could be inhibited by large doses of the growth hormone.
Bullough's findings and conclusions have been challenged by Gelfant
(1960a, b ) ; in particular, the theory that insulin and growth hormone
mediate their mitotic effects in mouse ear epidermis through the glucokinase reaction could not be confirmed by this author.
Convincing evidence that insulin also influences other pathways of
carbohydrate metabolism in culture was presented by Sidman (1956b)
who grew adipose tissue of the rat in the presence of this hormone.
Brown adipose tissue differentiates in culture in the absence of nervous
or endocrine influences and is determined while still part of the loose
mesenchyme (Sidman, 1956a). Sidman explanted mesenchyme of the
interscapular region of late foetal rats. After a few days the cells withdrew their amoeboid processes and aggregated into clearly demarcated
islands. The cells increased in size and deposited in their cytoplasm
small droplets of lipid which enlarged and coalesced; eventually,
typical adipose cells were formed. This occurred faster and more
profusely in cultures grown in serum than in defined medium.
The addition of insulin to the medium (0-04-4 /xg/ml) promoted
the synthesis of glycogen, accelerated the deposition of lipids and
prolonged the survival time of the tissue in culture (Figs. 1 and 2). This
effect was greater in cultures grown in serum than in synthetic medium
and the author contends that the serum may contain substances which
potentiate the action of the hormone. After 3-4 days in culture, glycogen
synthesis decreased even if fresh insulin was added to the medium, but
this fall could be mitigated by addition of glucose. The author claims
that this decrease in glycogen synthesis was not due to exhaustion of
essential metabolites but represents a shift in synthetic activity in the
direction of lipogenesis. The tissue retains the ability to synthesize
glycogen beyond a growth period of 4 days in vitro, since insulin still
stimulates glycogen synthesis when added to cultures after various
periods of growth in normal medium.
Haugaard, Haugaard and Stadie (1954) and Haugaard and Marsh
(1952) demonstrated the rapid binding of isotope-labelled insulin and
Chayen and Smith (1954) that of fluorescein-labelled insulin to many
tissues, including adipose tissue. Sidman (1956b) confirmed their
results and obtained evidence that the hormone was not only rapidly
bound but also remained effective for several days in vitro. Thus, cul-